Induced polarization logging - Borehole modeling, tool design and field tests

Induced polarization logging provides in situ Qv and S interpretations of shaly sands. An experimental IP tool was developed which measures quadrature conductivity and phase shift, in addition to conventional SP and resistivity measurements. Borehole geometry and inductive coupling effects were modeled for IP arrays using both analytic solutions and finite-element programs. Collectively, the programs handle the effects of borehole, invaded zone, and finite bed thickness. Inductive coupling in the 16" Normal array was found to be small for conductivity-frequency product as high as 10 mho-m""∗-Hz. The IP tool that was developed is a modular four electrode array which can operate in both frequency- And time-domain. Current and voltage are sampled simultaneously downhole and digitally transmitted to the surface in order to avoid capacitive coupling in the logging cable. Non-polarizable Ag-AgCl voltage electrodes are used to eliminate electrode polarization. Field tests of the IP tool were conducted at a Shell test well in Johnson City, Texas and in the Big Foot Field, Frio, Texas. The Johnson City well traverses a variety of clean to very shaly sands in a fresh water environment. Comparison of Qv's obtained from cores at Johnson City were in good agreement with log-derived values, over the entire range 0.0 to >1.0 meq/ml. The Olmos B sand in the Big Foot field is the shaliest producing sand in Shell's experience, with Q ' s up to 0.75 meq/ml. In the Olmos B test, good agreement was also obtained between IP log- And core-derived Qv and Sw values.