Determination of Residual Oil Saturation With the Borehole Gravity Meter

Abstract

Use of the borehole gravity meter (BHGM) to measure remaining oil saturation is a method new to the industry. The technique is described, and its applicability to Middle East reservoirs is discussed. Results of an extensive error analysis are presented. The method, which we call log-produce-log, consists of running a base BHGM before significant production, and a later BHGM after production. The remaining oil saturation is computed from the difference of the two BHGM measured bulk densities. The Middle East oil fields are ideal for application of this method. Porosities are high, crude is light, and connate water is dense (saline). The method is independent, for all practical purposes, of hole size, rugosity, number of casing strings, shale content, and acidization. The method also has a very large radius of investigation, 50 feet plus, which enables it to sense a far larger volume of the reservoir than any other method. As reservoirs are often heterogeneous in both porosity and fluid saturations, the remaining oil saturation thus determined would be more representative of the reservoir as a whole than any other technique. The disadvantages of the method are the need for a base BHGM log prior to significant production from the zone of interest, and a poor vertical resolution of about 10 feet. Results of a comprehensive and realistic error analysis are presented and show this technique as possibly the most accurate Sor method.