Deformation characteristics of a clayey interbed during fluid injection

Abstract

Surface deformation due to fluid withdrawal has long been observed at the surface above aquifers and oil reservoirs. Uplift associated with fluid injection has also been observed. Although the processes of subsidence and uplift are reversible in a poroelastic setting, the presence of clayey interbeds can result in ground deformation behavior non-reversible because of their low permeability and nonlinear behavior. In this investigation a Cam clay model is used in conjunction with a poroelastic model to simulate the presence of a laterally extensive lens-shaped clayey interbed in a sandstone aquifer during fluid injection. Spatial and temporal changes associated with this interbed during aquifer pressurization are captured and can be clearly differentiated from the deformation of the surrounding poroelastic aquifer formation. Results for pore pressures and ground surface deformation patterns can be categorized into three distinct time intervals: an early time interval where the aquifer is pressurized but not the interbed leading to a lower region above the interbed; an intermediate time interval in which the interbed pressures slowly increase, approaching the pressure of the adjacent aquifer, leading to a potentially large surface uplift above the interbed; and a late time interval in which pressure equilibration is achieved between the interbed and aquifer and a highly non-symmetrical irregular surface deformation pattern results, which may be higher or lower than the background depending on the interbed characteristics. Reservoir configuration is found to be an important factor influencing ground deformation behavior, with more obvious deformation always occurring in a laterally confined aquifer as opposed to a laterally infinite aquifer.