Geological Characterization of Texas Reservoirs

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Galloway, William E.

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Approximately 153 billion barrels of in-place oil have been discovered in Texas reservoirs. Assuming recovery efficiency continues to increase modestly, an estimated 61 billion barrels of this oil will be produced, largely by conventional primary and secondary recovery technologies. The remaining 90 plus billion barrels of oil represent a target of immense proportions. For comparison, most recent estimates by the U.S. Geological Survey are that only 6 to 22 billion barrels of additional recoverable oil are likely to be found by continued exploration in the State. Certainly not all of this unproduced oil can be recovered. Recovery efficiency of oil is limited by several factors. First, a portion of the oil contained within a reservoir, called the residual oil saturation, is not flushed from the rock because it is trapped in dead-end or isolated pores or has "wet" the mineral grains. This oil can only be moved from the reservoir by altering its physical characteristics or by artificially improving the ability of moving fluids to sweep it from the reservoir. Such oil is thus a potential target for the advanced, or so-called tertiary recovery processes. However, residual oil saturation can be measured and commonly ranges between 15 and 35 percent. Simple arithmetic shows that about one-fourth of the unrecovered 90 to 100 million barrels must remain in portions of the reservoirs that have not been drained in the course of conventional field development. Such oil is trapped in isolated compartments or lenses that were not tapped by wells drilled on conventional, regular spacings. Thus it constitutes a potential target for selective infield exploration and drilling. The objectives of this study were (1) to examine the geology and development history of the entire population of Texas oil reservoirs, (2) to improve our estimate of the amount of oil that remains as a target for strategic infield "exploration" and development, (3) to identify families of oil fields that offer the greatest potential for significantly improving statewide recovery efficiency by application of such infill programs, and (4) develop generic reservoir models that might be applied to improving recovery from the important families of less efficient reservoirs.


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