An Experimental Study on Conductivity of Unpropped Fractures in Preserved Shales

Abstract

Summary A great deal of evidence shows that hydraulic fracturing creates a large surface area of induced unpropped (IU) fractures that are too small to accommodate commonly used proppants and that, subsequently, close during production (Sharma and Manchanda 2015). Because of their enormous surface area, IU fractures can play an important role in hydrocarbon production if they can remain open during production. Therefore, the conductivity of these IU fractures under different stress conditions and when exposed to different fracturing fluids is of great importance. In this study, core-scale IU fractures were created with preserved shale samples from the Eagle Ford and Utica formations. Samples with different mineralogies were selected to represent a broad cross section of representative samples. Great care was taken to ensure that the shale samples were preserved because large changes in shale mechanical properties caused by sample desiccation have been observed. The fracture conductivities of unpropped fractures created in each of the shale samples were measured as a function of closure stress by using nitrogen or brine. The unpropped fractures were exposed to several water-based fracturing fluids including neutral brine, alkaline brine (pH 11, 12), and acidic brine (pH < 1), with or without clay stabilizers. The effects of fluid type, pH, clay stabilizers, shale mineralogy, and cyclic stress on IU-fracture conductivities were investigated. Batch tests also were performed to study the change of mechanical properties and fines production caused by fluid-shale interaction. Our results show that unpropped fractures yielded conductivities that were 2 to 4 orders of magnitude lower than those of propped fractures, and were more susceptible to closure stress. Exposure to water-based fracturing fluids decreased the unpropped-fracture conductivity by one order of magnitude. The primary mechanism for the decrease was shale softening caused by exchange of water and ions between the native fluid of shale and the exposed fracturing fluid. Shale softening was observed in exposure to all brines tested, regardless of their pH. In addition to shale softening, fines generation also contributed to the reduction of unpropped-fracture conductivity when shales were exposed to alkaline or acidic brine. Amine-based clay stabilizers were able to control the unpropped-fracture conductivity impairment by reducing the amount of clay-based fines. However, they were not as effective at stabilizing nonclay fines. Shale mineralogy affected the unpropped conductivities in two ways: It controlled the mechanical properties of the native preserved shale, and also affected the fluid-shale interactions. A clear correlation was observed between mineralogy and stress dependence. Clay-rich samples showed the most stress sensitivity in the presence of water or brine at neutral pH, whereas the calcite-rich samples showed less stress sensitivity. High clay content also resulted in lower restored conductivity after cyclic stress. Mechanical properties of shale such as hardness and Young's modulus, before and after fluid exposure, strongly correlated with the mineralogy of shales. Unpropped conductivity was more sensitive to cyclic stress than propped conductivities, and it dropped by 80% after one cycle of closure stress between 300 and 4,000 psi of closure stress. Clearly, it is shown that water-based fracturing fluids can affect conductivities of IU fractures in shales significantly, and these impacts need to be considered in the selection of fracturing fluids.