Sun Zepeng, Ni Yue, Wu Yuandong, Lei Yong
College of Resource and Environment, Shanxi Agricultural University, Taigu, Jinzhong 030801, China.
Shenzhen Institute, Peking University, Shenzhen 518057, China.
ACS Omega. 2022 Jul 25;7(30):26549-26559. doi: 10.1021/acsomega.2c02690. eCollection 2022 Aug 2.
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 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.
水力压裂与水平钻井相结合被广泛用于开发页岩气资源，大量压裂液被注入页岩储层。然而，压裂后压裂液不可避免地残留在储层岩石中，导致页岩孔隙系统发生改变，进而影响烃类生产效率。在这项工作中，重点研究了两种不同黄铁矿含量的页岩，即富黄铁矿（PR，牛蹄塘组）和贫黄铁矿（PP，下马岭组），以阐明压裂作业后黄铁矿氧化对孔隙结构的影响。采用滑溜水压裂液在100℃和50MPa条件下对页岩样品处理3天。利用场发射扫描电子显微镜（FE-SEM）和X射线衍射（XRD）确定表面形貌和矿物组成。进行低温N吸附以量化孔隙结构。结果表明，滑溜水处理后，黄铁矿氧化导致富黄铁矿页岩中的黄铁矿和方解石溶解，并产生许多溶解孔隙。矿物溶解导致中孔数量增加，总比表面积（TSSA）和总孔隙体积（TPV）增大，孔隙结构复杂性增强。另一方面，贫黄铁矿页岩在滑溜水处理后仅发生粘土膨胀。其孔隙表面粗糙度和孔隙结构复杂性随着纳米孔的损失以及TSSA和TPV的减小而降低。本研究结果加深了对黄铁矿氧化对孔隙结构影响的理解，并为基于页岩矿物组成特征优化压裂作业条件提供了新的见解。
