Reservoir Geomechanics and 4D Seismic Monitoring
Production of hydrocarbons changes reservoir pore pressure and alters the stresses acting on the reservoir and on surrounding rocks (Addis, 1997; Hettema et al., 2000; Hatchell et al., 2003; Sayers, 2005b). Decreased pore pressure resulting from depletion leads to an increase in the effective stress acting on the reservoir and can be accompanied by reservoir compaction, reduced porosity and permeability, casing deformation and failure, and surface subsidence. Other problems caused by production-induced stress changes include fracturing of the formation and the opening and closing of preexisting fractures, reactivation of faults, and bedding-parallel slip. Strong evidence for altered stress in and around reservoirs undergoing depletion is provided by seismic events resulting from production (Segall, 1989; Grasso, 1992) and by time shifts observed by using time-lapse seismic data (Hatchell et al., 2003).
Changes in stress can be particularly severe for unconsolidated formations such as deepwater turbidites. Figure 1 shows the pressure change computed for a deepwater Gulf of Mexico turbidite between 2004 and 2010. Such a pressure change leads to compaction of the reservoir, as shown in Figure 2 for one of the wells in this example. Vertical strain in the formation can lead to buckling of the casing, particularly in situations in which the casing is supported poorly, such as in the presence of a poor cement