Density logging is awell logging tool that can provide a continuous record of aformation'sbulk density along the length of aborehole. In geology, bulk density is a function of the density of the minerals forming a rock (i.e.matrix) and the fluid enclosed in the pore spaces. This is one of three well logging tools that are commonly used to calculate porosity, the other two beingsonic logging andneutron porosity logging

The tool was initially developed in the 1950s and became widely utilized across the hydrocarbon industry by the 1960s. A type of active nuclear tool, aradioactive source and detector are lowered down the borehole and the source emits medium-energygamma rays into the formation. Radioactive sources are typically a directional Cs-137 source. These gamma rays interact with electrons in the formation and are scattered in an interaction known asCompton scattering. The number of scattered gamma rays that reach the detector, placed at a set distance from the emitter, is related to the formation's electron density,^[1] which itself is related to the formation's bulk density (${\displaystyle {\rho }_{\text{bulk}}}$) via

${\displaystyle {\rho }_{\text{e}}=2{\rho }_{\text{bulk}}\frac{Z}{A}}$

where${\displaystyle Z}$ is the atomic number, and${\displaystyle A}$ is the molecular weight of the compound. For most elements${\displaystyle Z/A}$ is about 1/2 (except for hydrogen where this ratio is 1). The electron density (${\displaystyle {\rho }_{\text{e}}}$) in g/cm³ determines the response of the density tool.

The tool itself initially consisted of a radioactive source and a single detector, but this configuration is susceptible to the effects of thedrilling fluid. In a similar way to how thesonic logging tool was improved to compensate for borehole effects, density logging now conventionally uses 2 or more detectors. In a 2 detector configuration, the short-spaced detector has a much shallower depth of investigation than the long-spaced detector so it is used to measure the effect that the drilling fluid has on the gamma ray detection. This result is then used to correct the long-spaced detector.^[2]

Assuming that the measured bulk density (${\displaystyle {\rho }_{\text{bulk}}}$) only depends onmatrix density (${\displaystyle {\rho }_{\text{matrix}}}$) and fluid density (${\displaystyle {\rho }_{\text{fluid}}}$), and that these values are known along the wellbore,porosity (${\displaystyle \varphi }$) can be inferred by the formula

${\displaystyle \varphi =\frac{{\rho }_{\text{matrix}}-{\rho }_{\text{bulk}}}{{\rho }_{\text{matrix}}-{\rho }_{\text{fluid}}}}$

Common values of matrix density${\displaystyle {\rho }_{\text{matrix}}}$ (ing/cm³) are:

• Quartz sand - 2.65
• Limestone - 2.71
• Dolomite - 2.87

This method is the most reliable porosity indicator for sandstones and limestones because their density is well known.^[1] On the other hand, the density ofclay minerals such asmudstone is highly variable, depending ondepositional environment,overburden pressure, type of clay mineral and many other factors. It can vary from 2.1 (montmorillonite) to 2.76 (chlorite) so this tool is not as useful for determining their porosity. A fluid bulk density${\displaystyle {\rho }_{\text{fluid}}}$ of 1g/cm³ is appropriate where the water is fresh but highlysaline water has a slightly higher density and lower values should be used forhydrocarbon reservoirs, depending on the hydrocarbon density and residual saturation.

In some applications hydrocarbons are indicated by the presence of abnormally high log porosities.

Sonic logging

• Well logging
• Drilling technology