Thermal Fracturing Simulation for Flow Assurance in CCS

Abstract

Abstract CO2 injection into depleted gas fields causes long-term cooling of the reservoir. Therefore, even if injection pressure stays below the fracture initiation pressure, the cooled volume still creates an extensive stress disturbance that can induce propagation of large fractures over time. The enhanced injectivity after the onset of this thermal fracturing might jeopardize injection operations due to the risk of hydrate plugging in the injection well caused by the combination of low pressure and low temperature, and large fractures may also increase the risk of loss of containment. Modeling the fracture evolution provides an estimate of these effects and their timing. Coupled simulation of CO2 injection provides the thermal fracture dimensions for a given uncertainty in the reservoir parameters. Simplified stress modelling is applied in the thermal fracture reservoir simulation, but a full 3D geomechanical model that was developed for fault slip analysis provides accurate estimates of the stress state after depletion and the subsequent evolution of the stresses during CO2 injection. For computation efficiency, sector models were used with locally refined grids to accommodate fractures in the reservoir simulation model. It was verified that the fracture models match the full-field simulation under matrix flow conditions. The fracture simulations were developed in close relation with flow assurance modeling to determine the operational windows that avoid hydrate formation while maintaining the required injection target. Thermal fracture propagation by CO2 injection into the depleted Dutch offshore gas field has been simulated by using coupled simulation approach. The model has been developed with geomechanical properties and stresses obtained from various sources in neighboring fields. It was found that stress, thermal expansion coefficient, modulus and permeability distribution are the principal parameters that determine fracture growth. The forecast of thermal fracture propagation yielded in some cases very long fractures reaching compartment boundaries. Injectivity was enhanced by up to a factor of 4, which is significant for flow assurance. The coupled modeling of thermal fracturing provides mitigating measures in case the temperature and pressure drop into the hydrate formation window.