Advancements in horizontal drilling with hydraulic fracturing have enabled commercial oil production from tight oil reservoirs. However, the primary recovery factor remains very low, usually less than 15%. It is a big challenge to supply formation drive energy to sustain production. After hydraulic fracturing, there is often pre-mature water breakthrough or gas channeling when we inject water or gas. Therefore, CO2 huff-n-puff becomes an attractive option to improve oil recovery in tight oil reservoirs.
Based on typical reservoir and fracture properties in Ordos Long-7 tight oil reservoir, a compositional reservoir simulation model with hydraulic fracture network was established to evaluate the performance of CO2 huff-n-puff enhanced oil recovery (EOR) method. Through numerical simulation, we perform sensitivity study to explore the impacts of operation parameters such as CO2 injection rate, injection time, soaking time, number of huff-n-puff cycles on EOR performance. Some of these parameters have rarely been investigated for recovery in tight reservoirs, such as in-situ fluid composition, fracture pore volume and hydraulic fracture characterization. Furthermore, correlation analysis is used to evaluate the performance of CO2 huff-n-puff process.
In this study, we find that hydraulic fracture morphology and fracture conductivity can have a large impact on the performance of CO2 huff-n-puff. Due to the existence of natural fractures in tight reservoirs and the stimulated reservoir volume, CO2 huff-n-puff can not only mobilize the crude oil near the well, but also have certain recovery effects on the remaining oil between the adjacent wells. Simulation results show that the most important parameter is number of cycles, followed by CO2 injection rate and soaking time. It is found that the optimum injection pressure of CO2 huff-n-puff process can be set around the minimum miscibility pressure (MMP) for CO2 and the crude. We set the soaking time period to be 30 days, injection rate to be 150 ton/day, number of cycles to be 4 for optimized oil recovery. The incremental oil recovery factor after one cycle is 1.59%, and the output-to-input ratio is 1:1.75.
The findings in this work have the potential to advance our understandings of the role of CO2 EOR in developing unconventional oil reservoirs, which will benefit both the energy industry and the environment with the potential benefit of CO2 geological sequestration.