Time-lapse Acoustic Monitoring of Fracture Alteration in Marcellus Shale

During hydraulic fracturing of unconventional reservoirs, injected fluids and proppants induce a series of chemo-mechanical alterations that impact fluid flow. There is a wide range of alteration types from geochemical reactions to proppant emplacement, which requires understanding how various alteration processes and possible feedback upon each other impact fluid flow within the stimulated rock volume. In this study, we conducted a time-lapse acoustic monitoring of fracture propping and acidizing on clay-rich (carbonate-poor) Marcellus shales. We simulated fracture propping by filling an artificial fracture with sand proppant in one sample and geochemical alteration by treating an artificial fracture with 15 v.% hydrochloric acid at 7.8 MPa pressure and 80 °C for three weeks in another sample. Acoustic P- and S-wave velocities were measured before and after fracture treatment. In the fracture acidizing experiment, microstructure and fracture permeability were also measured. Our experimental results show that proppant emplacement leads to an average of 3% decrease in S_∥-wave (polarized parallel to fracture) velocity, and fracture acidizing results in an average of 2% decrease in S_⊥-wave (polarized perpendicular to fracture) velocity. Geochemical alteration in fracture acidizing creates an altered zone that reduces fracture stiffness and permeability. The increase in fracture compliance implies that coupling of fracture acidizing and propping could cause proppant embedment issues. The changes in S-wave velocities show potential for their use in monitoring fracture presence in the stimulated rock volume. This work forms the basis for future analysis of the combined effects of geochemical fracture alteration and proppant emplacement on acoustic and transport properties.