Laurent Mosse

Quantification of Remaining Oil Saturation Using a New Wireline Dielectric Dispersion Measurement - A Case Study from Dukhan Field Arab Reservoirs

The accurate determination of remaining oil saturation (ROS) for Qatar Petroleum’s Dukhan oil field, under production since the 1940s, is a key requirement for an ongoing revision of the field development plan. In 2009 several dedicated observation wells were drilled and cored. Extensive wireline data acquisition of nuclear, resistivity, acoustic, and nuclear magnetic resonance (NMR) data was performed with the objective to quantify the remaining oil saturation from log measurements and compare these logbased results with remaining oil saturations from core analysis. The Dukhan field is under water flooding for secondary recovery, and changing water salinities resulting from the mixing of salty formation and fresh injection water make any resistivity-based calculation of remaining oil saturation a difficult task. Faced with these challenges Qatar Petroleum chose to field-test a new wireline dielectric dispersion tool to obtain resistivity independent remaining oil saturation information. A second objective was to test the estimation of Archie’s cementation factor m from dielectric dispersion measurements, as Qatar Petroleum had evidence that m varies vertically and laterally across the reservoir. Remaining oil saturations obtained from the dielectric dispersion tool are presented and compared with resistivity-based values, NMR diffusion-relaxation analysis results, and core-derived Dean-Stark oil saturation measurements. The results confirm the ability of the dielectric dispersion tool to give a reliable measurement of remaining oil in place. Estimation of Archie’s cementation factor m appears to confirm expected variations especially in the Arab-C reservoir in certain parts of the field, with the dielectric dispersion based remaining oil saturation agreeing with the observed fluid production. Introduction Dukhan is a large mature field located onshore Qatar, approximately 80 km west of Doha. The field which was discovered in 1941 forms a north-south plunging anticline approximately 70 km long by 8 km wide and has more than 750 wells penetrating the main producing reservoirs. The Upper Jurassic (Kimmeridgian) Arab-C and Arab-D are the main reservoirs in terms of size and production potential. The Middle Jurassic Uwainat contains smaller amounts of oil and gas, whereas non-associated gas is present in the Permo-Triassic Khuff reservoir. The Dukhan field is divided into four inter-connected sectors that extend from North to South: Khatiyah, Fahahil, Jaleha and Diyab (Fig. 1). The division into sectors does not reflect subsurface segmentation of reservoirs. Rather, it is based on allocation of wells to surface facilities. Fig. 1: Dukhan field and location of example wells A, B, and C.