Cyclic gas injection huff-n-puff in multi-stage fracturing horizontal wells to improve recovery of shale oil and gas reservoirs: Taking Eagle Ford Shale in North America as an example

In the case of dissolved gas drive development, the prediction of primary oil recovery of depleted horizontal wells in shale oil and gas reservoirs by multi-stage hydraulic fracturing is only 5%-10%, so how to improve the oil recovery is a difficult problem to face. In recent years, the pilot test and large-scale field application of Cyclic Gas Injection Huff-n-Puff in Multi-Stage Fracturing Horizontal Wells to enhance shale oil and gas recovery have achieved remarkable results in The Eagle Ford Shale in North America. In order to accelerate the efficient development and utilization of unconventional oil and gas resources in China, based on the investigation of relevant literature and combined with the existing successful experience in North America, the technology of huff and Puff gas injection to improve shale oil and gas recovery was examined from the aspects of action mechanism, numerical simulation and field test, and some specific suggestions are presented. The results show that large-scale gas injection Huff-and-Puff could effectively prolong the life cycle of wells and increase the ultimate oil and gas recovery by 30% to 70% compared with primary depletion development. The reservoir with low dissolved gas oil ratio, gas injection huff and puff to improve recovery factor is injected into the mechanism of gas diffusion in the fracture reservoir in contact with the surplus oil and dissolved, make its volume expansion, lower crude oil viscosity and surface tension, improve the liquidity of crude oil, for the initial high dissolved gas oil ratio of volatile oil reservoirs and condensate gas reservoir, evaporation of oil phase components into injected gas is the more important oil increase mechanism, while for condensate gas reservoirs, evaporation of condensate near wellbore to increase production is the main oil increase mechanism. The mechanism of oil increase above determined that only a large amount of gas injection could achieve obvious effects. The injected natural gas is dry gas with 90% methane content. The large-scale application of injection gas Huff-n-Puff in North America benefits from cheap natural gas that is cheaper than CO2. Due to the high price of domestic natural gas, using CO2 Huff-n-Puff could be considered. CO2 Huff-n-Puff can reduce greenhouse gas emissions, and problems, e.g., its capture, storage, and pipeline transportation could be solved.