Dissolution and Deformation of Fractures Lab Experiments and Numerical Simulations
Environmental Engineering Seminar
Civil & Environmental Engineering
University of California, Irvine
Simulating flow in fractured reservoirs during enhanced oil recovery and CO2 sequestration, where local porosity changes may significantly alter permeability, remains a fundamental challenge. Geometrical alteration in fractures during flow, caused by mineral dissolution, may reduce the contact area between fracture surfaces and affect 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 closer caused by mechanical deformation. Additionally, injection of large volumes of CO2 perturbs the target formation from chemical and mechanical equilibrium leading to the possible creation or enhancement of leakage pathways threatening the integrity of competent structural seals that prevent leakage of geologically sequestered carbon.
Here, we present experimental results and numerical simulations aimed at quantifying the role of coupled chemical alteration and mechanical deformation on fluid flow in fractured calcium carbonates (CaCO3) under reservoir conditions. We scan the fracture surfaces, before and after the flow experiments, using a high- resolution optical profilometer to map changes in aperture fields. Flow of brine equilibrated with CO2 at 60 °C at pore pressure of 14 MPa leads to significant dissolution. The dissolution is controlled, to first order, by the dimensionless Damkohler number Da. We vary Da in experiments by changing the flow rate through fractured cores and observe a transition in dissolution from low (0.1 ml/min) to high (20 ml/min) flow rates. At low flow rates, dissolution causes the formation of large-scale channels largely aligned with the fractures. At higher flow rates, dissolution occurs more uniformly over the fracture surfaces. However, the area of contacting asperities, controlled by the significant mechanical stresses (28 MPa) 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. Also comparison of the simulated dissolved aperture fields with measured aperture fields after flow-through shows qualitative agreement. We 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 the coupled response of chemo-mechanical deformation in fractured reservoirs under varied stress and flow conditions.
If time permits, we will present results of a flow experiment in fractured anhydrite (CaSO4) core that investigates the potential for leakage pathways to grow over time under reservoir conditions. At high flow rates, the pressure gradient within the core increased slowly for a period of 4 days followed by a rapid increase in differential pressure corresponding to a two-order-of-magnitude decrease in permeability. During the experiment, the diameter of the core decreased by ~300 μm at the inlet and a skin of gypsum (CaSO4
Dr. Jean Elkhoury is a postdoctoral scholar at Civil and Environmental Engineering at UC Irvine. He received the equivalent of a BS and an MSc in Physics from Universidad Simón Bolívar, Venezuela. After graduation, he worked for a few years in exploration geophysics (field and research). He came to the US for graduate school at UCLA where he received a Ph.D. in Geophysics. Prior to joining the Subsurface Processes Lab at UCI, he was a postdoc in the Seismological Lab at Caltech.