Rock Physics Analysis and Time-lapse Rock Imaging of Geochemical Effects Due to CO2 Injection into Reservoir Rocks

Abstract

Time-lapse seismic monitoring is a widely used method for studying time-evolving processes of the Earth’s subsurface in a wide range of applications (i.e., hydrocarbon production, fluid-migration in tectonically active regions, and environmental engineering). The pivotal concept of this method is that changes in seismic velocity and density depend on the variation of the properties of the rock frame and the fluid permeating it in response to changes of physical parameters such saturation, pore fluid pressure, temperature, and stress. Increasingly, however, time-lapse seismic monitoring is called upon to measure and quantify subsurface changes caused by geochemical processes due to the injection of fluids unlikely to be in chemical equilibrium with the host rocks (i.e., surfactants, CO2, bacterial growth promoters, chemical oxidants). This paper springs from a series of laboratory experiments and high-resolution images monitoring changes in microstructure, transport, and seismic properties of rock samples when injected with CO2. Results show that CO2 injection into a brine-rock system induces precipitation and dissolution in the host rock, which permanently change the rock frame. These alterations change the baseline transport and elastic properties of the rock frame and thus the input parameters of rock physics models and reservoir simulators.