Veins in Paleo-reservoir as a Natural Indication of Coupled Changes in Pore Pressure and Stress, Salt Wash Graben of SE Utah, USA

Abstract:

Hydrofracturing associated with elevated fluid pressure coupled with changes in stress has been crucial in enhancing the production and recovery of hydrocarbons. Furthermore, it is also an important issue to access the efficiency and stability of long-term CO₂ geologic storage reservoirs. Veins are mineral-filled extension fractures developed along the plane of σ₁-σ₂ and perpendicular to σ₃, and the fluid pressure must exceed σ₃applied to the plane when the vein opens. Therefore, vein is a well-known natural analogue for fluid migration in a paleo-reservoir. In the Salt Wash Graben of SE Utah, CO₂-charged vein systems hosted in the bleached Entrada Formation are well developed and examined to understand the conditions of fluid pressure and stress during the injections of CO₂-charged fluid. Based on color and relative cross-cutting relationship in the field, veins are subdivided into two sets; sub-vertical black mineral-rich veins and orthogonal calcite veins that have previously been described as ‘grid-lock fractures’. The vein distribution and fluid leakage along through-going fractures in mechanic units allow us to determine the stress regime and driving stress condition through 3D-Mohr circle reconstruction. The results of this statistical analysis for the veins show that the orthogonal veins indicate a ‘stress transition’ with maximum principal stress direction changing from vertical to NNW-SSE sub-horizontal which coincides with the current regional stress regime. The possible causes of the stress transition can be considered. The process of repeated sealing, reactivation and localization of veins within the bleached zone is a natural indication of a coupled change in pore pressure and stress in the reservoir. Thus, an understanding of the effect of stress changes due to the volumetric injection of CO₂ in the subsurface as well as a knowledge of how pre-existing fractures affect fluid flow with respect to elevated pore pressures in layered rocks are important for reservoir characterization and efficient site selection of geologic CO₂ storage in the subsurface.