Wireline Gyro Surveying
Field of the Invention
This invention relates to method of gyro surveying a borehole, wireline equipment for use in a borehole, and a gyro survey instrument for use in a wireline core drilling operation. In particular, the invention has application to the field of gyro surveying for mineral exploration and mining with wireline core drilling applications.
Background of the Invention
In the minerals exploration and mining industry it is commonplace practice to map the geological resource by drilling a large number of inclined boreholes and extracting core samples from them. Core samples are solid cylinders of geological material, typically 3m long, that geologists can analyse to determine the composition is of the geology in which the borehole has been drilled. The extent of the geological resource can be mapped by analysing core samples extracted from the many boreholes. This activity takes place primarily during the exploration drilling phase to validate the extent of a mineral resource for investment purposes.
Unlike some types of drilling, the primary aim of core drilling is not to make a hole, but to extract core samples. A core drill string is a series of connected long hollow tubes (called drill rods or drill pipes), with a barrel at the end connected to a special core bit at the bottom of the hole. As the drill operator rotates the drill string and applies downward pressure to it, abrasion from the core bit cuts into the rock, pushing the cylindrical core into the core barrel. As the drill moves further into the Earth, the drill operator adds rods onto the top end, thereby increasing the length of the drill string Wireline core drilling is a special type of core drilling, most commonly used for minerals exploration. The principal difference between wireline core drilling and conventional core drilling is the method of removing the core samples. In conventional core drilling, the entire drill string and core barrel must be removed from the hole in order to retrieve the core sample. This is time-consuming, as the drill rods must be removed one at a time and the borehole can be several kilometres deep.
With wireline core drilling, a core sample can be removed from the bottom of the hole without removing the drill string from the borehole. Known wireline core drilling equipment is illustrated in Figs. 1 and 2. This removal is achieved by means of a core barrel 1 that fits inside the drill string, which can be raised and lowered. A core barrel 1 that contains a core sample 2 can be retrieved back to surface and an empty core barrel can then be lowered into the borehole to enable core drilling to continue. The empty core barrel is lowered through the centre of the drill rods in the borehole and latches into the final section of the drill string immediately behind the drill bit 3. As shown more clearly in Fig. 2, the core barrel 1 typically includes a core barrel stabiliser 9, core catcher 10 and adaptor 11, the latter for providing an interface with a latching head assembly 12, as described below.
When the drill operator wants to remove the core, typically an "overshot" 4 (i.e. a female mating portion) is lowered on the end of an armoured cable 5, commonly referred to as a wireline or slick line, connected to a winch 8 at the surface. A swivel 13 is typically provided between the wireline 5 and overshot 4 to reduce torsional stresses. The overshot 4 attaches to a corresponding male mating part 6 (the "spearpoint") of the latching head assembly 12 at the back of the core barrel.
The wireline 5 is then winched back and the core barrel 1, which contains the core sample 2, disengages itself from the outer tube 7. It can then be brought to the surface by winching in the wireline 5. Typically, the same overshot arrangement is then used to lower an empty core barrel into the borehole. The core barrel may be lowered on the wireline all the way to the drill bit 3 or may be lowered part way on the wireline 5, typically to the water table, then released to free-fall or pumped the remaining distance to the drill bit 3 while the wireline 5 is retracted.
As part of the wireline core drilling process, there is a need to determine the depth, direction and inclination of the borehole at periodic intervals and also the rotational orientation of each core relative to the vertical. This, along with similar information obtained from other boreholes, allows the composition and extent of the surrounding geology to be determined. Typically the borehole is surveyed every 3am using a directional survey instrument. The survey instrument is lowered into the borehole on the slick line after the core barrel has been retrieved. The survey instrument provides a measure of the borehole direction relative to North (azimuth) and the angle from the vertical (inclination or "dip").
These directional survey instruments are typically magnetic compass type instruments, which measure the azimuthal direction of the borehole, i.e. with respect to magnetic North. To obtain this information however, the instrument must be in an area of non-magnetic disturbance. For wireline core drilling, the rods must be thin-walled to maximise the diameter of the core and are therefore typically made of high-tensile steel. Magnetic compass type instruments therefore cannot be used in close proximity to the rods.
This problem of magnetic interference is usually overcome by running stainless steel drill rods at the lower end of the drill string and by pulling back the drill string a few metres so that the drill bit is drawn back from the bottom of the hole. The magnetic instrument is then lowered into the borehole on the slick line after retrieving the core barrel into the exposed open-hole' section to obtain a survey, away from the magnetic interference effects of the drill string.
An alternative type of survey instrument incorporates gyroscopic rate sensors (hereinafter referred to as "gyros") to provide directional information, either by measuring the Earth's rate of rotation within the borehole or by measuring changes in direction downhole, relative to a known surface reference direction. These types of survey instrument are insensitive to local magnetic disturbances and do not require the use of any non-magnetic drill rods or require the drill string to be pulled back to provide an open hole section in which to run the instrument.
In all these cases, the directional survey instrument is deployed after retrieval of the core barrel and when drilling has stopped, and so therefore consumes expensive rig time.
The application of magnetic and gyro based directional survey instruments described above is well known. Gyro instruments are also frequently run after completion of the drilling process to verify the direction of a borehole and validate the accuracy of magnetic tools which have previously been run during the drilling phase. This ensures that any survey errors due to local magnetic interference within the borehole are minimised and that the direction of the borehole is verified independently of the drilling process.
Gyro instruments can also provide a higher level of accuracy than magnetic instruments. High accuracy gyro instruments are capable of sensing the component of Earth's rate of rotation about its spin axis that lies in the direction of each of the gyro's sensing axes. Such instruments can therefore determine the direction of true north from within the borehole using a technique commonly referred to as gyro-compassing. These instruments are often referred to as north-seeking as they do not require any directional alignment on the surface prior to their deployment into the borehole to determine its direction.
Borehole surveys may be categorised typically as single-shot surveys, multi-shot surveys or continuous surveys according to the following definitions. A single-shot survey is defined as the measurement of one or more sets of survey data taken at a single depth, typically at the bottom of the borehole. A multi-shot survey is defined as the measurement of one or more sets of survey data, taken at multiple discrete depths by pausing (i.e. keeping the instrument stationary in the borehole) during either the in-run or the out-run or both. A continuous survey is defined as the continuous gathering of survey data during the in-run or out-run or both, while the instrument continues its motion along the borehole, including, optionally, any periods when the instrument is at a constant depth. In all cases, the depth of each calculated survey point must be determined.
In the case of single-shot surveys, the depth may be determined from the length of wireline paid out at the survey point. Alternatively, where the single-shot surveys are taken at the bottom of the borehole, the pipe-tally (i.e. the number of drill pipes in the drill string) may be used to determine the depth.
In the case of multi-shot surveys, the depth of each survey point may be determined independently from the survey instrument but there must be a means by which each
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survey taken by the survey instrument can be associated with the corresponding depth measurement. This may be achieved by recording on the surface, the length of wireline paid out at each occasion that the winch is stopped and the times at which this occurs. Similarly, the survey instrument may record the time at which each survey is taken using a timer synchronised to the surface clock. The survey times and depth information may subsequently be matched to enable survey information to be calculated.
In the case of continuous surveys, the depth and time may be recorded periodically at the surface and the survey instrument may record the time periodically as continuous survey measurements are taken, using a timer synchronised to the surface clock. The continuous survey times may subsequently be matched with the depth information to enable survey information to be calculated.
Alternatively, the survey instrument may determine the depth autonomously, without the need to match with externally-derived depth information. This may be achieved by double-integrating the information derived from its accelerometers to determine the depth at each survey point or at known times, although this method is typically prone to significant cumulative errors. Alternatively, the survey instrument may incorporate or make use of an additional sensor such as a casing collar locator that can detect joints in the drill pipe. The depth can be determined from the total number of drill pipes passed. By combining these two methods, the depth can be determined more accurately.
Each survey point in a single-shot or a multi-shot survey is typically performed while the instrument is stationary in the borehole. A gyro that is substantially stationary with respect to the Earth will produce an output that is dependent on the angle between its rate sensing axis and the Earth's spin axis. The gyro output will also be subject to an unknown bias offset, which may vary slowly over time and may be highly temperature sensitive. The gyro output may also be sensitive to the magnitude and direction of any external forces such as gravity.
For convenience, it is helpful to define an axis convention that is typically, but not universally used in borehole survey instruments. The following axis convention is used hereinafter: The 7 axis is parallel with the instrument axis and is positive in the direction heading into the borehole.
The X axis is orthogonal to the Z axis The Y axis is orthogonal to both the X axis and the 7 axis such that the X, Y and Z axes form a conventional right-handed set If a gyro is rotated to different orientations about an axis that is orthogonal to its rate sensing axis, the component of its output attributable to Earth's rotation will vary sinusoidally with the rotation angle. Typically, this rotation is about the Z-axis and the rotation may be achieved either by means of a rotating platform within the instrument upon which the gyro is mounted or by rotating the instrument within the borehole. In the latter case, the twisting of the wireline or some other means may be used to rotate the instrument in the borehole, thereby removing the need for a rotating platform within the instrument.
Thus if a gyro whose rate sensing axis lies in the XY plane is rotated about the Z-axis to different angular positions, its output will vary sinusoidally with the rotation angle as shown in Fig. 3. The amplitude and phase of this sinusoid are dependent upon the orientation of the instrument with respect to the Earth's spin axis. The phase is also dependent on the arbitrary starting angle of the rotation.
The mean value of the sinusoid is the unknown bias offset and a well-known technique for determining its value is to calculate the mean of any pair of gyro output values taken from diametrically opposite orientations of the sensing axis (i.e. outputs separated by 1800 in Fig. 3). Once calculated, the bias offset can thereafter be subtracted from the gyro output to reveal the rate of rotation in the sensing axis.
To determine the azimuth of the instrument (i.e. the bearing relative to true north in the horizontal plane), rotation rates from at least two different, typically orthogonal sensing axes are required, and these are typically in the X and Y axes. The rotation rates in the required sensing axes may be determined either from a dual-axis sensor or from separate sensors with the required axis orientations or from a single sensor that is rotated sequentially to the required orientations.
Additionally, to determine the azimuth of the instrument, the angle of the plane containing the rate-sensing axes relative to the horizontal is required. This may be determined using single or multiple gravity measuring devices such as accelerometers or inclinometers. Typically three orthogonal accelerometer axes are used to give an accurate estimate of the inclination angle, i.e. the angle of the instrument axis from the vertical. Typically one of these accelerometer axes is aligned with the instrument axis and the other two accelerometer axes are aligned perpendicular to the instrument axis coincident with the gyro sensor axes. Other configurations with fewer than three accelerometer axes are also possible. For example, if the total gravitational force is known, the acceleration in the third axis can be calculated from two orthogonal accelerometer axes. Alternatively, a single accelerometer, mounted such that its sensing axis may be rotated to orthogonal positions, may be used to determine the gravitational force in two orthogonal axes sequentially. The acceleration in the third axis can then be calculated.
The components of Earth's rate of rotation in the vertical and north-south directions are dependent upon the local latitude. There is no component in the east-west direction. These components of Earth's rate of rotation can be specified mathematically as: E =Qcos(A) and E =Qsin(A) where: E and E are the north and vertical components of Earth's rate of rotation o is Earth's rate of rotation (15.04°/hour) A is the local latitude A well-known mathematical gyrocompassing technique is typically applied to determine the azimuth from measurements of rotation rates in at least two orthogonal axes along with knowledge of the local latitude and a measure of the Earth's gravitational field seen on the rate sensing axes. This involves projecting the rotation rate measurements into the horizontal plane so that the angle relative to true north can be determined. At high latitudes, the component of Earth's rate in the horizontal plane is small, which leads to a reduced accuracy of the calculated azimuth.
The gyrocompassing technique outlined above may be performed about any axis and is not restricted to being performed about the instrument axis. It may also be performed in more than one axis if the instrument incorporates multiple rotary platforms, An alternative surveying method is to run the instrument into and/or out of the borehole in a continuous fashion. Rather than attempting to measure Earth's rate of rotation, although this may also be incorporated, the rotation rate sensors measure the rate of change of direction as the instrument travels along the borehole. By mathematically integrating the rate measurements, the borehole direction can be determined at any point. As a continuous survey, this has the advantage of being significantly faster to execute than a multi-shot survey in which the instrument must be paused at each survey point. Such a survey method would typically use a gyrocompass type measurement as previously described to establish a start or end direction for the survey but could alternatively use an external direction reference.
High accuracy gyroscopic based survey systems are typically more complex to operate than magnetic survey instruments and are often run by specialists after completion of a drilling phase to verify the direction of the borehole, previously obtained from magnetic survey instruments. These gyro-based instruments typically send survey information to the surface as the instrument is lowered into the borehole and subsequently retrieved from the borehole, thereby providing a larger number of surveys than would be obtained by the periodic running of magnetic based instruments. These systems also usually require additional equipment and personnel on site including depth control systems and surface computing.
Low performance gyroscopic-based multi-shot type survey instruments currently exist, but these instruments have insufficient sensitivity and accuracy to determine the direction of the borehole by measuring the Earth's rotation rate and therefore rely on an external reference direction to establish an initial borehole direction.
Traditional high accuracy gyro instruments are also fragile, as they are typically based upon mechanical spinning mass gyro sensor technology. They are therefore confined to being run only after completion of a coring / drilling phase, when drilling has stopped. It is believed that there are currently no high accuracy gyro-based sensor instruments available on the market that are not based upon mechanical spinning mass sensors.
As prior art may be mentioned:
AU 2010249163, (Parfitt), US 5806195, (Uttecht et al), US 7296638, (Beach et al), US 4611405, (Van Steenwyk), and EP-B1-20681 18, (Watson) However, in contrast to the present invention, none of these disclose the use of a directional survey instrument deployed with the core barrel or as part of the core barrel retrieval assembly to take surveys.
Instruments currently exist that can be integrated into the core barrel retrieval assembly but can only determine the inclination of the borehole; not the direction.
One example of such a system is made by Pajari Instruments Ltd, details of which can be found at http://www.QaiarLcomlsurvey methods/overshot.htmk Aim of the invention It is an aim of the present invention to overcome the above problems, and thus eliminate the need to deploy and retrieve borehole survey tools between retrieval of one core and commencement of drilling the next core.
Another aim of the present invention is to eliminate the need for post-drilling high accuracy gyro survey instruments to be run.
In accordance with the invention, this aim is achieved by conducting a high accuracy gyroscopic-based borehole survey during a wireline core drilling operation, in direct contradiction to accepted convention. To enable this, ruggedised rotation rate sensor technology, such as ruggedised gyro sensor technology, may be implemented.
The present disclosure proposes an alternative method of use of a gyro survey instrument, which is based upon a ruggedised rotation rate sensor technology.
These may be specially-packaged mechanical spinning-mass rate sensors, such as dynamically-tuned or rate-integrating type, M icro-Electro-Mechanical System (MEMS) rate sensors or solid-state sensors such as optical rate sensors, which are capable of withstanding the vibration and shock encountered during the coring / drilling process and have sufficiently high performance to enable the accurate measurement of the Earth's rate of rotation within the borehole and therefore determine the direction of the borehole with respect to true North. A suitable form of rotation rate sensor is a gyro sensor, as is known in the art per se. For convenience, the term "gyro sensor" is used in the following text, but it should be understood that other types of rotation rate sensor may equally be used.
The use of MEMS or solid-state sensors is advantageous due to their inherent mechanical robustness but it may be possible to mount a mechanical gyro such that it is isolated from the potential shock and vibration forces seen while retrieving a core or drilling, which would otherwise damage the sensor element.
There may be any number or type of gyro sensors in a gyro-based instrument and in the case where there is more than one such sensor, they may be of different types.
Typically there will be either one or two rotation rate sensors mounted on a rotary platform within the instrument such that their rate sensing axes remain orthogonal to the instrument axis. Alternatively, gyro sensors may be mounted at any known angle relative to the instrument axis and their outputs mathematically adjusted according to the mounting angle to establish a coordinate set with respect to the Earth's gravitational field and Earth's spin axis. Additionally, there may be a gyro sensor, whose sensing axis is parallel to the instrument axis, which is advantageously also mounted on the rotary platform.
Summary of the Invention
According to this invention from one aspect, there is provided a method of gyro surveying a borehole, the borehole being subject to a wireline core drilling operation which comprises the steps of: a) lowering drilling equipment comprising a drill bit releasably connected to a core barrel into the earth, the drill bit cutting a core of earth material, the core being received within the core barrel; b) retrieving the core barrel and its received core from the borehole; and c) fitting an empty core barrel to the drill bit; wherein the gyro surveying takes place during the drilling operation.
According to this invention from a second aspect, there is provided wireline equipment for use in a borehole comprising: a) drilling equipment comprising a drill bit releasably connected to a core barrel, the drill bit for cutting a core of earth material, the core being received within the core barrel, b) a carriage apparatus for moving the core barrel within the borehole, wherein the wireline equipment comprises a gyro sensor.
According to this invention from a third aspect, there is provided a gyro survey instrument for use in a wireline core drilling operation, comprising a gyro sensor within a housing, the housing have connection means specifically adapted for connection to at least one of a latching spearpoint, swivel head, core barrel, core-barrel adapter and overshot.
The gyro surveying may be performed by a gyro survey instrument, which may be located in or attached to the carriage apparatus functioning as either a deployment means or as a retrieval means or it may be located in or attached to a core barrel adaptor, which is connected to the top of the core barrel, or it may be located in the core barrel itself, whereby the core barrel may have to be extended to accommodate the gyro survey instrument. Advantageously, when the gyro survey instrument is located in the core barrel, the gyro survey instrument may further comprise a core orientation device. For the avoidance of doubt, the term "carriage apparatus" as used herein may be taken to include deployment means for lowering an empty core barrel toward the drill bit, retrieval means for raising an at least partially-filled core barrel away from the drill bit, or a means capable of performing both these deployment and retrieval functions, all of these means being known per se.
