Phase field method to model mixed-mode fracturing in fluid saturated porous reservoir

                                        Hydraulic fracturing is a useful stimulation technique to create cracks in unconventional reservoirs and enhance the effective fluid transmissivity to recover gas from natural gas reservoirs or heat from geothermal reservoirs. However, due to fracturing the overall strength and load bearing capacity of the reservoir is compromised. This may be a serious concern if the reservoir is below a dam or any other massive structures. In such case significant settlement can take place as the result of mixed mode fracturing inside the reservoir which might already have natural fractures. Phase field method based on the formulation of mixed-mode fracturing has been adopted in the present work for modeling fracture propagation in a saturated porous medium when subjected to fluid pressure and increasing overburden load. A numerical method has been developed using Finite element method (FEM) for solving the displacement and damage field, and Finite volume method (FVM) for solving flow field due to its flux conservative nature which is automatically satisfied for each FVM cell. Our FEM code alone has been first validated for modelling mixed mode fracturing considering a single fracture as a notch against the published experimental and numerical results for elastic medium subjected to compressive load. In this method, the notch does not have any material and computational mesh is refined around the notch as commonly done by others. We have later developed an alternative method where the pre-existing crack is modelled as a fully damaged zone. In this method, a structured and uniform grid can be used to obtain same fracturing pattern and load-deflection curve. The alternative method has a few advantages such as it can be easily applied for reservoir with many cracks without any grid refinement around the pre-existing cracks, and it can be easily coupled with FVM code for modeling fluid flow. Our numerical modelling code is capable to simulate fracturing along with the branching and merging effects. We have simulated load-bearing capacity of a fractured reservoir subjected to increasing overburden load. The reservoir is considered to consists of many randomly oriented but poorly connected natural fractures. The load-bearing capacity and load-deflection curves are compared for reservoirs with and without hydraulic fracturing. The simulations have been performed for different set of natural fractures to understand the effect of fracture density and other fracture network properties on the load-deflection curves.