Imbibition of Oxidative Fluid into Organic-Rich Shale: Implication for Oxidizing Stimulation

摘要

A large amount of fracturing fluid enters a well of a shale gas reservoir to create a fracture network, but the recovery of fracturing fluid is generally less than 30%. Fracturing fluid from the hydraulic fractures usually invades the microfractures and matrix by spontaneous imbibition during the shut-in. Recent studies show that the water-rock interaction may induce shale structure failures, which can significantly affect imbibition rate. Due to the presence of oxidizable compositions (e.g,, pyrite and organic matter (OM)), oxidation easily induced the structure failures and dissolution pores. However, its effects on imbibition of water into the shale is poorly understood. In this study, imbibition experiments of deionized water (DI water) and oxidative fluid under no confining pressure conditions were conducted to determine the imbibition characteristics; shale cubes (1 cm X 1 cm X 1 cm) and crushed samples (380-830 mu m) were treated by DI water and oxidative fluid for revelation of the change in the composition and the associated dissolution structures and explanation of the imbibition characteristics of oxidative fluid in shale. The results show that the final amount of oxidative fluid imbibed is higher than that of DI water; oxidation-induced microfractures during the imbibition lead to a "phase step" of the normalized imbibed volume vs time curve and "S" characteristic of the normalized imbibed volume vs square root of time (sqrt time) curve. These differences are mainly caused by the improvement of the imbibition pathway and the increase of water retention space by oxidation. After the oxidation treatment of crushed shale samples for 48 h, lots of oxidation-induced microfractures and dissolution pores were observed by field-emission scanning electron microscopy. Combining the analysis of X-ray diffraction (XRD) and atomic absorption spectroscopy (AAS) found that the dissolution pores seemed to strongly contribute to the loss of calcite, dolomite, and pyrite. Results from mercury injection capillary pressure analysis showed that the oxidative dissolution could lead to a high porosity and good connectivity of nanoscale pores networks in shale cubes. Moreover, oxidative dissolution decreased the barriers of microfracture propagation according to the decrease of zeta potential in the shale-water system and, meanwhile, accelerated the release of clay hydration forces to induce microfractures. The results indicate that the coordinative effect between spontaneous imbibition and oxidative dissolution may play a significant role in increasing the gas supply ability of nanoscale pores and microfractures, thus achieving oxidizing stimulation of shale formation to enhance shale gas recovery.