Where the gyro survey instrument is located in or attached to the core barrel carriage apparatus, single-shot surveys may be taken: i) when an empty core barrel is latched in place behind the drill bit using carriage apparatus functioning as a deployment means; ii) when carriage apparatus functioning as a core barrel retrieval means is latched onto a full core barrel; or iii) at both i) and ii).
Advantageously, the taking of measurements may be triggered by automatically detecting the above instances, using suitable triggering means. This may be by detecting, respectively, the detachment or attachment of the core barrel carriage apparatus to the core barrel adaptor; detecting the change in tension on the wireline; detecting the cessation of vibration such as may be due to motion along the borehole; detecting the cessation of any rotation in the borehole such as may be due to wireline twisting during payout or retraction, or by any other means. Alternatively, measurements may be taken by the gyro survey instrument based on a timer synchronised to a surface clock.
Where the gyro survey instrument is located in or attached to the core barrel carriage apparatus, multi-shot or continuous surveys may be taken: i) while lowering an empty core barrel into the drill string for connection to the drill bit using a carriage apparatus functioning as a deployment means; ii) while raising the deployment means after deployment of the empty core barrel in i); iii) while lowering a carriage apparatus functioning as a retrieval means toward a full core barrel; iv) while raising the retrieval means with the full core barrel attached; or v) a combination of the above.
In the case of multi-shot surveys, measurements may be triggered by automatically detecting the instrument's motion state, e.g. becoming stationary in the borehole, using suitable triggering means. This may be by detecting the cessation of vibration such as may be due to motion along the borehole; detecting the deceleration that occurs prior to stopping; detecting the cessation of any rotation in the borehole such as may be due to wireline twisting during payout or retraction, or by any other means. Alternatively, measurements may be taken by the gyro survey instrument based on a timer synchronised to a surface clock.
Where the gyro survey instrument is located in or attached to a core barrel adaptor or is located within the core barrel itself, a large number of single-shot surveys may be taken: i) while the empty core barrel is stationary at the bottom of the borehole before drilling commences, while the carriage apparatus functioning as a deployment means is returning to the surface; ii) while the full core barrel is stationary at the bottom of the borehole after drilling stops, while the carriage apparatus functioning as a retrieval means is lowered; or iii) both of the above.
Advantageously, these single-shot surveys may be averaged to generate a more accurate survey result. Advantageously, the taking of measurements may be triggered by automatically detecting the empty core barrel's motion state, e.g. when it becomes stationary at the bottom of the borehole and the commencement of drilling, or when the drilling has finished and the full core barrel ceases to be stationary at the bottom of the borehole, using suitable triggering means. Detecting when the empty core barrel becomes stationary at the bottom of the borehole may be achieved by detecting the detachment of the core barrel deployment means from the core barrel adaptor; detecting the cessation of vibration such as may be due to motion along the borehole; detecting the cessation of any rotation such as may be due to wireline twisting during payout, or by any other means. Alternatively, measurements may be taken by the gyro survey instrument based on a timer synchronised to a surface clock. The commencement of drilling may be detected by detecting the vibration caused by the drilling process. Detecting when the drilling has finished may be achieved by detecting the cessation of vibration such as may be due to the drilling process. Detecting when the full core barrel ceases to be stationary at the bottom of the borehole may be achieved by detecting the attachment of the core barrel retrieval means to the core barrel adaptor, detecting the vibration such as may be due to the motion along the borehole, detecting any rotation such as may be due to wireline twisting during retraction or by any other means.
Where the gyro survey instrument is located in or attached to a core barrel adaptor or is located within the core barrel itself, multi-shot surveys may be taken during lowering of the empty core barrel (or core barrel with adaptor) provided that its descent is halted periodically. Survey measurements may be triggered by automatically detecting the instrument becoming stationary in the borehole, using suitable triggering means. This may be by detecting the cessation of vibration such as may be due to motion along the borehole; detecting the cessation of any rotation such as may be due to wireline twisting during payout, or by any other means.
Alternatively, measurements may be taken by the gyro survey instrument based on a timer synchronised to a surface clock.
Where the gyro survey instrument is located in or attached to a core barrel adaptor or is located within the core barrel itself, multi-shot surveys may also be taken during drilling, providing that the drilling process is periodically halted. Survey measurements may be triggered by automatically detecting the cessation of vibration due to interruptions in the drilling process or by any other means, using suitable triggering means.
Where the gyro survey instrument is located in or attached to a core barrel adaptor or is located within the core barrel itself, multi-shot surveys may also be taken during raising of the retrieval means and full core barrel (or core barrel with adaptor) provided that the wireline retraction is halted periodically. Survey measurements may be triggered by automatically detecting the instrument becoming stationary in the borehole, using suitable triggering means. This may be by detecting the cessation of vibration such as may be due to motion along the borehole; detecting the cessation of any rotation such as may be due to wireline twisting during retraction, or by any other means.
Where the gyro survey instrument is located in or attached to a core barrel adaptor or is located within the core barrel itself, continuous surveys may be taken: i) while lowering an empty core barrel (or core barrel with adaptor) for deployment behind the drill bit; ii) while raising the deployment means before drilling commences iii) while drilling; iv) while lowering the retrieval means after drilling has finished v) while raising a full core barrel (or core barrel with adaptor); or vi) a combination of the above.
In this case the survey may be referenced to one or more gyrocompasses or an external direction.
Where the gyro survey instrument is located in or attached to the core barrel retrieval means, it may additionally have an electrical connection to the surface via a conducting wireline. In this case, the gyro survey instrument may use the wireline to receive power or command signals from the surface. The gyro survey instrument may also use the conducting wireline to communicate survey measurements back to the surface, for example in real time.
Where the gyro survey instrument does not have a means of communicating with surface equipment while it is deployed in a borehole, the gyro survey instrument may comprise data processing means to sample and process survey data and a local memory to store the survey data measurements, which may then be downloaded when the gyro survey instrument is brought to the surface.
