Drilling fluid invasion
Encyclopedia
Drilling fluid invasion is a process that occurs in a well being drilled with higher wellbore pressure than formation pressure. The liquid component of the drilling fluid (known as the mud filtrate) continues to "invade" the porous and permeable formation until the solids present in the mud, commonly bentonite
, clog enough pores to form a mud 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 called phase trapping occurs. This is when a fluid enters a formation that is below its irreducible 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 for well logging
. In many cases the "depth of investigation" of a well logging tool is only a few inches (or even less for things like sonic logs
), and it is quite possible that drilling fluid has invaded beyond this depth. Therefore the readings that are taken that are influenced by formation fluids are measuring mud filtrate properties rather than formation (in situ) fluid properties.
Bentonite
Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...
, clog enough pores to form a mud 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 called phase trapping occurs. This is when a fluid enters a formation that is below its irreducible 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 for well logging
Well logging
Well logging, also known as borehole logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface or on physical measurements made by instruments lowered into the hole...
. In many cases the "depth of investigation" of a well logging tool is only a few inches (or even less for things like sonic logs
Sonic logging
Sonic logging shows a formation’s interval transit time, designated Dt. It is a measure of a formation’s capacity to transmit sound waves. Geologically, this capacity varies with lithology and rock textures, notably porosity....
), and it is quite possible that drilling fluid has invaded beyond this depth. Therefore the readings that are taken that are influenced by formation fluids are measuring mud filtrate properties rather than formation (in situ) fluid properties.
See also
- Resistivity loggingResistivity loggingResistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current...