Numerical Simulation of Fluid Induced Fracturing: Micro-seismicity and Fluid Type Dependency

To monitor the integrity of the storage formation in relation to CO2 injection, several methods and technologies will be employed, including microseismic monitoring. Injection of CO2 leads to increased pore-pressure which in turn can activate existing weaknesses, e.g. faults, at the risk of leakage. Presented is a hydro-mechanical model that describes single-phase fluid flow in a deforming porous media with an existing weakness that can fracture. In the intact porous media, the fluid flow is governed by Darcy's law and the geomechanical behavior is described by Biot's theory of linear poroelasticity. The weakness zone is defined as a zero-thickness domain and the governing equations are obtained by integrating the governing equations of the intact porous media over the virtual thickness of the weakness. This makes possible the seamless transition of an intact porous media formulation to an open fracture formulation as the weakness zone activates, deteriorates and eventually ruptures. The deterioration of the weakness zone is described using the Cohesive Zone Model (CZM). The hydro-mechanical model is used to study the impact of different fluid types on activation and fracturing of a weakness, in particular mode-I, tensile failure, and potential implications for micro-seismicity. Firstly, it was confirmed that fluid type matters, e.g. under the same pressurization, different fluids (different mobility and compressibility) propagates a fracture at different velocities. Additionally, three hypotheses from observations in the literature was tested. One hypothesis was confirmed, as simulation results show that for a high-viscosity fluid, a strong pore pressure gradient occurs ahead of the fracture tip, in the propagation direction, thus driving the fluid flow and the propagation of the fracture, favoring longer fractures. However, for the other two hypotheses, the hydro-mechanical model could neither confirm that low-viscosity fluids will tend to produce smaller but more abundant fractures (as reported in the literature), nor reproduce observations of lower breakdown- and initiation pressure of rocks for low-viscosity fluids. The reason for this discrepancy could be differences in the premises of the observation and the case studied here, or due to other factors not considered in this study, e.g. chemical effect, differences in wetting properties of the rock, or that super critical CO2 is not representative for typical low-viscosity fluids.