Where the gyro survey instrument does not have an electrical connection to the surface equipment while it is deployed in a borehole, the gyro survey instrument may comprise a local power source such as a battery pack.
The gyro survey instrument may comprise any number of accelerometers, which may be configured such that the inclination of the instrument, and hence the inclination of borehole, can be determined. Additionally, the gyro survey instrument may comprise any number of gyro sensors which may be of any type of gyro sensor technology, such as specially-packaged mechanical spinning mass sensors, micro-electrical-mechanical (MEMS-type) sensors, or solid-state sensors such as optical gyro sensors. The gyro sensor(s) may be configured such that in conjunction with the inclination measurement, the azimuth of the instrument, and hence the azimuth of the borehole, can be determined. As the gyro survey instrument may need to endure the vibration and shock of the core drilling process, it advantageously comprises one or more ruggedised gyro sensors.
The present invention therefore provides various advantages, including economic benefits: In the current methods used in the mineral and mining exploration industry, a core sample is taken typically every 3m and a survey is taken typically every 30m. The frequency of survey samples is largely dictated by the desire to maximise drilling time. For a typical borehole, surveyed over a depth range from 300m to bOOm, there would be a total of around 25 surveys. With slick line deployment rates at typically 20m per minute, this would take an average of more than 1 hour per survey.
Thus the total time spent surveying would be around 30 hours per borehole, which constitutes a significant additional cost to the drilling program.
The proposed system would eliminate this additional time required for surveying leading to considerable cost savings for every borehole and would further eliminate the need to conduct any post-drilling surveys.
Additional benefits of these methods include: 1. Continuous surveys to be taken during wireline core drilling operations, which has never previously been realisable in the industry.
2. The accuracy of surveys can be improved through the use of high-accuracy gyro-based survey instruments.
3. The number of surveys that can be obtained per borehole is an order of magnitude larger than the number obtained using current survey methods.
This large increase in the density of survey data effectively further increases the accuracy of the survey data by applying averaging to reduce the effect of any measurement errors.
For both geologists and investors in exploration companies, this presents a significant advantage, as accurate survey data could be obtained without a time penalty each time a core sample is retrieved from the hole.
Brief Description of the Drawings
The invention will now be described with reference to the accompanying drawings, of which: Fig. 1 schematically shows, in sectional view, a known wireline core drilling equipment assembly located within a borehole; Fig. 2 schematically shows, in sectional view, the wireline core drilling equipment assembly of Fig. 1 in more detail; Fig. 3 schematically shows an idealised output from a gyro sensor in dependence on rotation angle; Figs. 4a-d schematically show four embodiments of the present invention in sectional view; and Figs. 5a-d schematically show the embodiments of Figs. 4a-d in oblique view.
Detailed Description of the Invention
Figs. 4 and 5 show four specific embodiments of the present invention, with the embodiment of Fig. 4a corresponding to that of Fig. 5a etc. Embodiment 1 (Figs. 4a, 5a) Fig. 4a shows wireline core drilling equipment including a conventional core barrel 21, with a core barrel adapter 22 and latching spearpoint 23 attached, and a carriage apparatus 24 which incorporates an overshot 25, a gyro survey instrument 26 and a local power source 27 such as a battery pack or the like. The gyro instrument 26 is shown in more detail in Fig. 5a. The carriage apparatus 24 may comprise a retrieval apparatus for removing a "full" core barrel, i.e. one containing a core of earth material, subsequent to a drilling phase, or alternatively a deployment apparatus for deploying an empty core barrel to replace a previously-retrieved full core barrel.
Indeed, in many instances the same equipment is used for both operations.
In this embodiment, the gyro survey instrument 26 comprises a local power source 27, and, as shown in Fig. 5a, power conditioning means 28; sensor equipment (described in more detail below); data acquisition means 29, data processing means and data storage means 31.
The sensor equipment comprises one or more accelerometer sensors 33 having a total of one or more sensing axes and one or more rotation rate sensors 34, here comprising gyro sensors, having a total of one or more rotation rate sensing axes.
Fig. 5a also shows a wireline-tension sensing means 36, a depth sensing means 38, a motion sensing means 37 and a seat sensing means 39 (for detecting when a spearpoint is connected or latched into the overshot). A non-conducting swivel 40 is provided at the top of the carriage apparatus 24 for connection to a non-conducting wireline 41.
The accelerometer sensors 33 and gyro sensors 34 are mounted on a rotary platform 32, which is driven for rotation by a motor 35 about the axis of the instrument. In alternative embodiments (not shown), the accelerometer and gyro sensors may be mounted in fixed orientations within the instrument.
In a preferred embodiment there exist a total of three orthogonal accelerometer sensing axes, which are preferably, but not necessarily, mounted such that one axis is parallel with the instrument axis. Optionally there may be only two orthogonal accelerometer axes, in which case, the acceleration in the third axis can be calculated based upon the known Earth's gravitational force. Optionally there may be only one accelerometer axis, mounted on a rotary platform 32 such that its sensing axis is at a fixed angle to the axis of the rotary platform, which may be rotated such that gravitational force may be determined in two axes sequentially.
Advantageously, this accelerometer axis would be orthogonal to the axis of the rotary platform 32. The acceleration in the third axis can then be calculated based upon the known Earth's gravitational force. In alternative embodiments, there may be more than three accelerometer axes, thereby providing varying degrees of redundancy, error detection or accuracy improvement.
Preferably there exist two rotation rate sensing axes which are, advantageously, both orthogonal to the axis of the rotary platform 32. This may be by means of a two-axis gyro sensor or by means of two single-axis gyro sensors mounted orthogonally.
Optionally there may be only one single-axis gyro sensor, mounted such that its sensing axis is orthogonal to the axis of the rotary platform 32, which may be rotated such that the gyro sensor is aligned sequentially in two orthogonal orientations.
