SECONDARY RECOVERY

Water flooding is one of the most common and efficient secondary recovery processes. Water is injected into the oil reservoir in certain wells in order to renew a part of the original reservoir energy. As this water is forced into the oil reservoir, it spreads out from the injection wells and pushes some of the remaining oil toward the producing wells. Eventually the water front will reach these producers and increasingly larger quantities of water will be produced with a corresponding decrease in the amount of oil. When it is no longer economical to produce these high water-ratio wells, the flood may be discontinued. As mentioned previously, average primary recoveries may be only 15% of the oil in the reservoir. Properly operated waterfloods should recover an additional 15% to 20% of the original oil in place. This leaves a substantial amount of oil in the reservoir, but there are no other engineering techniques in use now that can recover it economically. In most cases, oil reservoirs suitable for secondary recovery projects have been produced for several years. It takes time to inject sufficient water to fill enough of the void spaces to begin to move very much oil. It takes several months from the start of a waterflood before significant production increases take place and the flood will probably have maximum recoveries during the second, third, fourth, and fifth years after injection of water has commenced. The average flood usually lasts 6 to 10 years.

Water floods have been highly successful in the Wyoming Basin and probably account for 75% of the total production from the area. Flood recoveries will generally be an additional 80% to 100% of the primary production. There are no special problems with floods in the Wyoming Basin. Ample supplies of salt water are generally available and injection pressures are not too high - 1500 PSI or less. Corrosion is minimal and no expensive, high-pressure equipment is involved. Sufficient potential flood properties are available on reasonable terms - especially smaller areas owned by independent operators who do not have the finances to support the installation of properly engineered secondary recovery operations. Water floods in the Wyoming Basin should return 2 to 3 times their cost and are considered to be low-risk prospects.