Role of molecular diffusion in heterogeneous, naturally fractured shale reservoirs during CO2 huff-n-puff

Abstract

Shale reservoirs are characterized by high degree of heterogeneity and ultra-low matrix permeability. Natural fracture (both macro and micro) systems are important to increase the performance of shale reservoirs by providing flow path for the hydrocarbon from the nanopores in the matrix to the wellbore. Configurations of natural fractures also have significant implications for the recovery performance of shale gas and oil reservoirs. CO2 huff-n-puff or cyclic injection has been suggested to be a feasible way to recover oil from shales and the flow process is complex. Molecular diffusion is an important transport mechanism of the mixture of CO2 and oil, especially in tight formations. Based on these knowledge, various configurations of the natural fracture system in shale reservoirs with the dual porosity and dual permeability model were constructed, regarding Dykstra-Parsons (DP) coefficient and correlation length. Dykstra-Parson (DP) coefficient ranges from 0.40 to 0.78, and correlation length ranges from 50 ft to 3000 ft. Based on the geological setting of the Middle Bakken formation and the Bakken live oil PVT data, compositional models were performed to simulate the CO2 huff-n-puff process. CO2 injection begins after the primary recovery factor reaches 3% and both multiple-cycle and single-cycle of injection were investigated regarding the oil recovery performance and CO2 injectivity behavior. In the multiple-cycle scheme the period of one cycle is short; the injection, soak and production time are 10 days, 10 days and 100 days, respectively. In the single-cycle scheme, the period of one cycle is long, and gas injection volume is set to be constant with the soak time of 100 days. The impact of molecular diffusion during the huff-n-puff process is compared in different simulation scenarios. Findings in this work reveal the importance of the configuration of the pre-existing natural fracture system for the performance of the huff-n-puff process. Reservoir heterogeneity hampers primary production. However, during the puff production process, the negative impact of heterogeneity is less as the correlation length increases when the molecular diffusion is taken into account. There exists a threshold correlation length value wherein heterogeneity becomes favorable for improved oil recovery instead of hampering oil recovery, after taking into account of the molecular diffusion, which is approximately 1500 ft in this simulation study. The value of correlation length also affects the performance of multiple-cycle and single-cycle huff-n-puff processes differently because CO2 injectivity behavior is different in reservoirs characterized by different DP coefficients and correlation length: there exists a threshold time point when a longer correlation length begins to favor the injectivity, and the threshold time point is delayed for a scenario with a larger DP coefficient.