The Prévost group develops simulation tools that capture the effects of coupling between fluid flow, thermal and geomechanical effects. Since the inception of CMI, the group’s Dynaflow model, which offers a modular, hierarchical approach for multiphysics simulations, has been adapted to simulate aquifer geochemistry and multiphase flow as well as to predict physical stresses induced by CO2 injection. The model has recently been applied to study CO2 injection impacts at BP’s In Salah facility in Algeria.

 


Predicting length of fractures in caprock

The Dynaflow model has unique capabilities to simulate fluid flow fully coupled with thermal and geomechanical effects, allowing prediction of the stresses induced in the caprock by continuous injection of CO2 in an aquifer. This year, the researchers showed that the temperature of injected CO2 significantly affects these stresses. Particularly when CO2 is injected at temperature 40-50°C below the ambient temperature, the stresses in the caprock become tensile and may overcome the tensile strength, causing fracturing of the caprock.

The initial length of fractures is relatively small. However, the team found that the fractures will propagate driven by the high fluid pressure in the aquifer. In collaboration with Howard Stone, Prévost and colleagues have developed an analytical model capable of predicting the resulting fracture length based on the pressure and stress data calculated by Dynaflow. They estimate the length of a hypothetical fracture at the In Salah site to be of the order of 25 − 35 m within 10 years after fracture initiation (Figure 17). On such a length scale there is a risk that a fracture may connect to a leaky fault and become a pathway for CO2 leakage

Figure 17. Effects of injection temperature on the longitudinal stresses in the caprock. Effective horizontal stresses in the aquifer and caprock around the injection well after 10 years of continuous injection of CO2 at 90°C, 50°C and 40°C. (Ambient initial temperature=50°C; positive values correspond to tensile stresses.)

If a CO2 -brine mixture migrates upward, it reaches regions where the pressure and temperature are lower than that of the critical state. Therefore, the migrating mixture may start boiling. Current work is focused on implementing a flash calculations module for the CO2 -water system. Including such a module in Dynaflow will allow the modeling of the flow of the boiling mixture.