Prodcuction

Oil Production
Once an accumulation of oil has been found in a porous and permeable reservoir, a series of wells are drilled in a predetermined pattern to effectively drain this "oil pool". Wells may be drilled as close as one to each 10 aces (660 ft. between wells) or as far apart as one to each 640 acres (1 mile between wells) depending on the type of reservoir and the depth to the "pay" horizon. For economic reasons, spacing is usually determined by the distance the reservoir energy will move commercial quantities of oil to individual wells.

The rate of production is highest at the start when all of the energy from the dissolved gas or water drive is still available. As this energy is used up, production rates drop until it becomes uneconomical to operate although significant amounts of oil still remain in the reservoir. Experience has shown that only about 12 to 15 percent of the oil in a reservoir can be produced by the expansion of the dissolved gas or existing water.

Secondary Recovery
Waterflooding 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.