Influencing Factors and Application of Spontaneous Imbibition of Fracturing Fluids in Lacustrine and Marine Shale Gas Reservoir

Abstract

Lacustrine shale is a terrestrial shale typically with high clay mineral content and poor matrix brittleness. The plasticity of the matrix makes it difficult to form complex fracture networks either naturally or through stimulation. Furthermore, strong vertical and lateral heterogeneity results in significant differences in lithology and stress between layers. It is difficult to hydraulic fracture these heterogeneous, clay-rich shales and obtain large stimulation volumes, leading to poor stimulation. Because lacustrine shale has high clay mineral content and high heterogeneity, the effect of spontaneous imbibition on stimulation and the flow back rate is particularly important. This study focuses on the imbibition properties of lacustrine shale fracturing fluid using scanning electron microscopy (SEM) observations, nuclear magnetic resonance (NMR) tests, and linear expansion rate tests to determine the differences in the capacity of spontaneous imbibition between lacustrine and marine shales. The characteristics and influencing factors of spontaneous imbibition of the lacustrine shale are determined, and technical countermeasures to improve the complexity of fracture networks of lacustrine shale reservoirs are suggested. The results show that the main influencing factor is the ability of imbibition and diffusion of fluid. A clay stabilizer should be added to the fracturing fluid to avoid formation damage caused by the hydration and expansion of clay minerals. The addition of cleanup additives is not conducive to the spontaneous imbibition of shale samples. The expanding accelerant fracturing fluid can cause the clay minerals of the lacustrine shale to further expand, making it hard to release the internal expansion stress, thus forming a burst fracture. The fracturing fluids of different formulations should be divided into three slugs and pumped into a lacustrine shale reservoir as the preloading fluid to take full advantage of the different properties of the various fracturing fluids.