Technical Note Deformation characteristics of a clayey interbed during fluid injection Xuejun Zhou ⁎, Thomas J. Burbey National Energy Technology Laboratory—Regional University Alliance (NETL-RUA), Department of Geosciences, Virginia Tech, United States abstract article info Article history: Received 1 October 2013 Received in revised form 17 September 2014 Accepted 5 October 2014 Available online 14 October 2014 Keywords: Ground deformation Fluid injection Cam clay model Poroelasticity FEM Abaqus 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 associat- ed with this interbed during aquifer pressurization are captured and can be clearly differentiated from the defor- mation 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 aqui- fer 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. Published by Elsevier B.V. 1. Introduction Land deformation due to fluid withdrawal or injection has long been observed over many oil reservoirs and aquifers systems (Tolman and Poland, 1940; Yerkes and Castle, 1969; Vasco et al., 2001; Bell et al., 2002; Teatini et al., 2011a,b). As the prospect of CO 2 sequestration pro- gresses as an important environmental issue, gaining knowledge about aquifer-system response to waste storage becomes vitally important (Vasco et al., 2010). With the development of sophisticated surface monitoring techniques such as InSAR and GPS, careful monitoring of surface deformation signals during fluid injection can yield critical infor- mation about the geometric configuration and characterization of the host reservoir (Fielding et al., 1998; Vasco, 2004). For historical reasons, the literature associated with land subsidence due to fluid withdrawal is far more abundant than those associated with land uplift due to fluid injection. In a poroelastic hypothetical frame- work, these two processes are opposite to each other and reversible. Exceptions include formations with high clay content because clays often result in non-reversible and nonlinear deformation behavior. These systems are more difficult to evaluate than purely elastic systems because a hydrodynamic (time) lag response typically occurs in systems containing clayey interbeds due to their low permeability. Consequent- ly, their response to an imposed confining stress is dependent on the preconsolidation stress history because their virgin compaction curve and rebounding curve often do not coincide. In this investigation an interbed refers to a relatively thin lens- shaped unit composed of fine-grained clay material with a much lower permeability and relatively higher porosity than the surrounding host aquifer formation, which is composed of sandstone (Figure 1). The hydrodynamic lag has been observed during fluid withdraw- al as the clayey interbeds drain more slowly than the surrounding coarser aquifer material. This leads to a time-delay between the ex- traction of the groundwater and the occurrence of land subsidence (Shearer, 1998). The time lag for consolidation can range widely from several days to many decades or even longer (Bell et al., 1986; Hoffmann et al., 2001; Li et al., 2006; Shi et al, 2007). This hydrodynamic lag effect may also be expected during fluid injection, but it is unclear whether the mechanics of the deforma- tion response will be similar to that of groundwater withdrawal. The goal of this technical note is to investigate the hydrodynamic lag and the surface deformation response caused by clayey inter- beds during fluid injection and to correlate the deformation re- sponse with the pore pressure evolution in both the clayey interbed and the aquifer. Engineering Geology 183 (2014) 185–192 ⁎ Corresponding author at: Mewbourne School of Petroleum & Geological Engineering, The University of Oklahoma, 100 E. Boyd Street, SEC 1210, Norman, OK 73019, United States. E-mail addresses: zhouxj@ou.edu, zhxjun@yahoo.com (X. Zhou). http://dx.doi.org/10.1016/j.enggeo.2014.10.001 0013-7952/Published by Elsevier B.V. Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo