1.
n. [Formation Evaluation]
The distance from the
borehole wall into the
formation that the
mud filtrate has penetrated. The term assumes equal
invasion on all sides of the borehole. It is the diameter of the circle thus formed, with the center being the center of the borehole. The diameter of invasion affects whether a log measures the
invaded zone, the
undisturbed zone, or part of each zone. The term is closely related to the depth of invasion, being twice the depth of invasion plus the borehole diameter. Diameter of invasion is a more appropriate
parameter for describing the response of azimuthally symmetric measurements such as
induction, laterolog and propagation
resistivity.
The term is well-defined in the case of a step
profile of invasion. In the case of an
annulus or a transition zone, two diameters must be defined, corresponding to the inner and outer limits of the annulus or transition zone. When the invasion
model is not specified, the term usually refers to the outer limit of invasion.