Representation and scaling of faults in fluid flow models

Abstract

Fault representation and scaling in flow models are examined with respect to fault zone properties, the accuracy with which they can be determined, and how these variables and fault geometries can be incorporated realistically in to flow models. Outcrop data show that fault displacement/thickness ratios and permeability vary widely. For simple single fault models, results for numerical models are compared with analytical and statistical methods. Representation of a fault as a transmissibility surface conflates the effects of four variables -fault zone thickness and permeability, grid-block size and matrix (host-rock) cell permeability. Random spatial variation of transmissibility factor values is well represented by a uniform transmissibility factor which is the arithmetic mean of the values representing log-normally distributed permeability and thicknesses. Realistic ranges of fault zone thicknesses can be represented without grid-block refinement by an upscaling method based on simple transformation of transmissibility factor curves derived from a range of coarse grid-block models. Sub-seismic faults have significant effects on effective permeability of model volumes at kilometre scales only when the faults are assigned a permeability less than c. 0.001 of the matrix permeability.

Structural heterogeneity and permeability in faulted eolian sandstone: Implications for subsurface modeling of faults

“…These may exhibit significant variability at the outcrop to oil field scale (Foxford et al, 1998;Walsh et al, 1998;Beach et al, 1999;Hesthammer and Fossen, 2000;Shipton and Cowie, 2001). The lowest permeability case, flow perpendicular to the fault zone (transverse flow), is considered for these analyses.…”

Structurally complex reservoirs: an introduction

“…This work therefore provides a rationale for the successful application of these approaches, even though the deformation mechanism which it implicitly assumes may not be correct. Similarly, detailed modelling of fluid flow through a realistic representation of fault damage zones by Harris et al supports the conclusion that the simpler, commonly used approach of harmonically averaging volume-weighted fault-rock permeability, is a useful first approximation in assessments of the flow impact of subseismic faults (Walsh et al 1998).…”

“…Similarly, the width and internal geometry of fault damage zones has been well characterized in numerous outcrop analogues (Antonellini & Aydin 1994;Hesthammer et al 2000;Billi et al 2003;Odling et al 2005) and can be predicted by selecting the right analogue and control parameters such as fault displacement and host-rock lithology. Once the geometry and petrophysical properties of the small-scale features are defined (see below), their flow implications can be simulated by explicitly modelling the observed fractures or a stochastic representation of them (Heath et al 1994;Manzocchi et al 1998;Walsh et al 1998;Harris et al 1999Harris et al , 2003.…”

“…decreasing effective permeabilities by more than c. 20%) when fault rock permeabilities are at least two orders of magnitude below those of the reservoir host rocks (e.g. Manzocchi et al 1998;Walsh et al 1998). For less permeable fault rocks, cross-fault flow decreases rapidly with an increase in flow tortuousity, until flow is dominated by the connectivity of sealing faults (Walsh et al 1998).…”

Reservoir Compartment Assessment: A Case Study of Bangko and Bekasap Formation, Central Sumatra Basin Indonesia

“…The transmissibility is related to the hydrocarbon that can be drawn from the reservoir through fault plane. This means that higher permeability tends to be a leaking character, while lower permeability tends to be a sealing character [21][22][23]. Further, a comprehensive assessment of the reservoir compartment is carried out by combining sand-sand juxtaposition, SGR, and transmissibility for each reservoir sand layer.…”