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
Figures & Tables
This is a recording of the 2010 Distinguished Instructor Short Course (DISC) by Colin Sayers. The state of stress within the earth has a profound effect on the propagation of seismic and borehole acoustic waves, this leads to many important applications of elastic waves for solving problems in petroleum geomechanics. This course provides an overview of the sensitivity of elastic waves in the earth to the insitu stress, pore pressure, and anisotropy of the rock fabric resulting from the depositional and stress history of the rock, and introduces some of the applications of this sensitivity. The course will provide the basis for applying geophysics and rock physics solutions to geomechanical challenges in exploration, drilling, and production. See catalog #233A for the accompanying DISC book.