ABSTRACT

Monitoring thermo-chemo-mechanical processes geophysically — e.g., fluid disposal or storage, thermal and chemical stimulation of reservoirs, or natural fluids simply entering a new system — raises numerous concerns because of the likelihood of fluid-rock chemical interactions and our limited ability to decipher the geophysical signature of coupled processes. One of the missing links is understanding the evolution of seismic properties together with reactive transport because rock properties evolve as a result of chemical reactions and vice versa. Capturing this coupling experimentally is one of the missing elements in the existing literature. This paper describes recent advances in rock-physics experiments to understand the effects of dissolution-induced compaction on acoustic velocity, porosity, and permeability. This paper has a dual aim: understanding the mechanisms underlying permanent modifications to the rock microstructure and providing a richer set of experimental information to inform the formulation of new simulations and rock modeling. Data observation included time-lapse experiments and imaging tracking transport and elastic properties, the rock microstructure, and the pH and chemical composition of the fluid permeating the rock. Results show that the removal of high surface area, mineral phases such as microcrystalline calcite and clay appears to be mostly responsible for dissolution-induced compaction. Nevertheless, it was the original rock microstructure and its response to stress that ultimately defined how solution-transfer and rock compaction feed back upon each other. The change in pore volume to the applied stress, the permeability characterizing the formation, and the reactive transport of phases characterized by a high surface area were strongly coupled during injection, controlling how velocity evolved. In less stiff rocks, rock-fluid interactions led to grain-slip-driven compaction and a consequent decrease in velocity. In tight and stiff rocks, rock-fluid interactions led to minimal compaction, a larger increase in permeability, and crack opening. Nevertheless, the change in velocity of these tight rocks was almost negligible.

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