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Dual laterolog DLL makes use of a galvanic conduction principle to focus electrical currents into rock formations, therebyminimizingshoulderandboreholeeffectsinthemeasurement of formation resistivity. The tool includes two separate focusing systems: deep-sensing LLd and shallow-sensing modes LLs. Laterolog current-focusing systems were designed for operation primarily in vertical boreholes penetrating horizontal layers; only recently their design has been revised for operation in deviated wells in the presence of electrical anisotropy. We simulated three-dimensional3DDLLmeasurementsindipping,invaded, and electrically anisotropic formations and appraised the corresponding effects on apparent resistivity logs. Simulations were performedbycombiningtheuseofaFourier-seriesexpansionin anonorthogonalsystemofcoordinateswithanexisting2Dgoaloriented, higher-order, and self-adaptive finite-element method. This numerical algorithm yields accurate solutions in limited CPUtimebecauseonlyafewFouriermodesareneededtosimulate practical applications. For the calculation of focused currents, we introduced an embedded postprocessing method that incorporates a synthetic focusing principle to compute current intensitiesateachiterativestepofoptimalmeshrefinements.Our numerical method accurately simulates 3D DLL measurements in rock formations that exhibit extreme contrasts of electrical resistivity. Simulations indicate that LLs resistivity logs are more sensitive to both invaded and anisotropic layers than LLd resistivity logs. In deviated wells, shoulder-bed effects on apparent resistivitylogsincreasewithanincreaseofdipangle,andareemphasized across thin conductive layers. Electrical anisotropy effects on apparent resistivity logs increase substantially with dip angle.

geophysics

Simulation of DC dual-laterolog measurements in complex formations: A Fourier-series approach with nonorthogonal coordinates and self-adapting finite elements