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Drilling fluid invasion is a process that occurs in a well being drilled with higher wellbore pressure (normally caused by excessive mud weights) thanformation pressure. The liquid component of thedrilling fluid (known as themud filtrate, or spurt) continues to "invade" the porous and permeable formation until the solids present in the mud, commonlybentonite, clog enough pores to form amud cake capable of preventing further invasion.
If invasion is severe enough, and reservoir pressures are unable to force the fluid and associated particles out entirely when the well starts producing, the amount of oil and gas a well can produce can be permanently reduced. This is especially true when a process calledphase trapping occurs. This is when a fluid enters a formation that is below itsirreducible saturation of that fluid. Once the fluid is present, it is held in place by capillary forces and usually can not be removed.
Invasion also has significant implications forwell logging. In many cases the "depth of investigation" of a well logging tool is only a few inches (or even less for methods such assonic logs), and it is quite possible that drilling fluid has invaded beyond this depth. In these cases readings are strongly influenced by mud filtrate properties rather than pure formation (in situ) properties. This influence must be considered when interpreting the resulting logs.
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