Model Shows Computational Gains, Preserves Accuracy in Tight Rock EOR
Authors: Carpenter Chris 
Published: January 2024

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 3834855, “Advanced Dual-Porosity and Dual-Permeability Model for Tight Rock Integrated Primary Depletion and Enhanced Oil Recovery Simulation,” by Daegil Yang, SPE, Tyler Do, and Changdong Yang, SPE, Chevron, et al. The paper has not been peer reviewed.

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Discrete fracture or dual-porosity modeling has been widely adopted for tight rock simulation. To represent the complex fracture heterogeneity and simplify its characterization, the authors present in the complete paper a new simulation model to holistically optimize the tight rock reservoir completion, primary depletion, and enhanced oil recovery modeling and retain the injection mechanism and simplify the complex fracture characterization to enhance computational efficiency. The model enables users to characterize different properties in multiple regions. This tool will have broad applications in supporting asset-development decisions.

Introduction
Traditional dual-porosity, dual-permeability (DPDK) simulation incorporates microscale fracture characterization through the shape factor (matrix/fracture coupling), which offers a significant contact area for the gas/oil mixture to represent the field response reliably. However, its homogeneous fracture network cannot be used to optimize the completion design. The authors have developed what they refer to as the advanced DPDK (A-DPDK) simulation work flow to maintain heterogeneous fracture characterization and to simplify microfracture characterization. The work flow contains the following features:
- A newly derived shape-factor algorithm that can represent the scale and heterogeneity of the fracture network
- Local grid refinement (LGR) that accelerates the speed of computation (five-times-faster acceleration) and improves accuracy
- A fracture-characterization feature that serves as a helpful diagnostic tool to assign a flexible cutoff value for both propped fractures (PF) and unpropped fractures to enhance simulation speed and robustness
- A connected PF feature that can label the effective fracture region to simplify completion design work flow and prioritize reservoir simulation
The authors validated the new features implemented in the work flow systematically and demonstrated the benefits of the proposed work flow. The authors also applied the work flow in one gas-injection pilot and successfully history-matched the primary depletion and the gas-injection response.

Modeling Approach
The A-DPDK model design is aimed at modeling a complex fracture network using a structured grid system while maintaining the background natural fracture. In the modeling process, the main fracture network is screened and identified and its transmissibility calculated. Then, the microfracture is calculated to matrix mass transfer. This can be numerically modeled using a DPDK framework. The geometry of the hydraulic fracture network is preserved explicitly as grid cells in the fracture domain. Also, in the matrix domain, the enhanced permeability region (EPR), which represents the microfracture, is included. The mass transfer between matrix cells and the EPR is modeled as a grid-cell connection (transmissibility), and the mass transfer between the hydraulic fracture cells and the microfractures is modeled as a grid-cell connection as well. A new way of calculating the coupling term that models the mass transfer between the hydraulic fracture and the EPR and the matrix is introduced.