An Experimental Investigation of the Conductivity of Unpropped Fractures in Shales

Abstract

There is a great deal of evidence that shows that hydraulic fracturing creates a large surface area of induced unpropped (IU) fractures, that are too small to accommodate commonly used proppants and subsequently close during production (Sharma and Manchanda 2015). Due to their enormous surface area, IU fractures can play an important role in hydrocarbon production if they are allowed to remain open during production. Therefore, the conductivity of these IU fractures under different conditions of stress 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 shales. 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 since we had observed that shale desiccation results in large changes in mechanical properties. The fracture conductivities of unpropped fractures created in each of the shale samples was measured as a function of closure stress 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 were also performed to study the change of mechanical properties and fines production caused by fluid-shale interaction. Unpropped fractures demonstrated conductivities that were 3 to 4 orders magnitude lower than propped fractures, and were more susceptible to closure stress. Exposure to water-based fracturing fluids decreased the unpropped 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 conductivity when shales were exposed to alkaline or acidic brine. Amine-based clay stabilizers improved the unpropped conductivity by reducing the amount of clay-based fines. However, they were not as effective at stabilizing non-clay fines. Shale mineralogy affected the unpropped conductivities in two ways: it controlled the mechanical properties of the native preserved shale, and also impacted 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 4000 psi of closure stress. It is clearly shown that water-based fracturing fluids are able to impact conductivities of IU fractures in shales significantly, and these impacts need to be taken into account in the selection of fracturing fluids.

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