Mitigating water blockage in shale matrix by acidizing or oxidizing treatment: Evidence from transport pathway change and diffusivity enhancement

Abstract

A relatively large number of pH-sensitive minerals (e.g., calcite, dolomite, siderite) and redox-sensitive compositions (e.g., organic matter, siderite, pyrite, and chlorite) are present in gas-bearing shale formations. During hydraulic fracturing, some of these compositions can be dissolved under acidic or oxic condition and may generate critical transport pathways to bypass water blockage within the fracture-matrix interface. This paper reports the effects of 5% HCl or 15% H2O2 reactive fluids on transport pathway and methane diffusion capacity of Longmaxi shale samples. Experiments show that these sensitive minerals are distributed evenly throughout the shale matrix as dispersed or isolated grains. Reactivity differences between sensitive compositions and non-sensitive minerals cause heterogeneous dissolution at micrometer scale and generate substantial new pore space in the reaction zones, giving an overall pore volume increase by 4.6–6.6 vol % calculated using μCT data. Apart from dissolution pores, many oxidation-related microfractures with a spacing less than 10 μm are also observed in H2O2-treated shale. Due to resolution limitations of SEM and μCT imaging, these new transport pathways appearing as separated void spaces are actually connected by much smaller pores at nanoscale, leading to a greater than 3.7-fold increase in macropore diffusivity. Despite more water imbibition into the H2O2-treated shales, the largest increase in diffusivity is also observed post oxidizing treatment. Our study clearly demonstrates that the newly formed transport pathways can reduce the diffusion pathway length and tortuosity of nanopore systems, thereby significantly improving wet shales’ diffusivity. These observed changes in transport pathway and diffusivity indicate that acidizing or oxidizing treatment may be a possible method for mitigating water blockage within fracture-matrix interface of hydraulically fractured Longmaxi shale gas formation.