Effectiveness of embedded discontinuities technique in capturing geomechanical behavior in naturally fractured reservoirs

Abstract

Abstract This paper highlights the efficacy of the finite element method with embedded strong discontinuities in modeling discontinuities in porous media, specifically in the geomechanical behavior of Naturally Fractured Reservoirs (NFRs). The approach considers hydromechanical coupling and offers low computational cost. NFRs account for a significant portion of global reserves, representing approximately 60% of global oil reserves and 40% of gas reserves. Given that flow in NFRs is more complex than in conventional reservoirs due to the presence of multiple fractures, it's crucial to understand how pressure variations or effective stress during operations impact fracture closure and permeability of these reservoirs. To analyze this behavior, numerical simulation results using the proposed method were compared, under different liquid pressure depletion values, with the approach proposed by Oda, which is commonly used in commercial software for calculating fracture permeability tensors. This approach was enriched with Barton's fracture closure formulation and updates on rock matrix porosity and permeability. Four simulations were conducted: Firstly, a hypothetical scenario consistent with Oda's assumptions, where fractures are interconnected and span the entire grid cell, to validate the numerical hydromechanical model; subsequently, three representative sections of a Brazilian pre-salt carbonate reservoir were selected. The study confirms the efficacy of the technique of embedded strong discontinuities in calculating equivalent permeabilities in NFRs, considering geomechanical effects, especially in cells with high fracture frequencies and intensities. Furthermore, the relevance of analyzing the geomechanical behavior in NFRs is emphasized.

Keywords

• Naturally fractured reservoirs (NFRs)
• Discrete fracture networks (DFNs)
• Embedded strong discontinuities
• Equivalent permeability