Optionally there may exist, a third rate-sensing axis that is substantially parallel with the instrument axis. Gyro sensor 34 in Fig. 5a is shown as possessing three orthogonal sensing axes. If this gyro sensor is mounted on the rotary platform 32, it may be used to determine the rotation rate of the rotary platform when the instrument is stationary or the combined rotation rate of the instrument and the rotary platform when the instrument is non-stationary. If this gyro sensor is not mounted on a rotary platform 32, it may be used to determine the rotation rate of the instrument within the borehole. Optionally, there may be additional rotation rate sensing axes, thereby providing varying degrees of redundancy, error detection or accuracy improvement.
In alternative embodiments, one or more rotation rate sensors may be mounted on rotary platforms whose axes are not aligned with the instrument axis. Such rotary platforms may be incorporated instead of or in addition to the rotary platform whose axis is aligned with the instrument axis. Advantageously, rotary platforms, upon which one or more rate sensors are mounted, may also be arranged such that they are each mounted on another rotary platform, thereby providing the ability to gyrocompass in any axis.
The seat sensing means 39, whose purpose is to determine whether or not the overshot 25 has latched onto a spearpoint 23, may comprise an electrical contact or any form of proximity detector such as magnetic, optical, acoustic, capacitive or inductive sensors or means for detecting the impact that occurs as the overshot seats onto a spearpoint for example.
The depth sensing means 38, whose purpose is to estimate the depth of the instrument in the borehole, may comprise a sensor that can detect joints between drill pipes such as a casing collar locator or similar. Alternatively, the depth sensing means may operate by integrating the accelerometer outputs or a combination of these methods may be used, for example.
The motion sensing means 37, whose purpose is to detect whether or not the instrument is stationary within the borehole, may comprise means for processing data from the accelerometer sensors or comprise vibration sensors for more direct motion determination for example.
The data acquisition means 29 is operable to convert information derived from the sensor equipment into a format that may subsequently be processed by the data processing means or stored by the data storage means.
In this embodiment, the gyro survey instrument 26 may be configured to take single-shot, multi-shot or continuous surveys, processing the acquired sensor information before storing the information in the data storage means for subsequent retrieval on the surface. In alternative embodiments (not shown), the processing of this information may be performed on the surface after recovery of the carriage apparatus 24.
The gyro survey instrument 26 may take one or more single-shot surveys using as triggering means the seat sensing means 39, the motion sensing means 37 or the wireline-tension sensing means 36 or any combination thereof. Alternatively, the gyro survey instrument may take one or more single-shot surveys after the elapsing of defined time intervals using a timer synchronised to a surface clock, i.e. such that the triggering means comprises the clock (not shown).
The gyro survey instrument 26 may take multi-shot surveys consisting of survey points at any number of depths during the lowering or raising of the carriage apparatus 24. The motion sensing means 37 may be used to detect the cessation of motion and trigger the gyro survey instrument to take each survey point and at the same time, the depth sensing means 38 may be used to determine the depth at each survey point. Alternatively, the gyro survey instrument may be configured to take survey points (optionally, including depth information) at defined time intervals using a timer synchronised to a surface clock such that the timing of surveys coincides with the times at which the wireline winch is halted, thereby ensuring that the instrument is stationary in the borehole. A depth counter on the surface may also be synchronised to the surface clock so that the depth information is also known at each survey point. This may be subsequently used as the primary source or an additional source of depth information and may be time correlated with the corresponding survey data recovered from the instrument's data storage means after retrieval.
The gyro survey instrument 26 may take continuous surveys in which survey data is continuously sampled and recorded into the instrument's data storage means. In this case, the data samples from all of the instrument sensors (i.e. accelerometer 33, gyro sensor 34, depth sensing means 38) would be recorded in a manner that also allows the time at which each data sample was captured to be determined using a timer synchronised to a surface clock. Corresponding depth information may be derived from the depth sensing means 38 within the gyro survey instrument or from a surface depth counter that is also synchronised to the surface clock. The latter may be time correlated with the corresponding survey data recovered from the instrument's data storage means 31 after retrieval. All information may subsequently be processed to produce a continuous survey of the borehole for either the in-run or the out-run or both.
In alternative embodiments, one or more of the wireline-tension sensing means, depth sensing means, seat sensing means and motion sensing means may be omitted, with survey triggering being effected by alternative methods, for example using a simple time-based system as described above.
Embodiment 2 (Figs. 4b, 5b) In this second embodiment, the gyro survey instrument 26' is, similarly to the embodiment shown in Figs. 4a, 5a, again located within carriage apparatus 24' but here it is provided with an electrical connection to the surface via a conducting wireline 41'. This electrical connection provides power from the surface to the gyro survey instrument, and also provides a communications link to pass information from the instrument to the surface. Many components are similar to those of the first embodiment described with reference to Figs. 4a, 5a, and the numbering system used has been retained as far as possible.
As the electrical connection is used to supply power, the gyro survey instrument 26' need not incorporate a power source. Instead, it incorporates a conducting wireline head 42, whose purpose is to make the electrical connection between the conducting wireline and the instrument electronics.
As the electrical connection is used to provide a communications link between the gyro survey instrument 26' and the surface, the former need not incorporate any data storage means. Instead, it incorporates a communications interface 43, whose purpose is to provide a means by which information may be transferred from the instrument's electronics onto the conducting wireline 41', via conducting wireline head 42.
In this embodiment, the gyro survey instrument 26' may be configured to take single-shot, multi-shot or continuous surveys. The acquired sensor information may be transmitted to the surface without processing or it may be processed within the instrument prior to transmission to the surface. In the case where the bandwidth of the communications link is insufficient to carry information at the required rate, the acquired sensor information may be stored in the data storage means 31 within the instrument. In the case where the bandwidth of the communications link is sufficient to carry information at the required rate, the acquired sensor information may be communicated to the surface in real time so the data storage means 31 may be omitted.
The instrument may use the seat sensing means 39 or the motion sensing means 37 as a trigger. Alternatively, the gyro survey instrument 26' may take one or more single-shot surveys after the elapsing of defined time intervals using a timer synchronised to a surface clock.
