Field tests of CO₂ huff-n-puff technology in Nanchuan normal-pressure shale gas field

Due to the high proportion of adsorbed methane and weak formation energy, the recovery rate of normal-pressure shale gas reservoirs is generally less than 30%. SINOPEC took the lead in conducting CO₂ huff-n-puff field tests in the Nanchuan normal-pressure shale gas field in the Sichuan Basin, verifying the feasibility of CO₂ injection for enhanced recovery of marine shale gas. To promote this technology, a comprehensive study was carried out on the Nanchuan normal-pressure shale gas field, involving laboratory experiments, numerical simulations, and dynamic huff-n-puff analysis. The study analyzed the CO₂ competitive adsorption differences in different shale reservoirs, explored the CO₂ huff-n-puff characteristics in the field, and clarified the synergistic effects of CO₂ huff-n-puff to enhance shale gas recovery (ESGR) technology through multi-mechanisms of energy enhancement, displacement, and water-unlocking, aiming to guide well selection and program optimization. Using techniques such as electron microscope scanning, well logging interpretation, and isothermal adsorption experiments, the study revealed that the CO₂ competitive adsorption capacity of normal-pressure shale reservoirs in the Upper Ordovician Wufeng and the Lower Silurian Longmaxi formations of the Nanchuan area increased with decreased burial depth and formation pressure, and with increased porosity, TOC, and clay mineral content. The adsorption capacity of supercritical CO₂ was found to be 6 to 7 times higher than that of CH₄. After CO₂ huff-n-puff operations in shale gas wells, the daily gas production increased by 3.5 to 6.5 times, and the recovery rate increased by 1.9% to 3.1%. Based on pressure monitoring during the injection and soaking stages of two wells over three rounds of CO₂ injection, CO₂ mainly concentrated in the near-well micro-fractures. The diffusion distance, generally not exceeding 70 m, was related to formation pressure and the conductivity of fracture network. The process of CO₂ huff-n-puff can be divided into three stages: early rapid CO₂ flowback, early production increase, and mid- to late-stage stable production. The production increase mechanisms include early energy enhancement and supplementation, mid-stage expansion and expulsion assistance + water lock removal, and late-stage adsorption displacement + desorption promotion by partial pressure. The main influencing factors for increased huff-n-puff production are the degree of reservoir modification and recovery. Wells with poor fracturing effects in medium and deep layers had a higher gas exchange rate during the early and middle stages of CO₂ huff-n-puff, while wells with high recovery rates in shallow layers had a higher cumulative gas increase in the middle and late stages. Based on numerical simulations, it is recommended to prioritize wells with strong adsorption capacity, a recovery rate of 20% to 30%, poor liquid carrying capacity, and a shut-in pressure as close to 7 MPa as possible for field pilot tests. In the low-pressure and low-yield stage, small-scale multiple rounds of CO₂ huff-n-puff can be carried out in medium-deep wells for energy enhancement and expulsion assistance, while large-scale CO₂ huff-n-puff can be conducted in shallow wells to replenish formation energy and achieve enhanced recovery through adsorption displacement.