Effect of Reservoir Heterogeneity on Primary Recovery and CO2 Huff ‘n’ Puff Recovery in Shale-Oil Reservoirs

Summary An equation-of-state (EOS) -based compositional reservoir simulator, UT-COMP, is used to simulate both primary recovery and carbon dioxide (CO2) huff ‘n’ puff recovery in a shale matrix typical of the Bakken formation, to investigate the effect of reservoir heterogeneity on hydrocarbon recovery. Nonaqueous components are carefully lumped into seven pseudocomponents. Permeability fields with various heterogeneity and correlation lengths are generated. UT-COMP is able to solve the compositional model, despite the permeability difference between the fracture and matrix being six orders of magnitude. The effects of both primary recovery and CO2 huff ‘n’ puff recovery depend significantly on reservoir heterogeneity. In primary recovery, the recovery factor can be fit by a two-parameter exponential formula; higher heterogeneity reduces the rate coefficient in the formula. Permeability fields with identical or similar heterogeneity have similar rate coefficients, even if the correlation lengths are different, which implies that the recovery depends primarily on heterogeneity and is insensitive to correlation length. Multiple-cycle CO2 huff ‘n’ puff processes are simulated in both homogeneous and heterogeneous reservoirs. Recovery rate in the production stage rises to a peak value much higher than that in the primary recovery, and then declines dramatically. The peak recovery rate decreases with increasing huff ‘n’ puff cycles, resulting from depleted reservoir pressure and hydrocarbons. The final recovery factor in the huff ‘n’ puff recovery is lower than that in the primary recovery, because the incremental recovery in the production stage is unable to compensate the loss in the injection and shut-in stages. Use of a longer shut-in time does not help increase the recovery rate in the production stage, because CO2 migration into the shale matrix is very limited because of the low matrix permeability. Reservoir heterogeneity leads to a faster decline of recovery rate in the production stage.