Effects of subsurface fracture interactions on surface deformation

Abstract

Although the surface deformation resulting from the opening of a single fracture in a layered elastic half-space resembles the observed deformation at the InSalah site, it seems unlikely that only a single fracture is involved. This raises the question of how interaction among multiple fractures affects surface deformation. Finite element modeling is used to build a 3D model of a reservoir with multiple fractures. The interacting cracks and fractures give this model a more complicated stress state, and so any surface deformation would be different from that of a model with a single fracture. Geodetic monitoring of large-scale CO 2 sequestration provides a potentially powerful and cost-effective tool for interrogating reservoir structure and processes. For example, InSAR observations at the InSalah, Algeria sequestration site have mapped the surface deformation above an active reservoir, and helped delineate the effects of CO2 storage. The impact of interactions on individual fractures and the qualitative changes in the surface displacement and stress fields are considered and the importance of orientation, position and fracture area is investigated. It was found that when the crack locations are biased towards stacked parallel arrangements, then the shielding effect of interactions dominates, meaning that the overall stiffness of a representative volume increases. When collinear interactions dominate then the overall stiffness is reduced. These effects are then used to find a volume average and a continuum description of a solid with effective elastic properties. In this way a volume of fractured rock can be replaced with a representative volume with elastic properties that approximate the interaction effects.