New C/O Methodology Applied for Reservoir Monitoring in Atypical Casing Completion and Complex Lithology: A Case Study

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractCarbon Oxygen ratio (C/O) logging has been used in the oilfield for a long time as a solution to determine oil and water saturations independent of the formation water salinity. The traditional S w computation methodologies rely on the use of the tool under standard and uniform completions, and are based either on a data base characterization or on deterministic models. These techniques fail when the casing size, along the logging interval, is variable, or when the tool characterization is not available for unusual casing sizes.Uncertainties are also encountered in complex lithologies, particularly in the presence of carbonates, due to deposition or diagenesis. In this case, it becomes difficult to compute the correct carbon content of the rock to correct the final volume of oil estimated.This paper describes a case study with a very atypical completion including downhole sensors as part of the casing, and the presence of a variable carbonate volume in the reservoir rock matrix. It is part of a project to define the evolution of a water flooding between two nearby wells, one the injector and the other the monitoring well. Here, a new computation methodology to calculate the volume of oil using the openhole computed S w , as a reference, and different time lapse C/O logging passes to define the change in volume of oil in the reservoir during the water injection, is presented. This variation (∆V uoi ), applied to the openhole S w , enables the computation of precise saturation results, where it is possible to observe the quantitative change in fluid volumes in depth over a period of approximately six months in spite of the nonstandard completion type and complex lithology.