Impact of Pyrite Oxidation on the Pore-Structure Characteristics of Shale Reservoir Rocks under the Interaction of Fracturing Fluid

Abstract

Hydraulic fracturing combined with horizontal drilling is widely used to develop shale gas resources, and huge amounts of fracturing fluid are injected into shale reservoirs. However, the fracturing fluid is ineluctably retained in reservoir rocks after fracturing, resulting in the alteration of shale pore systems and further affecting the hydrocarbons production efficiency. In this work, two types of shales with different pyrite contents, namely, pyrite rich (PR, Niutitang Formation) and pyrite poor (PP, Xiamaling Formation), were emphasized to illustrate the effect of pyrite oxidation on pore structure after fracturing operation. Slickwater fracturing fluid was used to treat the shale samples for a period of 3 days, under the condition of 100 °C and 50 MPa. The field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) were utilized to determine the surface morphology and mineral composition. The low-temperature N 2 adsorption was performed to quantify the pore structure. The results showed that the pyrite oxidation induced the dissolution of both the pyrite and calcite and generated many dissolution pores for the pyrite-rich shale after slickwater treatment. The mineral dissolution led to an increase in the number of mesopores, enlarged the total specific surface area (TSSA) and total pore volume (TPV), and strengthened the pore-structure complexity. On the other hand, the pyrite-poor shale only experienced clay swelling after slickwater treatment. Its pore surface roughness and pore-structure complexity degraded with the loss of nanopores and the reductions in TSSA and TPV. The results of this study enhance the understanding of the impact of pyrite oxidation on the pore structure and provide new insight into the optimization of fracturing operation conditions based on shale’s mineral composition characteristics.

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“…Surface karstification brings iron ions into the dissolved pores, caves and fractures of the reservoir, and combines with the sulfur ions produced by the dissolution of gypsum to form pyrite. [8] Therefore, high-quality karst reservoirs in this area are generally filled with pyrite crystals, with a content of about 1%. The larger the crack and hole, the higher the degree of endomorphism of pyrite crystal and the higher the content.…”