Fractured Reservoirs: Orgin, Description, Evaluation
1991. "Fractured Reservoirs: Orgin, Description, Evaluation", The Integration of Geology, Geophysics, Petrophysics and Petroleum Engineering in Reservoir Delineation, Description and Management, Robert Sneider, Wulf Massell, Rob Mathis, Dennis Loren, Paul Wichmann
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Fractured reservoirs have been recognized for over 40 years. Generally they have been, and still are regarded as a special type of reservoir with only a limited contribution to the total oil/gas reserves. DRUMMOND (1964) estimated that only about 13% of all reserves are contained in such reservoirs. It is the purpose of this paper to challenge this view. In order to do this it is necessary to take a closer look at the different types of reservoirs that might be called fractured traps.
The fractures that are of particular interest are the joints observed in almost all sediments. SCHEIDEGGER (1979) states: “Joints are ubiquitous phenomena”. Thus their actual presence, with highly variable spacing, is not a problem as any field geologist can confirm. Generally the orientation of these joints is vertical, or high angle, to bedding.
In order to be effective fluid conduits such joints must be OPEN. For reservoir depletion this means that they must be open NOW. One may question this additional requirement and argue that a joint once formed will never again close perfectly. Just how do we justify the necessity for gaping joints? Surely there are laboratory experiments that point in that direction. However, the simplest proof can be found in the process of artificial fracing, a daily operation in the oil patch for over 40 years, and a most successful one at that. The artificial fracture is produced in a positive (compressive) virgin stress environment by an excess pressure (injection pressure) within the fracture.