Representation of Fault Zone Permeability in Reservoir Flow Models
- E.A. Flodin (Stanford University) | A. Aydin (Stanford University) | L.J. Durlofsky (Chevron Petroleum Technology Company) | B. Yeten (Stanford University)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 30 September-3 October, New Orleans, Louisiana
- Publication Date
- Document Type
- Conference Paper
- 2001. Society of Petroleum Engineers
- 1.2.3 Rock properties, 5.5.3 Scaling Methods, 5.4.1 Waterflooding, 4.3.4 Scale, 5.5.8 History Matching, 1.6.9 Coring, Fishing, 5.5 Reservoir Simulation, 5.1.5 Geologic Modeling, 5.1.1 Exploration, Development, Structural Geology, 1.10 Drilling Equipment, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.1.2 Faults and Fracture Characterisation
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Faults can act as fluid flow barriers, conduits, or barrier/conduit systems in reservoirs. Their accurate representation in reservoir flow simulations is essential if realistic predictions are to be attained. In this work we compute the effective flow characteristics of faults using fine-scale field-based data. The faults we focus on are in porous aeolian sandstone and were formed by shearing along pre-existing joint zones. To find the bulk flow characteristics of the fault zones, we develop a computationally efficient upscaling methodology that combines numerical flow modeling and power averaging. By analyzing faults with different slip magnitudes, we are able to produce a relationship between fault permeability and fault slip. Slip magnitude is one of the few fault parameters that can be measured remotely in the subsurface and we show how it can be used to estimate the variation in permeability along a fault. We present three different flow simulation scenarios using variable fault properties derived using our new procedure. For each scenario, we present a second tuned case where we replace our variable fault-zone permeability by a fault with a constant permeability and width. In one case, we find no significant difference in flow response between the variable and constant permeability faults. The other two cases display differences, mostly with regard to breakthrough time and liquid production rates. Because the reservoir flows considered here are relatively simple, we postulate that the differences between the variable and constant permeability fault descriptions will become greater for more complex systems.
Faults are common features in oil and gas reservoirs. They can act to impede or enhance fluid flow dramatically,1 thereby playing an important role in reservoir performance.2 However, despite their strong impact on flow, typical reservoir simulation models represent faults in a highly simplified manner. Faults in these models are often used as adjustable parameters, with their gross impact on flow behavior "tuned" so the global model predictions agree with observed production data. The use of these models as predictive tools is therefore quite limited in many cases.
The purpose of this paper is to develop and apply a new procedure for assigning permeability values to grid blocks representing the fault zone in flow simulation models. We consider the case of faults in sandstone reservoirs. The grid block permeability values are determined using the results from detailed analog outcrop studies3 and from previously developed numerical solutions using a power averaging technique3,4 and a full numerical solution5,6 for computing fault zone permeabilities. These results provide an estimate of fault zone permeability, on the scale of 1-20 meters, as a function of the local fault slip magnitude. By combining these results with larger scale geologic measurements that provide estimates of the variation in fault slip over the length of the fault, we are able to estimate fault zone permeability along the entire fault. Through the application of this procedure, we demonstrate the impact of detailed fault zone descriptions on large-scale reservoir flows. Comparisons with large-scale flow results using a simple fault treatment, as commonly employed in current practice, are also presented. These comparisons demonstrate the qualitative improvements obtained using our procedure and the inaccuracies inherent in the simpler approaches.
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