Residual Oil Zone Recovery Evaluation and Forecast Methodology: A Wasson Field Case Study
Authors: Gupta Neha , Parakh Sameer , Gang Tao , Cestari Nicholas , Bandyopadhyay Parag 
Published: April 2022

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.