Coupled multiphase flow and geomechanics model for analysis of joint reactivation during CO2 sequestration operations

摘要

The initial and primary trapping mechanism for long term subsurface sequestration of CO2 is structural trapping beneath a low permeability caprock layer. Maintaining caprock integrity during injection operations is paramount to successful sequestration. Evaluation of jointed/fractured caprock systems is of particular concern to CO2 sequestration because creation of fractures or reactivation of joints can lead to enhanced pathways for leakage. In this work, a joint model is introduced to describe joint reactivation during injection of CO2. The model assumes equally spaced anisotropic joint sets with non-linear normal stiffness and linear shear stiffness. Normal displacement of the joints is mapped into a dynamically evolving effective anisotropic permeability tensor, assuming a cubic law for fracture permeability as a function of joint aperture. A model problem is presented to demonstrate features of the joint model and how it affects the coupled geomechanics and flow during injection of CO2 into deep saline aquifers. The model is used to demonstrate injection reservoir properties and injection rates that have potential for inducing leakage through the caprock due to overpressures associated with CO2 injection. In situations where pore pressure approaches or exceeds lithostatic pressure under a constant-rate, 30 year injection, the model indicates between 16 and 20% of injected CO2 could leak past the primary caprock after 50 years. The model also indicates a concomitant overpressure reduction that could signal caprock leakage during the injection.