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.

The use of CO2 injection to produce oil from the residual oil zone (ROZ) of the Wasson field in the Permian Basin has proven to be highly successful when an appropriate development plan is used. The significant volume of oil in place in the ROZ presents a large target for both reserves addition and CO2 sequestration. More than 60% of the ROZ potential lies beneath the already developed San Andres main oil column (MOC) area, which is under CO2 flooding with varying states of maturity, making it challenging to develop such projects efficiently and economically. Over the past 20 years, different pattern configurations (nine-spot, line drive, five-spot) and completion strategies (commingled injector, injection subsurface flow control devices, dual completion injection, dedicated and hybrid line drive) have been used at the Wasson oil development company (ODC) field to develop the ROZ. The results of these various pattern configurations and completion techniques and their pros and cons are discussed in this paper. Commingled production makes it more difficult to quantify incremental ROZ production and increases uncertainty in the performance forecast of future ROZ projects. The dedicated injectors provide better injection control to MOC and ROZ and improve CO2 utilization, especially where the MOC is mature. In this paper, we present one of the key findings from a detailed analysis of field history that caused Oxy to switch from the original dedicated ROZ development to a hybrid line drive pattern configuration. This novel strategy will have higher CO2 retention and more sequestration potential, better areal sweep efficiency for improved oil recovery, and lower capital and operating cost. It also reduces the likelihood of injector interference, provides a stable injection throughput for a long time, and results in a sustained oil and CO2 production plateau, which leads to more efficient utilization of plant capacity. Using ODC as an example, the total capital, F&D costs, and the number of new injection wells will be reduced by 33%, 35%, and 45%, respectively, for changing all the undeveloped patterns from the dedicated to hybrid line drive option. This novel development strategy improves the chance of promoting contingent resources (not currently considered to be commercially recoverable owing to one or more contingencies) to a higher category and offers higher returns with much lower F&D cost and shorter development time.