Effects of Imbibition and Compaction during Well Shut-In on Ultimate Shale Oil Recovery: A Numerical Study

Summary Shale wells are often shut in after hydraulic fracturing is completed. Shut-in often lasts for an extended period in the perceived hope of improving the ultimate oil recovery. However, current literature does not show a strong consensus on whether shut-in will improve ultimate oil recovery. Because of the delayed production, evaluating the benefits of shut-in in improving the ultimate oil recovery is crucial. Otherwise, shut-in would merely delay the production and harm the economic performance. In this paper, we use a numerical flow-geomechanical modeling approach to investigate the effect of imbibition on shut-in potentials to improve the ultimate oil recovery. We propose that imbibition is one of the strongly confounding variables that causes mixed conclusions in the related literature. The investigation methodology involves probabilistic forecasting of three reservoir realization models validated based on the same field production data. Each of the models represents different primary recovery-driving mechanisms, such as imbibition-dominant and compaction-dominant recovery. A parametric study is conducted to explore and identify the specific reservoir conditions in which shut-in tends to improve shale oil recovery. Ten reservoir parameters that affect the imbibition strength are studied under different shut-in durations. Comparison among the three models quantitatively demonstrates that shut-in tends to improve both the ultimate oil recovery and net present value (NPV) only if the shale reservoir demonstrates imbibition-dominant recovery. A correlation among ultimate oil recovery, flowback efficiency, and NPV also shows that there is no strong relationship between flowback efficiency and ultimate oil recovery. This study is one of the first to emphasize the importance of quantifying the imbibition strength and its contribution in helping recover the shale oil for optimum flowback and shale well shut-in design after hydraulic fracturing.