Khan Hasan Javed, Al-Abdrabalnabi Ridha, Al-Jawad Murtada Saleh
Department of Petroleum Engineering, King Fahd University of Petroleum and Minerals, Dhahran, Eastern Province, Saudi Arabia 34464.
ACS Omega. 2023 May 16;8(21):18626-18636. doi: 10.1021/acsomega.3c00543. eCollection 2023 May 30.
During hydraulic fracturing, the oxic hydraulic fracturing fluid physically and chemically alters the fracture surface and creates a "reaction-altered zone". Recent work has shown that most of the physicochemical changes occur on the shale fracture surface, and the depth of reaction penetration is small over the course of shut-in time. In this work, we investigate the physicochemical evolution of a calcite-rich fracture surface during acidized brine injection in the presence of applied compressive stress. A calcite-rich Wolfcamp shale sample is selected, and a smooth fracture is generated. An acidized equilibrated brine is then injected for 16 h, and the pressure change is measured. A series of experimental measurements are done before and after the flood to note the change in physicochemical properties of the fracture. High resolution computed tomography scanning is conducted to observe the fracture aperture growth, which shows an increase of ∼8.3 μm during the course of injection. The fracture topography, observed using a surface roughness analyzer, is shown to be smoother after the injection. The calcite dissolution signature, i.e., surface stripping of calcite, is observed by X-ray fluorescence, and mass spectrometry of the timer-series of the effluent also points in the same direction. We conclude that mineral dissolution is the primary mechanism through which the fracture aperture is growing. The weakening of the fracture surface, along with the applied compressive stresses, promotes erosion of the surface generating fines which reduce the fracture conductivity during the course of injection. In this work, we also highlight the importance of rock mineralogy on the fracture evolution mechanism and determine the thickness of the "reaction altered" zone.
在水力压裂过程中,含氧水力压裂液会对裂缝表面进行物理和化学改变,形成一个“反应改变区”。最近的研究表明,大多数物理化学变化发生在页岩裂缝表面,并且在关井期间反应渗透深度较小。在这项工作中,我们研究了在施加压缩应力的情况下,注入酸化盐水时富含方解石的裂缝表面的物理化学演化。选择了一个富含方解石的沃尔夫坎普页岩样本,并生成了一个光滑的裂缝。然后注入酸化平衡盐水16小时,并测量压力变化。在注水前后进行了一系列实验测量,以记录裂缝物理化学性质的变化。进行了高分辨率计算机断层扫描以观察裂缝孔径的增长,结果表明在注入过程中孔径增加了约8.3μm。使用表面粗糙度分析仪观察到的裂缝形貌在注入后变得更光滑。通过X射线荧光观察到方解石溶解特征,即方解石表面剥离,流出物的时间序列质谱分析也指向相同方向。我们得出结论,矿物溶解是裂缝孔径增长的主要机制。裂缝表面的弱化以及施加的压缩应力,促进了表面侵蚀,产生细颗粒,从而在注入过程中降低了裂缝导流能力。在这项工作中,我们还强调了岩石矿物学对裂缝演化机制的重要性,并确定了“反应改变”区的厚度。