An integrated approach to investigating the stimulation mechanisms of soaking in shale/tight oil reservoirs

Abstract

To investigate the impact of the soaking strategy on the entire shut-in, flowback, and production processes following hydraulic fracturing, we developed a core simulation system using nuclear magnetic resonance technology to track fluid migration across the entire process. This simulation system, with a specially designed core sample, enabled us to quantify the contribution of each stage to oil production and analyze the stimulation mechanism of soaking from an integrated perspective. Our results demonstrate that forced imbibition during the shut-in stage includes both countercurrent imbibition and cocurrent displacement. Increasing the pressure difference between the fracture and matrix strengthens the displacement effect and weakens the imbibition effect, leading to decreased oil recovery. In contrast, from an integrated perspective, increasing the pressure difference can enhance total oil recovery. This is because two additional stimulation mechanisms of soaking, namely, oil–water redistribution and formation pressure enhancement, can be observed only during the subsequent flowback and production stages and not during the shut-in period. As the pressure difference increases, the main stimulation mechanism of soaking changes from countercurrent imbibition to oil–water redistribution and formation pressure enhancement. Additionally, we observed that the soaking strategy can cause severe water blockage, hindering the full utilization of the formation pressure enhancement mechanism. This study reveals that the stimulation mechanisms in the shut-in stage have a wide-ranging impact that extends beyond this stage. Therefore, adopting an integrated approach is necessary to study the stimulation mechanism of soaking, breaking the traditional view of analyzing a single stage alone.