resistivity logging modeling history, techniques


Modeling Electromagnetic Tool Response

Barbara AndersonGerald MinerboMichael Oristaglio

Schlumberger-Doll ResearchRidgefield, Connecticut, USAT om BarberBob FreedmanFrank Shray

Schlumberger Well ServicesHouston, Texas, USA

Resistivity modeling was used as far back as1927, when Conrad Schlumberger first rea-soned how current from an electrodespreads out into the formations around aborehole. But he would have called it “the-orizing.” Characteristics were assigned tothe formation (the formation model) and thelaws of physics, usually in idealized form,were used to predict analytically theresponse made by some electrode configu-ration (the sonde, or tool, model) to themodeled formation. Both theoretical andexperimental modeling have passed throughmany stages since then.

Early experimental modeling used smallelectrodes in “infinite” saltwater baths.the role of modeling during the last http://www.360docs.net/doc/info-6d146e1ba8114431b90dd81f.html puterized modeling has reduced fromweeks to minutes the time required to cal-culate many effects of tool design changes.One can now systematically explore theeffects of environmental conditions such asborehole rugosity and caves, mudcake,invasion, dip, shoulder beds, and formationanisotropy on resistivity tool responses.

The latest stage in this evolution is aimedat providing rapid, low-cost log interpreta-tion through the use of fast computers withlarge, high-speed memories and efficientprograms. Recently, even massively parallelprocessing has been introduced to servethese goals. Some log interpretation by

Resistivity modeling is shortening the learning curve in gaining understanding of the reservoir.

Although almost as old as logging itself, resistivity modeling is an integral part of the latest developments,from steering horizontal wells to investigating the effects of anisotropy.

Later, tool responses were studied usingmock-up sondes in more realistic environ-ments created by using thin impermeablemembranes to separate waters of differentsalinity. For a number of years, a resistornetwork was used at the Schlumberger-DollResearch laboratory in Ridgefield, Connecti-cut USA. This network, consisting of tens ofthousands of electrical resistors, simulatedresistivities in borehole, invaded zone andvirgin formation. In addition, theoretical cal-culations of sonde responses to layered andinvaded formations generated books ofdeparture curves. This theoretical approachwas especially important for tools that hadlarge depths of investigation or were notreadily adaptable to laboratory http://www.360docs.net/doc/info-6d146e1ba8114431b90dd81f.html rge improvements in computing capa-bility have introduced qualitative changes in

interactive modeling is possible even onpersonal computers.1Program packages forsimple one-dimensional (1D) modeling arecommonplace; two-dimensional (2D) andthree-dimensional (3D) modeling are practi-cal in many special cases, although theygenerally require the use of mainframes orsupercomputers. Two-dimensional modelingpermits examination of axially symmetricradial variations—for example, treatingzero-dip layering and coaxial invasionsimultaneously. Three-dimensional model-ing also handles azimuthal variations suchas circumferentially irregular caves or inva-sion, sonde eccentering and dipping beds.

22Oilfield Review