An introduction to reservoir geomechanics

Production of hydrocarbons leads to changes in reservoir pore pressure, resulting in changes in the stress acting on the reservoir and surrounding rocks. A decrease in pore pressure due to depletion leads to an increase in stress carried by the load-bearing rock frame of the reservoir, and may be accompanied by microscale deformation mechanisms such as cement breakage at grain contacts, grain sliding and rotation, Hertzian cracking at point contacts, plastic deformation of clay and mica grains, and opening and closing of microcracks (Schutjens et al., 2004). The increasing stress acting on the rock framework may also lead to compaction of the reservoir, and may result in problems such as surface subsidence, casing deformation, reactivation of faults or bedding-parallel slip. Evidence for stress changes in and around reservoirs undergoing depletion is provided by seismic events resulting from production (Segall, 1989; Grasso, 1992), and by time shifts observed using time-lapse seismic (Hatchell et al., 2003). This paper provides a brief introduction to reservoir geomechanics and examines the implications for time-lapse seismic, which has the potential to become a useful tool for monitoring the stress changes that take place during hydrocarbon production.