Variations in rock properties caused by the invasion of the drilling fluid into the rock play an important role in petrophysical analysis.

Invasion is a process whereby drilling mud fluid is forced into the rock due to differential pressure. The drilling mud is made up of solid particles and ions dissolved in water. This water displaces the native formation water to some degree, and mixes with formation water that is not displaced. The distance to which some displacement and/or mixing occurs is called the invasion diameter, and the zone so disturbed is termed the invaded zone. The zone nearest the borehole, or flushed zone, is the portion of rock where the maximum amount of displacement and mixing has occurred. The balance of the invaded zone is sometimes named the transition zone, where the transition between maximum flushing and no invasion occurs. This is confusing, as there may be a vertical transition zone between oil and free water in a reservoir, so we avoid the use of transition zone in the invasion profile description.

The invasion process leaves behind the solid particles of the mud, which collect on the borehole wall. The resulting material is called mud cake, and may be 3 to 4 inches thick or very thin and difficult to detect. The mud cake thickness by definition is one half the difference between the bit size and the borehole diameter. If the hole is enlarged by erosion beyond the bit size during drilling, the mud cake thickness may be impossible to determine.

Mud cake is the sealing agent which slows down invasion. As a result, high permeability zones which allow quick buildup of mud cake, invade the least and low permeability zones invade the most or deepest. Non-permeable zones are not invaded. This sounds counter-intuitive, but that is what happens with a good quality drilling mud. Poor quality or native mud may not form a mud cake, and invasion will proceed in proportion to the permeability.

Drilling Fluid Invasion Model

The traditional abbreviations and definitions listed below describe conditions found within the near-wellbore environment:
    Rxo = resistivity of the flushed zone
    Ri    =  resistivity of the invaded zone
    Rt     = resistivity of the undisturbed zone
    Ro     = resistivity of the undisturbed zone which is 100% water saturated
    Rz      = resistivity of unknown mixture in the transition zone
    Rw     = resistivity of formation water
    Rm      = resistivity of mud
    Rmf    = resistivity of mud filtrate
    Rmc    = resistivity of mud cake
    Rs       = resistivity of surrounding beds
    Rsh     = resistivity of shale beds
    Dh       = borehole diameter
    Di        = invasion diameter
    Dj        = diameter of the flushed zone
    Bitz     = drill bit diameter (bit size)
    Hmc    = mud cake thickness
Computer program will use variations of these abbreviations to represent the same parameters.

Since the depth of investigation of logging tools varies, knowledge of the invasion profile is necessary in making assumptions about log analysis methods or parameters. This can sometimes be inferred from the relationship between shallow, medium, and deep investigation resistivity logs recorded in permeable water zones.

Most porosity-indicating logs read the flushed zone and part of the invaded zone, as do the gamma ray, SP, and shallow resistivity logs. Micrologs read mostly in the flushed zone. Residual hydrocarbons in the flushed and invaded zones, especially gas, will influence all shallow investigation logs.

Resistivity distribution in a radial direction from the borehole determines the response of resistivity logs to various invasion conditions. Some typical profiles are shown below.

Resistivity Response versus Depth of Investigation

Resistivity logs that measure different depths into the rock can be used to estimate the invasion profile. Results are used to judge the reliability of resistivity data, and to correct the log readings for the effects of invasion.

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