A Proposal To Improve recovery Efficiency

To substantiate our recommendations for secondary recovery by infill drilling and waterflooding, it was necessary to show that there is some relationship between spacing and ultimate recovery. This is a problem that has perplexed the industry since the problem that has perplexed the industry since the 1920's. The last serious attempt to find such a relationship was made in 1967 by the API Subcommittee on Recovery Efficiency. The 103 reservoirs analyzed by Craze and Buckley in 1945 were expanded to 312, and relationships between recovery efficiency, reservoir characteristics, and fluid characteristics were established for depletion-drive as well as water-drive reservoirs; however, a spacing/ ultimate-recovery relationship could not be determined, possibly because heterogeneity was not considered.Our recommendations for secondary recovery of oil do not involve new technology; however, they do require larger capital investments than do tertiary-recovery projects of similar size. Oil recovered by tertiary methods gets upper-tier prices because of the magnitude of the investment prices because of the magnitude of the investment required. If size of investment is the criterion used to obtain world prices, then modern infill-drilling programs more than meet this requirement. When mentioned in this paper, secondary recovery or infill drilling always means infill drilling combined with waterflooding to maintain pressures at or above saturation pressure. Such pressures at or above saturation pressure. Such infill drilling easily could result in spacing as dense as 20 acres per unit, where a unit consists of one production well and one injection well. This means that, with 20-acre spacing, there would be four times as many wells in a field as there are now with 40-acre spacing. Because one well now costs three times as much as it did in the early 1970's, requirements for receiving world prices for this method appear fully satisfied.The only methods effective for large-scale improvement of oil recovery are the secondary method of infill drilling as proposed here and the application of heat in viscous-oil reservoirs. There is at present no tertiary method ready to be used on a large scale. The CO2-WAG project in the SACROC Unit is the only CO2 project in operation on a large scale; a close look at the data on this unit causes considerable doubt about claims regarding the success of the project. project. Infill drilling, if done properly, can be used to recover at least as much oil as the U.S. already has produced. All reservoirs are heterogeneous to some produced. All reservoirs are heterogeneous to some degree i.e., not all portions of the reservoir are connected to one another. Infill drilling will permit production of oil from parts of the reservoir that production of oil from parts of the reservoir that might be bypassed by standard, low-density well spacing. Water must be injected in all portions of the reservoir simultaneously to keep the entire reservoir contents at least at, and preferably above, bubblepoint pressure. The water produced must be collected pressure. The water produced must be collected and cleaned for recirculation, and "short circuits" between injectors and producers must be located and repaired if possible. During production, some producers must be converted to injectors, and vice producers must be converted to injectors, and vice versa, depending on productive behavior in a particular portion of the reservoir. At all times, the particular portion of the reservoir. At all times, the engineer should determine the particular section of the formation into which water should be injected; the water's path from the injector to the producer should not be left up to formation permeability. For successful recovery, the number of producers should be equal to or less than the number of injectors, and the wells should be spaced on a regular pattern. Obviously, if the injection wells are widely spaced, only the most permeable channels are used. With regularly and densely spaced injectors, water will be forced into the less-permeable portions of the reservoir and will push the oil out. Openhole completions of water-injection wells never should be made, and such wells should not be fractured. If secondary recovery is carried out in this manner, operating costs will be far above normal; however, production also will be far above normal. production also will be far above normal. Although waterflooding is applicable everywhere except where heat is required to reduce oil viscosity, the most logical places to begin flooding operations are the limestone/dolomite reservoirs of New Mexico and west Texas. There are four reasons for this. p. 1164