MEASURING FORMATION RESISTIVITY THROUGH CASING AT ELK HILLS FIELD

Occidental of Elk Hills recorded formation resistivity through casing in ten wells at the Elk Hills Field, Kern County, California. Two types of tools were used to record the data. This case history describes the tools and theory used to make the measurements. The data are compared to open hole logs to validate the technique. The data are also applied to reservoir monitoring and surveillance questions. The new technique required some confirmation of the accuracy and precision of the measurement. The logs illustrate the effects of borehole size, cement thickness, open perforations, casing corrosion and casing scale. One log in particular compares a cased hole resistivity just after completion to the open hole induction resistivity. Despite the high signal-to-noise ratio, sufficient precision can be obtained to match open hole resistivity data. The study applies the logs to determine gas, oil, and water contacts in the 26R Reservoir. The data appear to reveal the contacts even in the presence of scale and open perforations. Another surveillance application in the Main Body B reservoir identifies water breakthrough in an existing waterflood. The lower resistivities are consistent with scale buildup in the same intervals. Reservoir monitoring is an important issue at Elk Hills Field. The resistivity data could improve the performance of an existing waterflood and determine moving oil-water, gas-oil contacts. Reservoir monitoring is difficult in existing wells because of the shallow investigation of typical cased hole logging tools. The through casing resistivity technique measures across a five-foot radius from the wellbore, allowing resistivity to be measured beyond the invaded zone. The results of this study show good agreement with open hole data but mixed success in application. Since reservoir monitoring is a significant and common issue, subsequent application and improvement of this technique is expected to continue.