SPE 122456 Conceptualization and Modeling of Flow and Transport Through Fault Zones

A physically based fault conceptual model is presented for modeling multiphase flow and transport processes in fractured rock of fault zones. In particular, we discuss a general mathematical framework model for dealing with fracture-matrix interactions, which is applicable to both continuum and discrete fracture conceptualization in fault zones. In this conceptual model, faults or fault zones of formations are conceptualized as a multiple-continuum medium, consisting of (1) highly permeable, large-scale and well-connected fractures, (2) low-permeability rock matrix, (3) various-sized vugs or large pore volumes, and (4) surrounding fractured or matrix formations on both sides. Flow through fault zones may be different from that through fractured reservoir rock, because of higer permeabilities and larger pore spaces in fault zones. In addition fault flow may be further complicated by non-Darcy’s and other nonlinear flow behavior because of large pore space. To account for such complicated flow regime, our model formulation includes non-Darcy flow, using the multiphase extension of the Forchheimer equation as well as descriptions for flow in parallel-wall fractures or tubes, based on solutions of flow through a parallel-wall, uniform fracture and Hagen-Poiseuille tube flow. The proposed fault flow model is discretized using an unstructured grid with regular or irregular meshes, followed by time discretization carried out using a backward, first-order, finite-difference method. The final discrete nonlinear equations are handled fully implicitly, using Newton iteration. The numerical scheme proposed is applicable to simulating multiphase fluid and heat flow as well as solute transport through the fractured fault zones and their interaction with surrounding rocks. The conceptual fault model is implemented into a general-purpose reservoir simulator, applicable to 1-D, 2-D, and 3-D simulation of multiphase flow in fault zones. As a demonstration example, we apply the model to simulate pressure and temperature responses in wells for a flow system controlled by faults. Introduction Since the 1960s, a number of numerical approaches and techniques have been developed and applied for modeling flow and transport processes in fractured reservoirs (e. g., Kazemi, 1969; Pruess and Narasimhan, 1985; Wu and Pruess, 1988). Even with the significant progress has been made towards understanding and modeling of flow and transport processes in fractured rock so far, most studies have focused primarily on naturally fractured reservoirs without taking into consideration of faults explicitly. Recently, characterizing fractured rock of faults or fault zones has received attention, because fault zones are found to be closely associated with and may dominate flow and transport processes in fractured reservoirs (Wu et al. 2004; 2006a; 2007a). Mathematical approaches developed for modeling flow through fractured reservoirs rely in general on continuum approaches, involving developing conceptual models, incorporating the geometrical information of a given fracture-matrix system, setting up mass and energy conservation equations for fracture-matrix domains, and then solving discrete nonlinear algebraic equations of mass and energy conservation. The commonly used mathematical methods for modeling flow through fractured rock include: (1) an explicit discrete-fracture and matrix model (Snow, 1965), (2) a dualand multiple-continuum method, including doubleand multi-porosity, dual-permeability, or the more general "multiple interacting continua'' (MINC) method (Warren and Root, 1963; Kazemi, 1969; Pruess and Narasimhan, 1985; Wu and Pruess, 1988), and (3) an effectivecontinuum method (ECM) (Wu, 2000). In addition to the traditional double-porosity concept, a number of triple-porosity or triple-continuum models have been proposed (Closemann, 1975; Wu et al. 2004a; Kang et al. 2006; Wu et al. 2007b) to describe flow through fractured rocks. In particular, Liu et al. (2003) and Camacho-Velazquez et al. (2005) present several new triple-continuum models for singlephase flow in a fracture-matrix system that include cavities within the rock matrix (as an additional porous portion of the