EOR Mechanisms and Applications of Thermal Composite Technologiesin Deep Heavy Oil Reserves

Abstract

Abstract The heavy oil reservoirs in China are predominantly deep-seated, with high heterogeneity and oil viscosity. Heavy oils situated at depths exceeding 900m constitute 62% of the nation's total reserves. By the increase of steam stimulation cycles, traditional thermal recovery techniques tend to yield diminishing benefits. Thus, Sinopec is establishing a series of thermal composite EOR technologies, primarily targeting deep, heavy oil reserves. In this study, we've engineered high-temperature and high-pressure micro-visualization experiments to elucidate the microscopic mechanisms underlying thermal composite oil recovery. Our findings show the presence of wall-adhering oil films and filamentary flows during low-temperature hot water flooding. Specifically, at 150°C, the phenomenon of oil-water slug displacement occurs. At 350°C, steam manifests as isolated, nebulous clusters within the oil phase, subsequently leading to the oil phase's adherence to the wall in a turbid state. Upon encountering small pores, steam induces capillary condensation, indicating its liquefaction and heat release in regions with low permeability, thereby enhancing thermal efficiency. Chemical flooding has been observed to yield fine oil-in-water emulsions at pore throats due to its occlusive action, optimizing flow capacity. Notably, the combination of high-temperature steam and chemical agents promotes spontaneous emulsification upon contact with crude oil, generating a flow of finer oil-in-water emulsions. Our "heat+CO2" findings show that at 0.3 MPa, gaseous CO2 tends to form bubbles, subsequently leaving an oil film after gas flooding. Contrastingly, after supercritical CO2 flooding at 10 MPa, almost no oil film residues are found on the wall. At 20 MPa, CO2 dissolution and extraction make the oil phase components segregate at later stage. The combined effects of "steam, chemical agents, and gas" have been identified to significantly enhance oil extraction rates. Based on our understanding of EOR mechanisms, we have conducted field tests. A thermochemical composite steam drive was executed in the highly-heterogeneous Ng5 block deep heavy oil reservoir in Shengli Oilfield, which has suffered from gas channeling induced by steam. This makes the horizontal displacement more uniform. The coefficient of variance in steam injection speed is significantly reduced to 0.13, surging daily oil extraction by up to fourfold. The recovery rate is increased by 27 percentage points, achieving 62.2%. For the challenges of steam injection impediments and crude oil recovery issues in the Zheng 411 block of deep super heavy oil in Wangzhuang Oilfield, a pilot scheme was launched to bolster thermal development through the combined effects of viscosity-reducing agents and CO2. Our data indicates that single-well production soared from 127 tons to 1,812 tons—a staggering 13.3-fold increment—with the oil-to-steam ratio peaking at 0.88.