Full Pressure Coupling for Geomechanical Multiphase Multicomponent Flow Simulation
Authors: Wilson Adam 
Published: June 2016

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 173232, “Full Pressure Coupling for Geomechanical Multiphase Multicomponent Flow Simulations,” by Florian Doster, SPE, Heriot-Watt University, and Jan Martin Nordbotten, SPE, University of Bergen, prepared for the 2015 SPE Reservoir Simulation Symposium, Houston, 23–25 February. The paper has not been peer reviewed.

Simulation of coupled flow and geomechanics is of rising importance as unconventional subsurface exploration pushes the operational envelope of geological media. Coupling schemes are commonly assigned to three categories: decoupled, iteratively coupled, and fully coupled. This paper focuses on iterative coupling and presents a new coupling scheme: full pressure coupling (FPC). Here, the geomechanics is solved fully coupled to a single-phase-flow problem using global pressure, and the resulting deformation is iteratively coupled to a multiphase multicomponent flow solver.

Introduction

Geomechanical response to fluid flow is important for several subsurface applications. Historically, geological subsidence has been seen as a response to petroleum production and a result of groundwater production. Recently, coupling of flow and geomechanics has received renewed attention, in the context of ground level uplift associated with the In Salah CO2- storage site and in the context of hydraulic fracturing for the purposes of enhancing geothermal systems and shale-gas production. The authors note a trend away from considering systems where a geomechanical response may be important toward using the geomechanical coupling directly in the engineering design. Simultaneously, there is a need for more-robust algorithms for handling strongly coupled hydromechanical systems.

The traditional approach to addressing coupled flow and mechanics uses a divide-and-conquer strategy, wherein the flow and mechanical systems are solved separately and coupled either loosely or through iterations. This strategy is motivated in part by the availability of established and robust commercial software for both multiphase flow and mechanical deformation. Although some of these coupled approaches can be shown to be stable and convergent, it is generally desirable to consider a tighter coupling between flow and mechanics.

When considering multiphase flow and deformation, the mathematical structure of the problem implies three main components: mechanical deformation, the fluid pressure equation, and mass transport. The traditional splitting algorithms propose to iterate between solving the mechanical system and then solving the pressure and transport systems together. However, the success of algorithms such as implicit pressure with explicit saturation/mass/component implies that pressure and transport are only weakly coupled. Consequently, this paper proposes a new splitting approach to multi phase flow and geomechanics, wherein the mechanical and pressure systems are solved together and iterate with the transport system. Thus, the pressure and mechanics are fully coupled.

Please see the complete paper for the model equations, the reformulation of the equations to obtain the full-pressure- coupling splitting, and the formulation of a discrete version of those equations using finite volumes and treating pressure and deformations implicitly and masses explicitly (FV-ImPDEM).

Illustrative Examples

Simple examples that are characteristic for CO2-storage scenarios illustrate the FV-ImPDEM algorithm. The chosen material properties are given in Table 1. The Young’s modulus is on the soft limit of sandstone, to illustrate the coupling effect. Also, the compressibilities of the fluids in most reservoirs are an order of magnitude smaller and the density of CO2 is normally not obtained for pure CO2. Also, note that the grains of the rock are assumed to be incompressible and all the compressibility of the rock is obtained through changing the packing of the grains.