Fault reactivation during CO2 sequestration: Effects of well orientation on seismicity and leakage

Injection or withdrawal of fluid at depth may trigger felt seismicity. Such human-induced seismicity is a key environmental concern related to the exploitation of natural underground resources. Thus, understanding how to avoid triggering felt earthquakes plays a crucial role in the success of underground anthropogenic activities, such as CO2 geological storage. In this work, we conduct 3D simulations of injection-triggered fault reactivation, in order to investigate the effects of well geometry on seismic rupture and CO2 leakage. We analyze two different cases of injection, through (1) a vertical and (2) a horizontal well. Simulation results for the vertical well show the fault pressurizing faster and more locally than for the horizontal well, resulting in a smaller seismic event. For the horizontal well, the pressure is distributed over a wider area along the fault, which requires a longer time to reactivate, but results in a larger event. Fault reactivation also produces changes in damage-zone and fault-core permeability, allowing the CO2 to leak from the injection zone through overlying caprock, toward shallower depths. Although the calculated fault permeability enhancement is similar for the two cases, results show a slightly higher leakage rate for the vertical well in the region close to the well itself, while the leakage resulting from injection through the horizontal well is more widely distributed.Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Thermal and capillary effects on the caprock mechanical stability at In Salah, Algeria

Victor Vilarrasa Riano

Thermo-mechanical effects are important in geologic carbon storage because CO2 will generally reach the storage formation colder than the rock, inducing thermal stresses. Capillary functions, i.e., retention and relative permeability curves, control the CO2 plume shape, which may affect overpressure and thus, caprock stability. To analyze these thermal and capillary effects, we numerically solve non-isothermal injection of CO2 in deformable porous media considering the In Salah, Algeria, CO2 storage site. We find that changes in the capillary functions have a negligible effect on overpressure and thus, caprock stability is not affected by capillary effects. However, we show that for the strike slip stress regime prevalent at In Salah, stability decreases in the lowest parts of the caprock during injection due to cooling-induced thermal stresses. Simulations show that shear slip along pre-existing fractures may take place in the cooled region, whereas tensile failure is less likely to occur. Indeed, only the injection zone and the lowest tens of meters of the 900-m-thick caprock at In Salah might be affected by cooling effects, which would thus not jeopardize the overall sealing capacity of the caprock. Furthermore, faults are likely to remain stable far away from the injection well because outside the cooled region the injection-induced stress changes are not sufficient to exceed the anticipated shear strength of minor faults. Nevertheless, we recommend that thermal effects should be considered in the site characterization and injection design of future CO2 injection sites to assess caprock stability and guarantee a permanent CO2 storage. (C) 2015 Society of Chemical Industry and John Wiley & Sons, Ltd

Wiley Periodicals, Inc2015

Impact of thermally induced stresses on fracture stability during geological storage of CO2

Lyesse Laloui, Victor Vilarrasa Riano

Geomechanical stability issues may arise due to induced thermal stresses because CO2 will generally reach the storage formation at a temperature lower than that of the reservoir. Cold injection will form a cold region around the injection well, which will induce thermal stress reduction. We simulate cold CO2 injection in deep saline formations in a normal faulting stress regime and investigate under what conditions thermal stresses may jeopardize the caprock sealing capacity by studying the effect of the heterogeneity of the thermal expansion coefficient between the reservoir and caprock. Furthermore, we use an elastoplastic constitutive model to account for inelastic deformation related to fracture instability. We find that the temperature difference should be limited in the presence of very stiff reservoirs, because the thermal stress reduction is proportional to the product of the rock stiffness, the temperature difference and the thermal expansion coefficient. Simulation results show that inelastic strain occurs in the cooled region within the reservoir, but fracture instability does not propagate into the caprock in the considered normal faulting stress regime. However, the cooled region of the lower portion of the caprock may experience yielding if the thermal expansion coefficient of the caprock is larger than that of the reservoir, because the thermal stress reduction in the caprock becomes larger than in the reservoir, which increases the deviatoric stress. Nevertheless, irreversible strain caused by cooling within the caprock are limited to a small region of the lower portion of the caprock and thus, the overall sealing capacity of the caprock is not compromised, so CO2 leakage is unlikely to occur because of cooling. (C) The Authors. Published by Elsevier Ltd.

Elsevier2016

Impacts of Thermally Induced Stresses on Fracture Stability During Geological Storage of CO2

Lyesse Laloui, Victor Vilarrasa Riano

Elsevier2017