Drilling fluid invasion is a process that occurs in a well being drilled with higher wellbore pressure (normally caused by excessive mud weights) than

formation pressure Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

. The liquid component of the

drilling fluid In geotechnical engineering, drilling fluid, also called drilling mud, is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also us ...

(known as the

mud filtrate A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a multiplayer real-time virtual world, usually text-based or storyboarded. MUDs combine elements of role-playing games, hack and slash, ...

, or spurt) continues to "invade" the porous and permeable formation until the solids present in the mud, commonly

bentonite Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-mon ...

, 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 Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...

occurs. This is when a fluid enters a formation that is below its

irreducible saturation In philosophy, systems theory, science, and art, emergence occurs when an entity is observed to have properties its parts do not have on their own, properties or behaviors that emerge only when the parts interact in a wider whole. Emergence ...

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, also known as borehole logging is the practice of making a detailed record (a ''well log'') of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (' ...

. In many cases the "depth of investigation" of a well logging tool is only a few inches (or even less for methods such as

sonic logs Sonic logging is a well logging tool that provides a formation’s interval transit time, designated as t, which is a measure of a how fast elastic seismic compressional and shear waves travel through the formations. Geologically, this capacit ...

), 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.

References

* Drilling fluid Well logging {{Petroleum-stub

See also

References