1. n. [Formation Evaluation]
The process by which mud filtrate, and sometimes whole mud, enters a permeable formation. The mud filtrate displaces some or all of the moveable fluids in the formation, leaving an invaded zone. The invasion process is complex. It is generally considered to start with a short initial spurt loss when the bit penetrates the rock. During this period, invasion depends on formation permeability, among other factors. A mudcake is soon formed, after which invasion is either described as dynamic, when mud is being circulated, or static, when it is not. In both cases, the volume of invasion depends little on formation properties and strongly on other factors such as mudcake permeability and differential pressure. However, the profile of the invasion front within the formation, both vertically and radially, does depend on formation properties. With high permeability and with different densities of filtrate and formation fluid, gravity can cause vertical movement of the filtrate, leading to different depths of invasion at the top and bottom of a zone. With two moveable phases (oil and water) and differing permeabilities, an annulus can be formed. The radial profile from the wellbore out to the undisturbed zone depends on permeability, with lower permeabilities leading to sharper transitions.
2. adj. [Formation Evaluation]
Pertaining to the influence of invasion on the response of logging measurements. For example, an invasion correction is the correction made to a deep-reading measurement such as an induction log for the effect of the invaded zone. The correction is based on a suitable invasion model, such as a step profile or transition zone model.