Reservoir Simulation Model Predicts EOR Performance on Alaska’s North Slope

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206247, “History Matching and Performance Prediction of a Polymerflooding Pilot in a Heavy Oil Reservoir on Alaska’s North Slope,” by Xindan Wang, Cody Keith, SPE, and Yin Zhang, University of Alaska Fairbanks, et al. The paper has not been peer reviewed. _ The complete paper discusses the first polymerflooding pilot to enhance heavy oil recovery on the Alaskan North Slope. After more than 2.5 years of polymer injection, significant benefit has been observed from the decrease in water cut, from 65% to less than 15% in project producers. The primary objective of the study was to develop a robust history-matched reservoir simulation model capable of predicting future polymerflooding performance. Pilot Description The pilot program targets an isolated fault block in the Schrader Bluff NB sands with two horizontal injectors (J-23A and J-24A) and two horizontal producers (J-27 and J-28). The producing lengths of the horizontal wells range from 4,200 to 5,500 ft, and the average interwell distance is approximately 1,100 ft. The project wells were put on waterflooding in June 2016, recovering approximately 7.6% of the original oil in place with a cumulative water injection of 0.09 pore volumes (PV). In August 2018, polymer solution injection commenced. The polymer solution, with a target viscosity of 45 cp, was introduced to both injectors by means of a polymer injection unit. By the end of December 2020, a total of 1.7 million STB of polymer solution had been injected, representing 0.08 PV in the flood pattern. A significant benefit of polymerflooding has been observed in the decrease in water cut in both producers. The oil production rate of Producer J-27 (supported by both injectors) increased continuously from 500 to 800 BOPD in April 2019. The oil production rate of Producer J-28 (supported only by J-23A to the north) also increased to approximately 700 BOPD in late 2019 and stabilized at approximately 500 BOPD in 2020. Reservoir Modeling Base Reservoir Model. A 3D reservoir model was developed for the pilot fault block. The selected active reservoir simulation model, including 23,848 active gridblocks, covers the potential sweep area of the project wells. The 3D view of the selected active reservoir simulation model is shown in Fig. 1. The approximate dimensions of the active reservoir simulation model are 6,900, 4,100, and 15 ft in the x, y, and z directions, respectively. The reservoir simulation model consists of eight layers in the z direction. High-Transmissibility Strips. The early water breakthrough and high water cut during the pre-polymer/waterflooding period indicate strong communication and short-circuiting behavior between the injecting and producing wells. To simulate this short-circuiting effect, four high-transmissibility strips at different locations and with different widths were introduced to connect the injectors and producers in the reservoir simulation model. Each strip was present in all eight layers of the model such that the strips in each layer were identical.