Geochemical and Geomechanical Alterations Related to Rock-Fluid-Proppant Interactions in the Niobrara Formation

摘要

Fracture conductivity degradation results from damage mechanisms and fluid interactions that occur during and after hydraulic fracturing treatments. Rock softening and associated proppant embedment are among the damage mechanisms affecting this degradation. The aim of this research study is to understand the physico- chemical interactions between fracturing fluid and formation to investigate the associated geomechanical property changes taking place in the Niobrara shale, a calcium carbonate rich formation, during and after hydraulic fracturing treatments. Experimental tests were carried out on Niobrara core samples to investigate the effects of chemical interactions between the formation, fracturing fluid, and proppant along with static and dynamic geomechanical property changes. The samples were characterized using X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), and Field-Emission Scanning Electron Microscopy (FESEM). Two sets of experiments were conducted: fluid chemical interactions with crushed rock and proppant, and geomechanical property variations in intact core samples. In the first set of experiments the changes in the composition of the solution were monitored as a function of saturation time. In the second set, the variations in the dynamic and static mechanical properties were examined in intact core plugs before and after they were saturated. Best practices for hydraulic fracturing fluid selection are established by incorporating the impact of the fracturing fluid on geochemical composition and geomechanical property changes in the formation. This study provides an insight into how each selected fluid formulation yields unique interactions with the mineral composition of the formation. Distilled water shows to be more prone to dissolve elements affecting the strength of the rock, whereas 2% brine solutions, especially 2% KCl, dissolve elements that could be the source of scales, fines migration, other types of undesired precipitations, as well as elements related to organic matter in the reservoir rock. A comparison of the dynamic and static elastic moduli and fluid chemistry data obtained pre- and post- treatment indicate that there is a correlation between softening of the formation and the chemical interactions taking place in the rock. FE-SEM images further support this interpretation. Hydraulic fracture treatment effectiveness in tight reservoirs can be improved by integrating multidisciplinary data. This study provides detailed geomechanical and geochemical analyses capturing associated changes in the rock and the fluid composition when they interact with each other. It also introduces a correlation between mineralogy and the mechanical properties of the rock proposing a simple approach to improve the fluid selection in hydraulic fracturing operations.