Apparatus and method for measuring the resistivity of underground formations

摘要

A method for measuring formation resistivity uses a tool (20) including a central current electrode (CE) and a series of voltage electrodes (A1-A13) arranged in pairs on either side of the current electrode (CE). A series of measurements is made using the current electrode (CE) with different numbers of pairs of voltage electrodes (Ai,Ai') maintained at a predetermined voltage so as to allow resistivity in the formation to be determined with different depths of investigation. In this manner these measurements are focused and are relatively unaffected by the borehole (22) or by adjacent layers in the formation. The central current electrode (CE) is segmented azimuthally into a series of electrodes, and the current flowing out of each azimuthal segment is measured separately. This can yield resistivity images of the formation surrounding the borehole (22). The azimuthal imaging capability can be used for stratigraphic or lithologic analysis of the formation and to detect fractures in the rock. The three-dimensional imaging capability can be used in deviated wells or horizontal wells to detect asymmetric invasion, or to locate a bed boundary close to the borehole.