Understanding Multimechanistic Gas-Water Flow in Fractured Reservoirs: Mapping of the Multimechanistic Flow Domain
Authors: Chawathe A. , Ertekin T. , Grader A. 
Published: December 2007

Abstract
The multimechanistic flow mechanism was proposed by Ertekin et al. in 1986. The principal hypothesis behind this mechanism was the transport of fluids under simultaneous influence of pressure and concentration gradients. In this study, we applied the multimechanistic flow concept to fractured reservoirs. We believe this application is relevant because multimechanistic flow may exist in naturally fractured gas reservoirs as well as coal seams. The development of the multimechanistic flow model, as applied to fractured reservoirs, is presented in detail in Chawathe et al. (1996), and the underlying physics of multimechanistic flow is explained in Chawathe et al. (1996). In this paper, we discuss identification of fractured systems undergoing multi-mechanistic flow.
Preliminary studies indicate that multimechanistic flow results in significant increase in cumulative gas production. One of the primary observations of this study is that it is not the fracture permeability by itself, but the ratio of the fracture to matrix permeabilities that influences cumulative gas production characteristics. In conclusion, we present a multimechanistic flow map which may assist the engineer in identifying fractured systems undergoing multimechanistic flow behavior.

Introduction
Fluid production through naturally fractured systems has been conventionally modeled using the dual-porosity, single-permeability (DPSP) concept. This concept is based on the sugarcube approximation of a fractured reservoir (Warren and Root, 1963). However, it does not capture the physics involved in intra-matrix flow transport. This happens because the DPSP concept involves solving the transport equation in the fractures only. The matrix blocks contribute to the flow in the form of passive sources/sinks (Gilman et al., 1983). In case of the proposed multimechanistic flow, it becomes necessary to include the matrix blocks in the flow modeling because of the intrinsic gas-water interactions in the matrix associated with this type of flow behavior. Keeping this in mind, we chose to model such systems using the dual-porosity, dual-permeability (DPDP) flow concept.
The DPDP formulation expresses the physical aspects of the fractured reservoir. The flow modeling, on the other hand, is generally done by substituting the momentum equation by Darcy's law to describe the superficial velocity vector in the diffusivity equation. This substitution addresses the convective (advective) aspects of the flow. This modeling practice has prevailed since most of the conventional models were designed to express liquid transport, such as oil, through porous systems. But is this adequate to express flow of gas through porous media? Probably not. We believe a better modeling approach should also involve the diffusion term especially when considering gas dynamics through tight, porous systems. The diffusion term is incorporated in the model by computing the vectorial sum of the corresponding Darcian and Fickian velocities. This sum is then substituted in the superficial velocity vector term in the diffusivity equation.

The Dual-Porosity, Dual-Permeability Multimechanistic Model
Although the details of the multimechanistic model development have been discussed in detail in Chawathe et al. (1996), we present some of the underlying equations here for completeness. The equations describing the flow of gas and water through the fractures as well as the matrix are derived based on the principle of mass conservation shown in Equation (1).
(Mass In) - (Mass Out) = (Mass Accumulated) (1)
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