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Fractured Reservoir Modeling and Interpretation |
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Course Description |
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All geological formations are fractured to some extent as a result of stress triggered by the Earth's crust weight, high fluid pressure, tectonic forces, and/or thermal loading. Fractures occur at a variety of scales from microscopic to continental and are important in hydrogeology, the earth sciences, and petroleum engineering; they are present in almost all oil, gas, geothermal, and water reservoirs. Fractures create traps, serve as conduits to oil and gas migration, and can behave as barriers or baffles to fluid flow. Naturally fractured reservoirs contain finite and infinite conductivity fractures in igneous, metamorphic, sedimentary rocks (matrix) and formations. In most sedimentary formations both fractures and matrix contribute to flow and storage, but in igneous and metamorphic rocks only fractures contribute to flow and storage, and the matrix has almost zero permeability and porosity.
Accurately modeling naturally fractured reservoir pressure transient behavior is important in hydrogeology, the earth sciences, and petroleum engineering, including ground-water contamination to shale gas and oil reservoirs. For more than 50 years, conventional dual-porosity type models, which do not include any fractures, have been used for modeling fluid flow in naturally fractured reservoirs and aquifers. They have been continuously modified to add unphysical matrix block properties such as matrix skin factor.
In general, fractured reservoirs are heterogeneous at different length scales. It is clear that even with millions of grid blocks, numerical models may not be capable of accurately simulating the pressure transient behavior of continuously and discretely naturally fractured reservoirs containing variable conductivity fractures. The conventional dual-porosity type models are obviously an oversimplification; their serious limitations and consequent implications for interpreting well test data from naturally fractured reservoirs are discussed in detail. These models do not include wellbore-intersecting fractures, even though they dominate the pressure behavior of naturally fractured reservoirs for a considerable length of testing time. Fracture conductivities of one to infinity dominate transient behavior of both continuously and discretely fractured reservoirs, but again dual-porosity models do not contain a single fracture. Our fractured reservoir model is capable of treating thousands of fractures that are periodically or arbitrarily distributed with finite- and/or infinite-conductivities, different lengths, densities, and orientations.
Appropriate inner boundary conditions are used to account for wellbore-intersecting fractures and direct wellbore contributions to production. Wellbore storage and skin effects in bounded and unbounded systems are included in the model. Three types of damaged skin factors that may exist in wellbore-intersecting fracture(s) are specified. With this highly accurate model, the pressure transient behavior of conventional dual-porosity type models are investigated, and their limitations and range of applicability are identified. The behavior of the triple-porosity models is also investigated. It is very unlikely that triple-porosity behavior is due to the local variability of matrix properties at the microscopic level. Rather, it is due to the spatial variability of conductivity, length, density, and orientation of the fracture distributions. Finally, we have presented an interpretation of a field buildup test example from a naturally fractured reservoir using both conventional dual-porosity models and our fractured reservoir model.
Fikri J. Kuchuk, Schlumberger Fellow, is currently chief reservoir engineer for Schlumberger Reservoir Characterization Group. He was a consulting professor at the Petroleum Engineering Department of Stanford University from 1988 to 1994 and taught advanced well testing. Before joining Schlumberger in 1982, Kuchuk worked on reservoir performance and management for BP/Sohio Petroleum Company. He holds B.S., M.S., and Ph.D. degrees from Technical University of Istanbul and Stanford University. Kuchuk is a Distinguished and Honorary Member of the Society of Petroleum Engineers, and a member of the Society for Industrial and Applied Mathematics, the Russian Academy of Natural Sciences, and the American National Academy of Engineering. He received the 1994 SPE Reservoir Engineering, 2000 SPE Formation Evaluation, and 2001 SPE Regional Service awards, as well as the Henri G. Doll Award in 1997 and 1999 and the Nobel Laureate Physicist Kapitsa Gold Medal. Kuchuk has been very active in professional societies, as SPE International Director At-Large and many other positions. He has published and presented more than 150 papers on formation evaluation, reservoir engineering, and geothermal. Kuchuk coauthored Transient Well Testing (SPE Monograph Series), and Pressure Transient Formation and Well Testing.
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