5.3.7 EOR Operations

When CO2 is stored as a component of EOR projects, the initial depletion of the reservoir, and all the influences associated with exploitation, production, and EOR processes may affect the integrity of bounding seals. Jimenez et al., 2005 studied the integrity of the Weyburn system under EOR-CO2 storage conditions using a mechanical earth model. A geomechanical analysis of the system was carried out using the pressure information in an explicit coupling where pressures were used as input for a geomechanical model. The large stiffness of the reservoir and the low pressure gradients lead to a minimum distortion of the reservoir and bounding seals, and insignificant changes in the in-situ stresses. Consequently, the hydraulic integrity of the caprock is preserved throughout the pre-CO2 injection history of the reservoir, and the uniformity of caprock deformations has little influence on wellbore hydraulic integrity. In addition, pressures were increased synthetically to assess the performance of the reservoir post-EOR or in the actual CO2 storage phase. It was found that hydraulic fracturing will be due to the mechanism of failure instead of shear failure. Therefore, the authors proposed that in order to maximise the volume of CO2 stored, it will be necessary to adjust the operation of the field to raise the pressure as uniformly as possible and maintain good control of injection pressures.

Vidal-Gilbert et al., 2009 modelled the geomechanical behaviour of an oil field reservoir that is used as a CO2 storage medium. In situ stresses and rock mechanical properties were determined using a 3D model. The pressure data obtained from reservoir simulations were integrated as input for a geomechanical model. Hence, the coupling is called a one-way coupling. The stress changes extracted from the model were also combined with a Mohr-Coulomb analysis to determine the fault slip tendency.