Multiporosity/multipermeability approach to the simulation of naturally fractured reservoirs

Abstract

This paper presents an array of deformation-dependent flow models of various porosities and permeabilities relevant to the characterization of naturally fractured reservoirs. A unified multiporosity multipermeability formulation is proposed as a generalization of the porosity-or permeability-oriented models of specific degree. Some new relationships are identified in the parametric investigation for both single-porosity and dual-porosity models. A formula is derived to express Skempton's constant B by Biot's coefficient H and relative compressibility •b*. It is found that the recovery of the original expression for Skempton's constant B is largely dependent on the choice of 0', representing relative compressibility. The dual-porosity/dual-permeability model is evaluated through an alternative finite element approximation. The deformation-dependent fracture flow mechanism is introduced where the rock matrix possesses low permeability and fracture flow is dominant. A preliminary study of the reservoir simulation identifies the strong coupling between the fluid flow and solid deformation. curve where the important fluid interchange between fractures and matrix blocks occurs. In general, interporosity flow has been described by two mechanisms. The first is the simple quasi-steady state model proposed by Warren and Root [1963], followed by the more sophisticated unsteady state models of various versions [Kazemi, 1969; deSwaan-O., 1976; Duguid and Lee, 1977; Kucuk and Sawyer, 1980; Najurieta, 1980; Chen et al., 1985]. Some noticeable differences in terms of pressure distributions in the transition period result from the two different mechanisms. The transient flow and deformation behavior in a porous medium may result from changes in either the fluid pressure or total stress boundary conditions applied to the system. It is the admissibility of changes in total stress within the system (which may result from natural tectonic changes or human activities) that describes the essence of coupled deformation-dependent flow behavior within porous media and sets it apart from decoupled diffusive (flow) systems. Comprehensive coupling between stresses and pore pressures was first rationalized by Biot [1941] and later adopted in many applications to specific deformation flow systems [Ghaboussi and Wilson, 1973; Zienkiewicz et aI., 1977; Simon et al., 1984; Lewis and Schrefier, 1987; Detournay and Cheng, 1988]. In naturally fractured reservoirs where the medium consists of discrete fractions of varying solid compressibilities and permeabilities, a multiporosity/ multipermeability approach appears more appropriate.It is important to correctly characterize the behavior of naturally fractured reservoirs. For example, the exceptionally high oil rate recovered in the initial stages of reservoir production may lead to overestimating well production by assuming a higher storage to exist than exists in reality. It was assumed that the high matrix block storage would 1621

Generalized Biot's theory and Mandel's problem of multiple‐porosity and multiple‐permeability poroelasticity

“…For this reason, generalizations to this model has been the subject of studies which consider either flow [Gwo et al, 1995] or coupled flowdeformation processes [Bai et al, 1993] in naturally fractured rocks. Shale is also known to comprise multiple-fracture networks ranging from the nanoscale to the sequence/systems tract scale in size and hydraulic conductivity [Slatt and Abousleiman, 2011].…”

A New Coal-Permeability Model: Internal Swelling Stress and Fracture–Matrix Interaction

“…Because coal matrix and fractures (cleats) have dramatically different mechanical properties, this interaction can have a significant effect on permeability changes under certain conditions, as will be demonstrated in this study. Although considerable effort has also been made with respect to modeling mechanical deformation processes and their coupling with flow processes in dual-continuum systems (fractured rock) (e.g., Wilson and Aifantis 1982;Bai et al 1993;Berryman and Wang 1995), these studies have focused on developing governing equations for coupled liquid flow and mechanical processes, rather than determining relationships between permeability and other related properties. Second, the previously discussed permeability models also generally assume that a change in the length of a matrix block (resulting from swelling or shrinkage) causes an equal but opposite change in the fracture aperture.…”

A triple-continuum approach for modeling flow and transport processes in fractured rock

“…Following the pioneering work by Barenblatt et al (1960) and Warren and Root (1963), several triple-porosity models have been proposed in the literature (e.g., Closmann, 1975, Wu andGe, 1983;Abdassah and Ershaghis, 1986;Bai et al, 1993) for describing flow through fractured rocks. has recently presented a new triple-porosity model for flow in a fracture-matrix system including cavities within the lithophysal rock matrix (as an additional porous portion of matrix) at Yucca Mountain.…”