The instrument 26' may use the motion sensing means 37 to detect the cessation of motion and trigger the gyro survey instrument to take each survey point and at the same time, the depth sensing means 38 may be used to determine the depth at each survey point. Alternatively, the gyro survey instrument may be configured to take survey points (optionally, including depth information) at defined time intervals using a timer synchronised to a surface clock such that the timing of surveys coincides with the times at which the wireline winch is halted, thereby ensuring that the instrument is stationary in the borehole. A depth counter on the surface may also be synchronised to the surface clock so that the depth information is also known at each survey point.
The gyro survey instrument 26' may take continuous surveys in which survey data is continuously sampled and, where a communications link to the surface with adequate bandwidth exists, may transmit data to the surface in real-time. In this case, the data samples from all of the instrument sensors would be transmitted to the surface in a manner that also allows the time at which each data sample was captured to be determined using a timer synchronised to a surface clock.
Where the communications link to the surface has insufficient bandwidth to enable reliable communications at the required rate, the continuously sampled survey data may be recorded into the instrument's data storage means 31. In this case, the data samples from all of the instrument sensors would be recorded in a manner that also allows the time at which each data sample was captured to be determined using a timer synchronised to a surface clock.
Corresponding depth information may be derived from the depth sensing means 38 within the gyro survey instrument 26' or from a surface depth counter that is also synchronised to the surface clock. The latter may be time correlated with the corresponding survey data recovered from the instrument's data storage means after retrieval. All information may subsequently be processed to produce a continuous survey of the borehole for either the in-run or the out-run or both.
In an alternative, advantageous, embodiment, the communications interface 43 may also provide a means by which information may be received from the surface, thereby enabling bidirectional communications between the gyro survey instrument and the surface. In this case, the instrument need not incorporate a motion sensing means, a seat sensing means or a depth sensing means. This is because in this case, the instrument may be commanded from the surface in real-time and may therefore operate with less autonomy than that required for previous embodiments. It may, for example, take one or more single-shot surveys when commanded to do so, the collection of survey points obtained at multiple depths making up a multi-shot survey.
In further alternative embodiments, the electrical connection to the surface may be operable to supply power from the surface to the gyro survey instrument but no communications link. Where no communications link to the surface exists, the continuously sampled survey data may be recorded into the instrument's data storage means. In this case, the data samples from all of the instrument sensors would be recorded in a manner that also allows the time at which each data sample was captured to be determined using a timer synchronised to a surface clock. The acquired sensor information may be stored in the data storage means within the instrument.
Alternatively, the electrical connection to the surface may be operable to provide a communications link only, with no power transfer capabilities. In this case, a local power source such as a battery or the like would be required at the instrument as for the embodiment shown in Figs. 4a, 5a.
Embodiment 3 (Figs. 4c, 5c) In this third embodiment, a gyro survey instrument 26" is attached to the core barrel system rather than the carriage apparatus 24" as described in the preceding embodiments. Many components are similar to those of the first embodiment described with reference to Figs. 4a, 5a, and the numbering system used has been retained as far as possible. Fig. 4c shows wireline core drilling equipment consisting of a conventional retrieval system which incorporates an overshot 25, together with a conventional core barrel 21, with a core barrel adapter 22 attached, above which is attached the gyro survey instrument 26" and a latching spearpoint 23. The gyro instrument is shown in more detail in Fig. 5c.
In this third embodiment, the gyro survey instrument 26" comprises a spearpoint-force sensing means 44, whose purpose is to determine the tensile or compressive force applied to the spearpoint, which may change during deployment or retrieval of the core barrel 21. Typically, when an overshot hits the spearpoint 23, there may be a compressive force and, once seated, when the wireline 41 is tensioned there will be a tensile force on the spearpoint. Advantageously the force sensing means 44 could operate continuously (not just during deployment and retrieval) to detect these changes.
In this embodiment, the gyro survey instrument 26" may be configured to take single-shot, multi-shot or continuous surveys, optionally processing the acquired sensor information before storing the information in the data storage means for subsequent retrieval on the surface.
The gyro survey instrument 26" may take one or more single-shot surveys using as a trigger, the seat sensing means, the motion sensing means or the spearpoint-force sensing means or any combination thereof. A large number of single-shot surveys may be taken throughout the period while the carriage apparatus functioning as a retrieval means is retracted after deployment of the empty core barrel, if applicable, or throughout the period after drilling while the retrieval means is lowered to retrieve the core.
The gyro survey instrument 26" may take multi-shot surveys consisting of survey points at any number of depths during the lowering or raising of the core barrel 21.
The motion sensing means 37 may be used to detect the cessation of motion and trigger the gyro survey instrument to take each survey point and at the same time, the depth sensing means 38 may be used to determine the depth at each survey point. A depth counter on the surface may also be used to determine the depth at each survey point which may be subsequently used as the primary source or as an additional source of depth information and may be time correlated with the corresponding survey data recovered from the instrument's data storage means 31 after retrieval.
The gyro survey instrument 26" may take continuous surveys in which survey data is continuously sampled and recorded into the instrument's data storage means 31. In this case, the data samples from all of the instrument sensors would be recorded in a manner that also allows the time at which each data sample was captured to be determined using a timer synchronised to a surface clock. Corresponding depth information may be derived from the depth sensing means 38 within the gyro survey instrument or from a surface depth counter that is also synchronised to the surface clock. The latter may be time correlated with the corresponding survey data recovered from the instrument's data storage means 31 after retrieval. All information may subsequently be processed to produce a continuous survey of the borehole for either the in-run or the out-run or both.
The gyro survey instrument 26" may also record data from its sensors into its data storage means 31 throughout the drilling step (i.e. step c in the wireline core drilling operation). Although the recorded data may be subject to increased noise due to the vibration from the drilling step, the long duration of this step allows a long data integration period, thereby presenting an opportunity to obtain accurate survey data during the drilling step.
In alternative embodiments, one or more of the motion sensing means, depth sensing means or spearpoint-force sensing means may be omitted. For example, an instrument set up for continuous surveying where the depth is determined by surface equipment using synchronised timers as set out above need not include any of these components.
