Integrated Well-Network-Design Mode Developed for CO2 EOR and Storage

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 25000, “Integrated Well-Network-Design Mode for CO2 EOR and Storage and Its Application in L Reservoir,” by Zangyuan Wu, PetroChina, CNPC, and China University of Petroleum; Yongliang Tang, PetroChina and CNPC; and Liming Lian, CNPC, et al. The paper has not been peer-reviewed. Copyright 2025 International Petroleum Technology Conference. _ This paper addresses the challenge of adjusting late-stage production in waterflooded reservoirs and proposes an integrated well-network-design mode for CO2 enhanced oil recovery (EOR) and storage. The well network is designed according to the full life cycle of carbon capture, use, and storage (CCUS), meeting the requirements of both CO2 EOR and storage phases. Overview of Geological Development of L Reservoir The L reservoir structure is in a zone consisting of a series of long-axis anticlines trending nearly east/west. The structure includes multiple development blocks on the plane, is intact overall, and has good sealing conditions. The L oil reservoir is divided vertically into two sublayers, L-1 and L-2. Based on the statistical analysis of core-sampling experimental data, the L-1 sublayer has an average porosity of 16.2% and a permeability of 39.3 md, classifying it as a medium-porosity, low-permeability reservoir. The L-2 sublayer has an average porosity of 19.5% and a permeability of 193.4 md, classifying it as a medium-porosity, medium-permeability reservoir. The reservoir properties of L-2 are significantly better than those of L-1. The crude oil from the L reservoir has good properties, characterized by low viscosity, low pour point, medium-to-low sulfur content, high wax content, and medium-to-high asphaltene content. CO2 EOR and Sequestration Mechanisms The conditions of the L reservoir make it well-suited for CO2 EOR. At the start of CO2 injection, because of the strong solubility of crude oil, CO2 can dissolve into the oil quickly upon pressurization. Therefore, the gas/oil ratio (GOR), expansion coefficient, and saturation pressure increase gradually. The dissolution of CO2 into the crude oil and the extraction of CO2 from the crude oil lead to a gradual decrease in viscosity. The expansion coefficient increases from 1.0000 to 1.3978, an increase of 40%, and the viscosity of the crude oil decreases from 3.617 to 1.052 mPa·s, a reduction of 70%. This indicates that, after CO2 is injected into the reservoir, it can achieve effective expansion and viscosity reduction. To characterize the properties and water-content differences of the L-1 and L-2 sublayers, suitable cores were selected based on measured porosity and permeability data. Special long-core-displacement experiments were conducted, including continuous gas driving after waterflooding, water-alternating-gas (WAG), cyclic gas injection, and CO2 + hydrocarbon-slug-displacement experiments. The oil-displacement efficiency of miscible CO2 injection exceeds 80% in all cases, and different gas-injection methods after waterflooding can enhance oil-displacement efficiency significantly. Both cyclic CO2 injection and WAG after waterflooding achieve efficiencies greater than 90%, and both can control the rise in the GOR effectively; continuous CO2 injection without profile modification and CO2 + hydrocarbon-slug injection, where the hydrocarbon is a nonmiscible component, affect the oil-displacement efficiency of these two injection methods.