A Scalable Massively Parallel Dual-Porosity Dual-Permeability Simulator for Fractured Reservoirs with Super-K Permeability

Abstract

Abstract Modeling Middle East giant carbonate reservoirs with fractures and super-k conductivity present unique challenges to conventional simulation approaches. Fractures in these giant oil fields can exist as fracture swamp neighboring fault zones. These regional/local fractures can interact with Super-K layers (high permeability stratiform) which form extremely high conductivity to fluid flow. Typically, meaningful simulation for such an oil field requires the use of millions of grid cells involving thousands of wells. These reservoirs are not the classical dual porosity where matrix permeability is poor and does not form a part of the global flow path. Dual porosity dual permeability (DPDP) formulation allowing a flexible regional representation of both single porosity and dual porosity behavior on a cell by cell basis is more suitable. A parallel dual porosity dual permeability simulator has been developed to efficiently solve multi-million grid cell fractured reservoirs problems with super-k fracture permeability. To be effective, the simulator has to engage tens to hundreds of processors in a highly scalable manner. This paper discusses the formulation, the data design and solution procedures involved in this development. The parallelization paradigm is the mixed approach with MPI (Message Passing Interface) and OpenMP (Open Message Passing). These parallelization approaches are open standards supported by most major hardware vendors. This allows for easy portability among various hardware architectures. However, the data design and solution methods discussed herein are amenable to MPI only or other parallelization approaches as well. The mixed paradigm approach is flexible and allows various combination and permutation of MPI processes and OpenMP threads to be used to solve a problem. Results are presented for a multi-million cell fractured reservoir simulation with super-k involving thousands of wells and over 60 years of history match. We present simulation results, computational efficiency and parallel scalability on the IBM Nighthawk II, as well as on the PC Xeon Linux cluster hardware. Introduction This paper describes a novel parallel algorithm for the solution of the general dual porosity dual permeability (DPDP) system. The targets are the giant Middle East oil fields where complex geological features such as large open faults, super-k permeability zones and fracture swamps. These fields have been subjected to peripheral water injection for many years where premature water breakthrough have occurred in parts of the reservoirs. The nature of super-k permeability and its interactions with vertical faults was previously discussed by Shahri et.al.1. Attempts to capture the correct flow physics for these reservoirs in 3D reservoir simulation using conventional single porosity simulator was described by Phelps et.al.2. Later, Dogru et.al.3 applied a multi-million cell model in a parallel simulator to refine the definition of these geologic features. Classical dual porosity simulators treat fractured reservoir problems where the natural fractures and/or vugs are the primary conduits for fluid flow. The porous rock matrix is typically very tight and primarily acts as storage with negligible global flow capacity. Many papers were written about the formulation of these simulators4–6. Many refinements7–9 were also proposed to capture flow physics such as capillary continuity, gravity drainage, forced and free imbibitions, and viscous gradient, associated with matrix fracture flow and exchange mechanisms. The significance of the dual porosity dual permeability concept in relation to flow physics of complex fracture reservoir systems were highlighted in Fung1-,11.