Assessment of Cold Water Injection Geomechanical Effects on Brazilian Pre-Salt Reservoirs and on Its Saline Caprocks

Abstract

AbstractCold water injection creates a cooled rock region around the well that can reduce the horizontal stresses acting on the reservoir and, also, the injection pressure to initiate and propagate a fracture. However, the impact of thermal stresses in saline caprocks, where creeping occurs, is not well understood yet. This study aims at modeling the cooled region created by cold water injection and evaluate the thermally induced stresses in both reservoir and saline caprock.In order to assess the evolution of the pressure and temperature in the reservoir and in the caprock during 16 years of cold water injection, reservoir simulations considering thermal effects were carried out. Various simulations were performed using different water injection rates and different thermal diffusivities for the reservoir and for the caprock. The model used in these thermal simulations comprehended not only the pre-salt carbonate reservoir, but also the salt rocks. Stress and strain were evaluated using a 3D finite element in a one-way coupling scheme, including the reservoir, overburden (salt and post-salt), sideburden and underburden.In the worst simulated case, after 16 years of cold water injection, the height of the cooled region in the salt was about 200 m above the top of the reservoir. As expected, horizontal stresses in the reservoir significantly decreased with cooling. On the other hand, horizontal stresses in the saline caprock did not change significantly, even though the thermal expansion coefficient of salt rocks is five times greater than that of carbonates. This result suggests that creeping in the salt relieves thermal stress induced by cooling. Thus, saline caprock integrity is not influenced by cold fluid injection in the reservoir, as long as salt creeping can be considered.Although cold fluid injection may cool the salt, stress state in the caprock remains unaltered. Thermal stress is relieved by the salt creeping at the expense of plastic deformations. Thus, the saline caprock integrity is not impacted by cold fluid injection.