Modeling Coupled Fracture-Matrix Fluid Flow in Geomechanically Simulated Fracture Networks
- Zeno G. Philip (Pinnacle Technologies) | James W. Jennings (U. of Texas at Austin) | Jon E. Olson (U. of Texas at Austin) | Stephen E. Laubach (Bureau of Economic Geology) | Jon Holder (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2005
- Document Type
- Journal Paper
- 300 - 309
- 2005. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 5.1.5 Geologic Modeling, 1.6.9 Coring, Fishing, 5.7.2 Recovery Factors, 5.8.6 Naturally Fractured Reservoir, 1.2.2 Geomechanics, 4.3.4 Scale, 1.14 Casing and Cementing, 5.5 Reservoir Simulation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.5.8 History Matching, 5.1.8 Seismic Modelling, 5.8.7 Carbonate Reservoir
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In conventional reservoir simulations, gridblock permeabilities arefrequently assigned values larger than those observed in core measurements toobtain reasonable history matches. Even then, accuracy with regard to someaspects of the performance such as water or gas cuts, breakthrough times, andsweep efficiencies may be inadequate. In some cases, this could be caused bythe presence of substantial flow through natural fractures unaccounted for inthe simulation. In this paper, we present a numerical investigation into theeffects of coupled fracture-matrix fluid flow on equivalent permeability.
A fracture-mechanics-based crack-growth simulator, rather than a purelystochastic method, was used to generate fracture networks with realisticclustering, spacing, and fracture lengths dependent on Young's modulus, thesubcritical crack index, the bed thickness, and the tectonic strain. Coupledfracture-matrix fluid-flow simulations of the resulting fracture patterns wereperformed with a finite-difference simulator to obtain equivalentpermeabilities that can be used in a coarse-scale flow simulation. The effectsof diagenetic cements completely filling smaller aperture fractures andpartially filling larger aperture fractures were also studied.
Fractures were represented in finite-difference simulations both explicitlyas grid cells and implicitly using nonneighbor connections (NNCs) between gridcells. The results indicate that even though fracture permeability is highlysensitive to fracture aperture, the computed equivalent permeabilities are moresensitive to fracture patterns and connectivity.
High-permeability fracture networks in a matrix system can createhigh-conductivity channels for the flow of fluids through a reservoir,producing larger flow rates and, therefore, larger apparent permeabilities. Thepresence of fractures can also cause early breakthrough of the displacing fluidand lead to poorer sweep efficiencies in displacement processes.A betterunderstanding of reservoir performance in such cases may be obtained byincluding the details of the fluid flow in fractures in a coupledfracture-matrix reservoir flow model.
It is very difficult to directly measure interwell fracture-network geometryin sufficient detail to model its effect on reservoir behavior. Thus, mostmodeling approaches have been statistical, using data from outcrop and wellboreobservations to determine distributions of fracture attributes such as fracturelength, spacing, and aperture to randomly populate a field. In this paper, weuse a mechanistic approach to generate the fracture patterns. Attributes of thefracture network depend on the applied boundary conditions and materialproperties.
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