Determining Formation Resistivity Anisotropy in the Presence of Invasion

Abstract

Abstract Resistivity anisotropy in both laminated shale-sand and clean sand formations is well documented. Tools that are sensitive to formation anisotropy are also well documented, and the leading contender for this type of measurement is the transverse induction array. Such an array, whose transmitter generates formation currents in the plane of the borehole axis, has a good sensitivity to the vertical resistivity of the formation, Rv. Invasion of mud filtrate into permeable formations has long complicated wireline log analysis. Interpretation of anisotropic formations will be no different. In most drilling environments, these formations will be invaded as readily as isotropic formations. Although we expect that invading mud filtrate from water-based mud will reduce the anisotropy, we also expect that invading oil-based mud (OBM) filtrate will increase it. Thus the anisotropic properties of each zone must be determined separately. A single-spacing induction array (of any orientation) cannot, by itself, separate uninvaded zone properties from those of the invaded zone. Multiple-spacing tools have been used for many years to make this separation. A multiarray triaxial induction tool is in field test, and invasion interpretation algorithms are under development. A fast analytic algorithm corrects shoulder effect on all nine triaxial couplings of each array, allowing Rv and the horizontal resistivity Rh to be determined at several radial depths in the formation. Rigorous 2D and 3D inversions are also used to evaluate beds on the order of 1 m thick in both vertical and deviated wells. Examples in both modeled formations and in actual formations demonstrate the methods. Introduction Induction tools (along with laterolog tools) have been the standard resistivity devices for borehole geophysics for about 50 years. The nearly geometric response of the induction array has made it easier to interpret than the non-geometric response of the laterolog tools. Conventional induction tools are built with coils that have their magnetic moments along the tool axis. The resulting sensitivity to the formation is in a direction perpendicular to the wellbore axis. When the beds are dipping, or when the well is deviated, the response is more complicated, but not in a way that allows determination of the dip angle.