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Petrophysical Signatures of Fluid Injection in Mudstones and Shales

Abstract

Over the past decade, fluid injection activity has increased substantially for the purposes of wastewater management, hydraulic fracturing and carbon capture and storage. Anthropogenic processes of fluid injection decrease the effective stress on rock by increasing pore pressures, in a process known as unloading. The fate of fluid injected at high pressures, evolution of pore structure in rock, and fluid migration pathways upon induced unloading remain largely unknown. We have developed analogs for three possible pore structure evolution pathways in mudstones upon fluid injection: dilation, micro-fracture network growth, and fracture propagation. Permeability of the modeled rock is calculated at different stages of fluid injection constrained by the developed fluid accommodation mechanisms using Lattice Boltzmann simulations. Numerical analogs are correlated with laboratory measurements of permeability on mudstone samples under stress as they are subjected to controlled fluid injection. Compressional wave velocity (Vp), shear wave velocity (Vs) and porosity are measured, along with permeability, at varying effective stress conditions. Initial results show the evolution of the dominating fluid accommodation mechanism from dilation in early injection stages, to microfracture growth at higher pore pressures, and finally to fracture generation as pore pressure nears fracture. Evolution of elastic properties of mudstones with fluid injection (Mavko et. al, 2009) and induced crack density (Schunbel et. al, 2003) are also calculated from acoustic velocity signatures. Nuclear Magnetic Resonance (NMR) measurements of relaxation times (T2) from mudstone samples have been planned to image pore structure with greater confidence. Linking laboratory measurements of velocities (Vp and Vs), permeability and NMR with numerical models will help us identify the pathways of pore structure evolution during unloading, how they are influenced by rate of injection, and the implications they have for fluid transport and rock failure. This study will help improve understanding of fate of injected water, evolution of pore and bulk properties of mudstones upon fluid injection and contribute to optimizing hydraulic fracturing techniques.