Fluid-shale-proppant interactions and the degradation of hydraulic fracture conductivity in the Niobrara Formation

Abstract

Abstract Conductivity is a major design goal in hydraulic fractures since it is a measurement of the fracture’s flow capacity. Placing the proper proppant type with the right fluid type is critical due to the different interactions taking place within the formation and proppant agent. However, fluid and proppant selection is usually based on economical and practical considerations. The majority of the research studies investigate the rock-fluid interactions related to clays, even though most shale plays don’t have high clay content. This study aims to understand the fluid, rock, and proppant chemical and mechanical interactions taking place in the Niobrara shale, a calcium carbonate rich formation, during hydraulic fracturing treatments. For this, coupled fluid chemical interactions, static and dynamic mechanical properties, and fracture conductivity of the Niobrara shale formation were investigated under reservoir stress and atmospheric conditions. Abstract The Niobrara samples used in this study were characterized using X-ray Diffraction (XRD), and X-ray Fluorescence (XRF), Field-Emission Scanning Electron Microscopy (FE-SEM), and Helical Computed Tomography Scan (CT-Scan). Three sets of experiments were conducted: (1) fluid chemical interactions with crushed rock and proppant, (2) geomechanical variations in intact core samples, and (3) chemical and mechanical monitoring under triaxial stress conditions. By studying these elements both individually and in combination, it is possible to understand the impact of fluid selection on geochemical and geomechanical changes in the rock. For the first set of experiments the associated changes in the composition of the solution were monitored as a function of time. For the second one, the variations of dynamic and static mechanical properties were examined in intact core plugs before and after they were saturated with different fluids. The last set of experiments was performed on a fractured and propped core under triaxial stress conditions. The special design implemented allows for the simultaneous acquisition of coupled stress, strain, ultrasonic compressional and shear wave velocities, flow data and fluid. Abstract The results from this study show that each fluid interacts with the elements present in the rock in a different way. Some fluids dilute elements related to the strength of the rock, while others dilute elements related to undesired precipitation or organic matter. Comparison of dynamic and static elastic moduli and fluid chemistry data pre- and post- treatment indicate that there is a correlation between the formation softening and the chemical interactions taking place between the fluids used in this study and the Niobrara formation. Moreover, conductivity damage effects vary not only with mineralogy of the rock, but also with the mineral distribution and where the fracture is located within the formation. Abstract Effective well performance in tight reservoirs can be accomplished through the integration of multidisciplinary data collection. This study integrates detailed geomechanical and geochemical analyses to capture the associated changes in the rock, proppant, and fluid when they interact with each other. A correlation between the mineralogy and mechanical properties of the core is introduced and a method to improve fluid selection in hydraulic fracturing operations is proposed.