Simulation study of water cut surge after CO[formula omitted] injection in tight oil reservoirs

Abstract

Tight oil contributed to 64% of total US oil production in 2019. However, recovery factors (RF) in tight oil reservoirs are low, typically less than 10% after primary depletion. Based on numerous published studies, gas huff-n-puff emerges as the most promising technique to push the RF beyond 10%. A recent pilot in the Wolfcamp shale confirmed the effectiveness of CO2 huff-n-puff; however, an unexpected water cut surge was also observed during the puff stage. A compositional modeling framework was hence implemented to investigate the reasons as well as the impact of such phenomena. To the best of our knowledge, it is the first time that such abnormal water cut behavior has been modeled for tight oil reservoirs. The fluid PVT and lab-scale model were established and tuned to obtain the critical inputs for the compositional model. A half-stage model of five fractures was then established as a base case, representing a typical completion design in this region. Its results demonstrated an improved oil RF from 7.96% of depletion to 12.16% after six cycles of CO2 huff-n-puff. And the improvement factor as 1.53 matched the published results of gas injection pilots in unconventional reservoirs. Based on the literature review, we found several possible mechanisms behind the water cut surge including underestimation of initial water saturation, interfacial tension (IFT) dependent relative permeability, reactivation of water-bearing layers, and re-opening of unpropped hydraulic fractures. Simulation-based sensitivity studies identified the re-opening of unpropped hydraulic fractures as the most plausible cause. The excessive water production was found to reduce the RF to 11.02% in contrast to a RF of 12.16% of the base case, marking the water management as a vital direction for future research.