The portion about a wellbore into which drilling fluid has penetrated, displacing some of the formation fluids. Invasion up to about a meter in diameter takes place in porous, permeable zones because the pressure of the drilling mud is greater than that of the formation fluids. See Figure I-6.
As mud penetrates into the formation, portions of the mud (mud cake) build on the formation wall, limiting further flow of mud fluid (filtrate) into the formation.
Directly behind the mud cake is a flushed zone from which almost all of the formation water and most of the hydrocarbons have been displaced by filtrate.
The invasion alters the distribution of resistivities and other properties and consequently the values that logs read. The depth of invasion is the equivalent depth in an idealized model rather than the maximum depth reached by filtrate.
In oil-bearing intervals, the filtrate may push a bank of formation water ahead of it to produce a relatively low-resistivity annulus, which is especially important with deep-investigation induction logs.