Measurement of Formation Resistivity Through Steel Casing

Abstract

B. Sh. Singer, O. Fanini, K.-M. Strack, L.A. Tabarovsky, Western Atlas Logging Services, 10201 Westheimer, Houston, TX 77042, USA, and X. Zhang, Electrical Engineering Dept., University of Houston, 4800 Calhoun Rd., Houston, TX 77024, USA Abstract Through casing resistivity measurements confirmed the original concept proposed by Alpin (Ref. 1) and recently revived and improved by Kaufman (Ref. 2) and Vail (Ref. 3). A commercial tool can be used for monitoring producing wells, waterflooding control, and searching for bypassed hydrocarbons in abandoned and active wells. The information on the resistivity of the formation is represented by the second spacial derivative of the casing voltage that is generally small. This makes the evaluation of possible distortions arising from different kinds of casing imperfections important. These imperfections include casing collars, corrosion, perforations, etc. The fact that only low frequencies can be used for the measurement allows us to propose fast and accurate modeling algorithms. Numerical evaluation shows that the distorting effect of casing imperfections is quite moderate. Typically, it does not exceed 10–20% of the measured apparent resistivity. The vertical resolution of the measurements is limited by the spacing between the voltage sensing electrodes. It can deteriorate due to the cement sheath that always exists around the casing. A significant distortion of the measurement occurs near the bottom of the casing. This distortion can reach 50% of the signal. To improve the readings, a simplified inversion algorithm has been developed. The most direct application of the algorithm is the monitoring of the pay zone resistivity. Introduction One of the major requests from the oil and gas industry to logging service companies is the measurement of the formation resistivity behind steel casing. Formation resistivity is the primary indicator of water, gas, and oil saturation. The through casing resistivity (TCR) tool would allow the industry to control pay zone depletion, waterflooding, etc. It would also make it possible to log producing and abandoned wells targeting the hydrocarbons bypassed in the open hole logging operations. Two factors are impeding application of the traditional logging approaches to measurement of formation resistivity behind a steel casing. The first and the major factor is the low resistivity of the casing steel that is typically about 0.2 m. A high magnetic permeability of the steel (40–110) is another obstacle for electrical logging of cased wells. At the frequency of 1 kHz, the skin depth in the steel is less than 0.1 cm; much smaller than the thickness of the casing wall. This means that an electromagnetic field created inside the casing will not penetrate into the formation. To energize the formation and to measure its inductive response. Ref. 4 proposed to saturate the casing with a strong magnetic DC-field. The relative magnetic permeability of a saturated casing equals one. This results in the increase of the frequency threshold. Ref. 5 proposed very low-frequency induction logging measurements so that magnetic saturation could be avoided. A number of authors concentrated their efforts on the development of a TCR tool based on low frequency galvanic energizing of the casing and the formation behind the casing. The concept of evaluating the current leaking from an energized casing into the formation was originated by Alpin (Ref. 1), who proposed a three-electrode apparatus measuring the second difference of the voltage inside the casing. As a practical solution, the concept was not implemented for almost 50 years, mainly due to two circumstances. P. 999