The Wasson Field in the Permian Basin has been the forerunner in the use of carbon dioxide (CO2) enhanced oil recovery (EOR) to tap the potential of the residual oil zone (ROZ). This field is one of the largest ROZ oil producers in the Permian with multi-billion barrels of oil in place, and it is a prime target for EOR as well as CO2 sequestration. Twenty-seven ROZ development projects implemented over three decades in three of the largest Wasson San Andres units (Denver, ODC, and Willard) comprise the scope of data analyzed for this paper. These projects targeted the ROZ pay in mature CO2 floods in the Main Oil Column (MOC) by utilizing existing wells and commingling production from both the MOC and ROZ to reduce costs. However, commingled production makes interpreting the incremental ROZ recovery challenging, which ultimately increases the uncertainty in predicting the technical and economic performance of future ROZ projects. This paper presents a reliable, geo science-driven forecasting technique for ROZ development based on a comprehensive study of the production and injection performance of the 27 ROZ projects. This study uses in-place volumes from a geological model that integrated log, core, and seismic data; historical production and injection data; multi-year zonal flow profiles; and established dimensionless forecasting methods. This paper presents a consistent methodology to: Estimate MOC performance through dimensionless analysis and deduce historical ROZ performance; and,Forecast ROZ ultimate recovery after history matching the resulting injection and production. The estimated ROZ oil recovery across the three Wasson units has been analyzed to establish correlations with the residual oil saturation (Sorw), reservoir quality index (RQI), reservoir heterogeneity, pattern configuration, waterflood maturity, and the water alternating gas (WAG) ratio of the CO2 injection. The key performance indicators of ROZ oil recovery have been determined to be the residual oil saturation and reservoir quality index. The study also shows that the average Sorwin the MOC after waterflooding operations can be higher than the Sorwin the ROZ post"natural" waterflood, resulting in higher oil recovery from the CO2 flood in the MOC than in the ROZ. A correlation has also been established between the ROZ and MOC oil recoveries as a function of floodable volumes using petrophysical properties, which can be applied to analogous ROZ development in mature MOC assets. Most published ROZ oil recovery estimation methods have used reservoir simulation models or analytical approaches like scaling the MOCoil recovery or use of analogous actual ROZ performance. These approaches have limited applicability and cannot be applied widely over different ROZ projects. This paper is the first study that utilizes voluminous historical field data from multiple ROZ projects spread over an extensive duration and acreage across the Wasson Field to estimate ROZ oil recoveries and then propose a novel approach to correlate and scale these estimated ROZ recoveries using petrophysical properties.
Bahrain oil field being the first oil discovery in the gulf region in 1932 is now in a mature stage of development. Crestal gas injection in the Mauddud reservoir has continued to be the strongest drive mechanism since 1938. Over the last five years, gas injection and fluid production rates have grown three folds with expanded drilling, workovers, and high volume lift activities however there are significant opportunities to increase oil production and optimize gas injection. Since 2009, a growing number of wells in Mauddud including newly drilled wells were being shut-in due to high gas to oil ratio (GOR) within the limited surface handling capacity. 30% of active producers accounting for 25% of oil production were shut-in by year 2014. Proper evaluation of this opportunity followed by the field implementation has helped to significantly improve the oil production rate, reserves base, and understanding of the Mauddud reservoir fluid dynamics. To manage the voidage replacement ratio (VRR) associated with the rapid increase in oil producers, gas injection in the Mauddud reservoir has grown from 180 mmscfd to 500 mmscfd between years 2009-2015. A phased optimization program has been implemented to identify the conformance issues, impact of faults, and for optimization of surveillance systems in order to maximize the oil recovery. The VRR optimization program has also shown opportunities to preserve the source gas from Khuff reservoir which is a valuable resource for the Kingdom of Bahrain’s future. This paper will describe the integration of gas shut-off workovers, selective gas compression, and VRR optimization as excellent gas management tools for maximizing recovery potential in the Mauddud reservoir.
Starting late 2011, high temperature and high-pressure steam pilots were initiated in two Bahrain carbonate reservoirs. The first thermal pilot started in December 2011 within the heavy oil Rubble Reservoir, in a 7 well horizontal and vertical well configuration, covering 1 to 3 acre spacing. The second pilot started in December 2012 within the light oil Mauddud Reservoir with a 9 well horizontal and vertical test configuration covering 5 acres. Both carbonate thermal pilots progressed through pilot expansion or modification phases over the last 4 years, and were both technically successful in mobilizing and recovering oil and successfully reduced residual oil to expected thermal recovery end points. Currently, the Rubble and Mauddud thermal projects are undergoing expansion for field economic appraisals.The focus of this paper is on the Mauddud Steam Pilot. The paper will describe the geologic environment, initial pilot design, changes made to the pilot through systematic analysis, and timely achieved final technical success results. The systematic analysis permitted successful pilot execution, pilot technical success and initiation of economic appraisal expansion in the amazingly short time period of approximately 2.5 years. Most astonishingly, this pilot is unconventional thermal recovery; there are no suitable analogs to tight fractured carbonate light oil thermal recovery.
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