Dissolution and deformation in fractured carbonates caused by flow of CO2-rich brine under reservoir conditions

Abstract

Geometrical alteration in fractures, caused by mineral dissolution, changes the contact area between fracture surfaces and affects the mechanical strength of fractures. It is difficult to determine the influence of dissolution on fracture porosity and permeability given the competition between fracture opening due to dissolution and fracture closure caused by mechanical deformation. Therefore, simulating flow in fractured reservoirs during enhanced oil recovery and CO2 sequestration, where local porosity changes may significantly alter permeability, remains a fundamental challenge. Here, we present results from experiments and numerical simulations that explore the influence of coupled geochemical alteration and mechanical deformation on calcium carbonate fracture geometry. We scan the fracture surfaces, before and after the flow experiments, using high-resolution optical profilometry to map changes in aperture fields. Flow of brine equilibrated with CO2 at 60°C and pore pressure of 14MPa leads to significant dissolution in fractured calcium carbonate cores. The evolution of the dissolution process depends, to first order, on the dimensionless Damkohler number Da. We vary Da in experiments by changing the flow rate through fractured cores and observe a transition in dissolution behavior over the range of flow rates from 0.1ml/min to 20ml/min. At large Da (small flow rate), dissolution causes the formation of large-scale channels aligned with the fractures. At small Da (large flow rate), dissolution occurs more uniformly over the fracture surfaces. However, the area of contacting asperities, controlled by the significant mechanical stresses (28MPa) and fracture surface roughness, constrains the spatial extent of the dissolution. We use the measured aperture fields as input to a single-species, reactive-transport model. Simulated Ca2+ dissolution agrees with temporal evolution of measured Ca2+ concentrations in outflow fluid. Comparison of the simulated dissolved aperture fields with measured aperture fields after flow-through shows qualitative agreement. We also use our reactive-transport model to explore the role of changing fluid properties and flow rates on dissolution and the resulting alteration of porosity and permeability beyond our experimental conditions. Our results emphasize the importance of length-scales in the coupled response of chemo-mechanical deformation in fractured reservoirs under varied stress and flow conditions.