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Reservoir Types and Characterization
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A reservoir is a subsurface rock structure, with sufficient size and closure that contains a three-dimensional network of interconnected void (pore) space and is overlain by a fine-grained water-saturated rock. Porosity and permeability are the key features of pore structure and are important factors in the production of hydrocarbons from the reservoir. Porosity represents the capacity of the reservoir rock to contain petroleum. Permeability represents the ability of the reservoir rock to transmit fluids. Together, they influence the fluid storage and movement. The reservoirs are classified according to the initial state of the fluids in the reservoir in three broad categories, including oil, gas-condensate, and gas reservoirs. The classification of the reservoir according to the drive mechanism is another useful approach. Additionally, the reservoirs can be classified as conventional or unconventional based on the technological requirements for development and production. The term unconventional reservoirs include a wide variety of reservoir types. They commonly include tight-gas sandstones, coalbed methane (CBM), shale gas and oil, heavy oil and tar sands, and gas hydrates. To estimate the productive potential of the reservoir and to maximize the economic payback, the reservoir characteristics—including porosity, permeability, fluid saturations, net formation thickness, pressure, fluid properties, and the structure of the reservoir—must be determined. Reservoir characteristics can be evaluated by the analysis of the information obtained from the direct and indirect measurements. Direct measurements in a reservoir are possible on fluid samples and on the reservoir rock samples (cores). The indirect measurements can be obtained by lowering a string of sensors at the end of a wireline into the well to obtain a continuous record of the subsurface rock properties (logs). Another indirect measurement involves a pressure transient test that can be interpreted to diagnose the condition of production and injection wells.
reservoir classification, reservoir characterization, unconventional reservoirs, heterogeneous reservoirs
West Virginia University, Petroleum & Natural Gas Engineering Dept., Morgantown, WV