f Analysis of D.C. logging through metal casing

The measurement of formation resistivity through metal casing can aid in characterizing existing reservoirs for effective recovery of oil and gas, as well as geothermal heat, without the cost and time of drilling new wells. Measuring the resistivity of adjacent fonnations may be useful to locate oil that was missed during the original logging prior to insertion of the casing. The method is also useful to monitor changes in resistivity caused by subsurface processes such as injection or leakage of contaminants from a waste site, flooding operations for enhanced oil recovery, or extraction processes of geothennal production. Several patents have recently been issued which describe methods and devices that are capable of measuring formation resistivity through casing, (Kaufman, 1989; Vail, 1989a, 1989b; Gard et al., 1989). Currently, it is only known that Vail has developed and tested such a device, called Through-Casing Resistivity TooI (TCRT). Kaufman (1990) investigated the behavior of the potential and its derivatives for a borehole with casing based on models of an infinite-length conductive pipe in a homogeneous medium. Schenkel and Morrison (1990) presented numerical results for a point souree of current in or beyond the end of a fixed length of casing in a homogeneous half-space. This study has now been extended using a surface integral equation approach can calculate the potentials for various electrode configurations in the presence of a finite-length casing and a layered medium.