DEVELOPMENT OF EFFICIENTLY COUPLED FLUID FLOW AND GEOMECHANICS MODEL FOR HIGHLY FRACTURED RESERVOIRS

Abstract

Number of unconventional developments have increased greatly in the recent years to meet the global demand on hydrocarbon usages. Completion work can be very challenging due to complex characteristic of unconventional reservoir, which directly affects production performance. A rapid decline in parent well production has recently been observed in many unconventional developments, which subsequently increases the number of infill wells. Hydraulic fractures created from infill wells tend to propagate towards the parent well as a result of reservoir depletion. The interference between parent and infill well fractures due to a tight spacing is the main cause of poor production performance in both parent and infill wells. Stress change can be observed as the reservoir depletes due to the poroelastic effect. This leads to complex fracture geometry created during infill well completion, which is difficult to predict and usually causes negative impact on well production. Therefore, it is important to be able to predict depletion-induced stress change in the reservoirs with complex fracture geometries. The prediction of fracture interference is sometimes not accurate compared to the field observation as most studies mainly focus on stress evolution in planar fracture geometries since it is difficult to model complex fracture geometries. Unstructured grids have been implemented to handle such problems, but it usually comes with high computational cost and less computational efficiency, which is not a good option when simulating a field-scaled reservoir. This has become the main motivation of this work, which is to develop a coupled geomechanics and fluid flow model to characterize stress evolution due to reservoir depletion in highly fractured reservoirs with high computational efficiency. In this dissertation, I have developed a coupled geomechanics and fluid flow using a well-known sequentially coupled method called fixed stress-split to capture stress change in both magnitude and orientation during reservoir depletion. The coupling method was selected to ensure stability of the simulation while maintaining low computational cost. Embedded Discrete Fracture Model (EDFM) was coupled with the model to gain capability in simulating complex fracture geometries using structured gridding system. This significantly improves computational efficiency as well as opens the possibility of exploring cases with complex fracture network. The simulator was developed based on an open-source code called Open source Field Operation And Maniputation (OpenFOAM), allowing the simulation to be conducted in full 3D without significantly impacting computational cost. The developed model was used to predict refracturing performance in a highly fractured reservoir as well as infill well completion in a multi-payzone reservoir. In addition, the model was coupled with complex fracture propagation model to study how heterogeneous stress state affects fracture geometry created during infill well treatment, which can greatly help predict fracture interaction and maintain production performance. Two-phase flow was also implemented to the model for some field case studies such as water injection. The results observed in this study suggest that fracture geometry is a main factor that affects stress change in magnitude and orientation. The presence of natural fractures and fracture spacing plays an important role in refracturing performance in highly fractured reservoirs. Critical time can be used to determine when the refracturing should be performed to ensure the successful results and obtain optimum refracturing locations. For the infill well completion in a reservoir with multiple pay zones, it is suggested that both parent wells should be placed in different layers to mitigate stress change in the infill zone. Fracture penetration effect should also be considered as it accelerates stress reorientation in the infill zone. Severe asymmetrical fracture geometries with the longer side being closer to the depleted zone can be observed in the infill well with short spacing when coupling fracture propagation model with the reservoir-geomechanics model. These results are crucial and can be a guideline for field operation in reservoirs with complex fracture network.