Geological Feature Aligned Unstructured PEBI Grids and Improved Fracture Modelling

Abstract Objective/scope Method to generate geological objects aligned unstructured PEBI grids is presented. Grid sensitivity analysis of the fractured reservoir is performed using unstructured PEBI grid where the fractures are modelled by the embedded discrete fracture model (EDFM). The PEBI grids generated are aligned with geological objects and are locally refined with smooth transition. Workflow and development in a massively parallel reservoir simulator are given. Further, comparative performance of different fracture modelling techniques is assessed. Methods, procedures, process In this work, unstructured grids are generated to honour discrete fractures with respect to control-points (centroids of control-volumes) instead of conventional discrete fracture model (DFM) where control-volumes are boundary aligned with fractures. The grids are also locally refined with smooth transition. Furthermore, fractures are modelled by the embedded discrete fracture model (EDFM) to account for the fracture and matrix flux transfer for flow simulation. Using EDFM on PEBI control point aligned and locally refined grids with smooth transition outperform the accuracy of conventional EDFM approach. The proposed PEBI grid generation and fracture modelling has been implemented within the framework of a massively parallel reservoir simulator and tested on large-scale models. Results, observations, conclusions The capabilities of PEBI grid generation technique where the centres of the PEBI cells are aligned with the geological objects (fracture planes) are demonstrated. We simulate fractured reservoirs using the PEBI grid and embedded discrete fracture modelling approach. The comparative performance of EDFM is assessed using control-points aligned PEBI-grids versus conventional non-control point aligned PEBI-grids. The result show that PEBI grid capture better flow resolution and pressure effects for the fractured reservoir than the conventional grids. Numerical results are also compared with discrete fracture modelling (DFM) approach which requires more involved control-volume aligned PEBI grids. The EDFM results on control-points aligned PEBI grids are comparable to DFM with improved accuracy. We also generate various grids for simulations with different level of refinement of PEBI around the fractures highlighting the capability of PEBI grid generation tool. The simulation results show that pressure transient effects because of fracture are better resolved by the refinement around the fractures. The refinement with smooth transition is especially useful for the highly conductive fractures and/or hydraulic fractures. Novel/additive information The novelty of the work lies in the development of control-point fracture aligned PEBI grid generation for the reservoirs with fractures simulated with embedded discrete fracture model (EDFM) is detailed. Use of control-point fracture aligned PEBI grids combined with EDFM enables to achieve accuracy comparable to DFM and does not require PEBI grid cells interfaces be aligned to fracture-planes. This work will prove a major breakthrough in unstructured PEBI grid deployment for large scale fractured simulation.