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.
Famous quotes containing the words drilling, fluid and/or invasion:
“Don’t spend your time in drilling soldiers, who may turn out hirelings after all, but give to undrilled peasantry a country to fight for.”
—Henry David Thoreau (1817–1862)
“There are no fixtures in nature. The universe is fluid and volatile. Permanence is but a word of degrees. Our globe seen by God is a transparent law, not a mass of facts. The law dissolves the fact and holds it fluid.”
—Ralph Waldo Emerson (1803–1882)
“We should have an army so organized and so officered as to be capable in time of emergency, in cooperation with the National Militia, and under the provision of a proper national volunteer law, rapidly to expand into a force sufficient to resist all probable invasion from abroad and to furnish a respectable expeditionary force if necessary in the maintenance of our traditional American policy which bears the name of President Monroe.”
—William Howard Taft (1857–1930)