Linking the Chemical and Physical Effects of CO2 Injection to Geophysical Parameters

This project aims to demonstrate techniques for quantitatively predicting the combined seismic signatures of CO2 saturation, chemical changes to the rock frame, and pore pressure. This will be accomplished (i) by providing a better understanding the reaction kinetics of CO2-bearing reactive fluids with rock-forming minerals, and (ii) by quantifying how the resulting long-term, CO2-injection-induced changes to the rock pore space and frame (e.g. porosity, permeability, mineral dissolution, and cementation) affect seismic parameters in the reservoir. This research involves laboratory, theoretical, and computational tasks in the fields of both Rock Physics and Geochemistry. Samples have been selected based on mineralogy (carbonates, sandstones, and calcite-cemented sandstones), porosity, pore and cement type. Ultrasonic Pand S-wave velocities are being measured over a range of confining pressures while injecting CO2 and brine into the samples. Pore fluid pressure will be varied and monitored together with porosity during injection. The measurement of rock physics properties will be integrated and complemented by those obtained via geochemical experiments to link the physical (e.g. porosity enhancement, selective dissolution and change in microstructure) and chemical processes (e.g. reaction type and dissolution rates) underlying the mechanisms triggered by CO2 injection. We will also develop computational and analysis tools needed to simulate multi-fluid flow at the pore scale while including dissolution effects. We previously reported laboratory results on carbonate samples showing that both P and Swave (elastic) velocities under dry and saturated conditions decrease over time, due to dissolution during injection of CO2-bearing brine. However, for similar injected volumes of fluid, the magnitude of these changes differs from one sample to another. It appears that the different responses of the rocks to CO2 injection are related, in part, to their initial microstructures. In this report, we begin by summarizing geochemical results from our team-members at Rice University. A key finding is the loss of the classical assumption regarding a single-valued “mean” rate or rate constant as a function of environmental conditions. Reactivity predictions and reservoir long-term stability calculations based on laboratory measurements are thus not directly applicable to natural settings without a probabilistic approach. A comprehensive stochastic model of dissolution has been developed by applying kinetic Monte Carlo methods that treat the complex dissolution process. The model has been tested with laboratory observations. Another key result is demonstration of the decoupling of surface area and its related density of reactive sites such as steps and kinks, from the overall rate: e.g., as diameter decreases, the diffusion length of mobile surface components will eventually exceed the spacing of step and kink sites, leading to diffusion limitations that are not predicted in rate models parameterized simply by surface area. This success of this approach also suggests a powerful strategy for the simulation of complex reservoir frameworks, in which diverse grains reside in complex contact with one another. We also summarize aspects of numerical simulation of flow in complex pore microstructures, with particular emphasis on microporosity as might be in encountered in carbonate rocks or sandstones with clay. The flow is not only determined by pore connectivity and tortuosity, but also by the absolute size of pores relative to the molecular mean-free path of the pore fluids. The flow is also coupled with the stress-dependent deformation of pore walls and adsorption or chemical reaction of the pore fluid with the mineral phase.