Effect of reservoir heterogeneity on improved shale oil recovery by CO ₂ huff-n-puff

An equation-of-state based compositional reservoir simulator, UT-COMP, is used to simulate the improved oil recovery by CO₂ huff-n-puff in a shale matrix typical of the Bakken Formation. Non-aqueous components are carefully lumped into seven pseudo components. 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. Multiple cycles of CO ₂ huff-n-puff are simulated and compared with production by primary depressurization. The oil is first contact miscible with the injected CO ₂ under the reservoir pressure. Simulations show that primary recovery outperforms CO₂ huff-n-puff in an ideally homogenous reservoir because injected CO₂ moves deep into the reservoir without much increase in near-well pressure, while CO₂ huff-n-puff outperforms primary recovery if there exists a low-permeability region which keeps CO₂ in the near-well bore injection region. In the latter case, the final recovery using CO₂ huff-n-puff is higher than that from primary depressurization; the recovery in a single cycle is about 3.3%, which increases to 3.5% in about 3 cycles. The recovery factor after 1000 days by primary depressurization with production pressure of 1000 psi is 11.6%, which is in agreement with existing studies. The recovery increase can be fit by a two-parameter exponential function and the rate coefficient is found to be insensitive to correlation length, while depends mainly on reservoir heterogeneity. A linear relationship between heterogeneity and rate coefficient is obtained. This work is the first to investigate the effect of heterogeneity on improved hydrocarbon recovery by CO₂ huff-n-puff, and will be valuable in understanding the coupling between shale properties and oil recovery.