resistivity logging modeling history, techniques

Barbara Anderson Gerald Minerbo Michael Oristaglio

Schlumberger-Doll Research Ridgefield, Connecticut, USA T om Barber Bob Freedman Frank Shray

Schlumberger Well Services Houston, Texas, USA

22Oilfield Review

Later, tool responses were studied using mock-up sondes in more realistic environ-ments created by using thin impermeable membranes to separate waters of different salinity. For a number of years, a resistor network was used at the Schlumberger-Doll Research laboratory in Ridgefield, Connecti-cut USA. This network, consisting of tens of thousands of electrical resistors, simulated resistivities in borehole, invaded zone and virgin formation. In addition, theoretical cal-culations of sonde responses to layered and invaded formations generated books of departure curves. This theoretical approach was especially important for tools that had large depths of investigation or were not readily 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 on personal computers.1Program packages for simple one-dimensional (1D) modeling are commonplace; two-dimensional (2D) and three-dimensional (3D) modeling are practi-cal in many special cases, although they generally require the use of mainframes or supercomputers. T wo-dimensional modeling permits examination of axially symmetric radial variations—for example, treating zero-dip layering and coaxial invasion simultaneously. Three-dimensional model-ing also handles azimuthal variations such as circumferentially irregular caves or inva-sion, sonde eccentering and dipping beds.

Resistivity modeling was used as far back as 1927, when Conrad Schlumberger first rea-soned how current from an electrode spreads out into the formations around a borehole. But he would have called it “the-orizing.” Characteristics were assigned to the formation (the formation model) and the laws of physics, usually in idealized form,were used to predict analytically the response made by some electrode configu-ration (the sonde, or tool, model) to the modeled formation. Both theoretical and experimental modeling have passed through many stages since then.

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

The latest stage in this evolution is aimed at providing rapid, low-cost log interpreta-tion through the use of fast computers with large, high-speed memories and efficient programs. Recently, even massively parallel processing has been introduced to serve these 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.

Modeling Electromagnetic Tool Response