Embodiment 4 (Figs. 4d, 5d) In this fourth embodiment, the gyro survey instrument 26" forms part of an assembly that is deployed inside the core barrel 21", which is extended in length to accommodate the instrument. Fig. 4d shows wireline core drilling equipment consisting of a conventional retrieval system which incorporates an overshot, and an extended core barrel, inside which is a gyro survey instrument. The gyro instrument is shown in more detail in Fig. 5d. In this fourth embodiment, many components are similar to those described in respect of embodiment 3 above, and similar numbering has been used where possible. In this embodiment, the gyro survey instrument 26" additionally incorporates a core orientation device 45, typically located at the lower end of the instrument.
In this configuration, the empty core barrel 21" (with gyro survey instrument 26" inside) would initially be lowered into the borehole in readiness for commencement of drilling the next core sample. When the core barrel reaches the bottom of the borehole, the core orientation device 45, in conjunction with other sensors in the instrument, is operable to determine and store information relating to the orientation of the next core, while it is still part of the formation. This may include mapping the profile and/or recording an image of the surface of the next core. At the same time, the gyro survey instrument 26" may take measurements from its other sensors so that the orientation of the instrument may also be determined. The core orientation may then be determined from the instrument orientation and the orientation of the surface profile and/or image of the core. Advantageously a gyro survey instrument enables the core orientation to be determined at any angle of inclination, including the condition in which the core is at or near vertical.
As drilling proceeds and the new core sample progresses up the barrel, the gyro survey instrument would be displaced upwards inside the core barrel 21" by the core sample. In this configuration therefore, the aforementioned extension of the core barrel enables the drilling equipment to achieve a standard length core sample.
In this embodiment, the gyro survey instrument 26" may be configured to take single-shot, multi-shot or continuous surveys, optionally processing the acquired sensor information before storing the information in the data storage means 31 for subsequent retrieval on the surface.
The gyro survey instrument 26" may take one or more single-shot surveys using the motion sensing means 37 as a trigger. Alternatively, the instrument could be configured to take a survey and determine the core orientation information after a predefined time limit has elapsed or by any other means as a trigger. A large number of single-shot surveys may be taken throughout the period while the retrieval system is retracted after deployment of the empty core barrel, if applicable, or throughout the period after drilling while the retrieval system is lowered to retrieve the core.
The gyro survey instrument 26" may take multi-shot surveys consisting of survey points at any number of depths during the lowering or raising of the core barrel 21".
The motion sensing means 37 may be used to detect the cessation of motion and trigger the gyro survey instrument to record data at each survey point. A depth counter on the surface may be used to determine the depth at each survey point which may be subsequently time correlated with the corresponding survey data recovered from the instrument's data storage means 31 after retrieval.
The gyro survey instrument may take continuous surveys in which survey data is continuously sampled and recorded into the instrument's data storage means. In this case, the data samples from all of the instrument sensors would be recorded in a manner that also allows the time at which each data sample was captured to be determined using a timer synchronised to a surface clock. Corresponding depth information may be derived from the depth sensing means 38 within the gyro survey instrument or from a surface depth counter that is also synchronised to the surface clock. The latter may be time correlated with the corresponding survey data recovered from the instrument's data storage means 31 after retrieval. All information may subsequently be processed to produce a continuous survey of the borehole for either the in-run or the out-run or both.
The gyro survey instrument 26" may also record data from its sensors into its data storage means 31 throughout the drilling step. Although the recorded data may be subject to increased noise due to the vibration from the drilling step, the long duration of this step allows a long data integration period, thereby presenting an opportunity to obtain accurate survey data during the drilling step.
In an alternative embodiment, the motion sensing means may be omitted, with surveying or triggering reliant upon other methods as outlined for previous embodiments.
General Embodiments 1, 2 and 3 would commonly be referred to as back-end' systems as any orientation device fitted to the instrument could only measure the orientation of the core barrel rather than the core-sample itself. In other words it cannot determine if the core rotates within the barrel. Embodiment 4 would be commonly referred to as a front end' system as it has the ability to measure the orientation of the core-sample itself, while it is still part of the formation, irrespective of any rotation that may occur within the core barrel.
In all embodiments, a continuous survey may be performed during either the in-run or the out-run of the gyro survey instrument, or both. In either case, the instrument may be aligned to a known reference direction either before commencing the continuous survey or after completion or both. The known reference direction may be derived from a single-shot survey or from an external reference direction. Thus the instrument may be aligned on the surface, prior to the in-run, to an external reference direction or a single-shot survey and/or may be aligned to a single-shot survey at the bottom of the borehole at the end of the in-run. Similarly, the instrument may be aligned to a single-shot survey at the bottom of the borehole prior to the out-run and/or may be aligned at the end of the out-run to an external reference direction or another single-shot survey on the surface. Each continuous survey would need to be matched with depth information obtained either from the depth sensing means within the instrument or from the depth versus time recorded at the surface. Each time a continuous survey is performed, survey data is collected for the entire borehole and consequently all but the deepest few metres of the borehole would be surveyed many times over. Although each individual continuous survey may be subject to errors, the results from multiple continuous surveys of the same sections of the borehole may be combined to reduce these errors and thus generate an accurate survey.
In all embodiments, various processing methods could be incorporated to minimise the mean power consumption and thus increase the lifespan of the power source within the gyro survey instrument. For example, the instrument may operate predominantly in a low-power quiescent state, waking up periodically to check for the condition(s) that would trigger a survey. If the condition is not satisfied, the instrument would return to its low-power state for a predefined period without taking a survey. If however, the condition is satisfied, the instrument would take a survey then return to its low power state. Once the gyro sensor has taken the required number of surveys, it could switch off completely until it is returned to the surface.
As can be seen, all embodiments described eliminate the need to run an additional survey instrument (of any type or technology) in the borehole after retrieving the core barrel, thus enabling the drilling process to continue uninterrupted, upon deployment of the next empty core barrel into the core drilling assembly.
In this way, the gyro survey instrument may therefore be deployed as a constituent part of the existing wireline coring process, thus achieving significant reductions in drilling rig costs and hence the exploration drilling programs.
The above-described embodiments are exemplary only, and various other modifications and alternatives are possible within the scope of the invention.