![]() Additional recovery methods were needed to produce the large quantity of oil that remained. Very early on, it was recognized that in most reservoirs, only a small percentage of the original oil in place (OOIP) was being recovered during the primary-production period because of depletion of the reservoirs’ natural energy. In addition to the need to dispose of saline water that was produced along with the oil, several other factors made waterflooding a logical and economical method for increasing recovery from oil fields. Also, in the U.S., thousands of wells had been drilled that were closely spaced, so that the effects of water injection were more obvious and so were more quickly understandable. By the mid-1940s, the onshore US oil industry was maturing and primary production from many of its reservoirs had declined significantly, whereas most reservoirs elsewhere in the world were in the early stages of primary production. Much of waterflooding’s technology and common practice developed in the U.S. In the 1920s, besides the line flood, a "five-spot" well layout was used (so named because its pattern is like that of the five spots on a die). There, the initial "circle-flood" approach was replaced by a "line flood," in which two rows of producing wells were staggered on both sides of an equally spaced row of water-injection wells. Reinjection of water was first done systematically in the Bradford oil field of Pennsylvania, US. By the 1930s, reinjection of produced water had become a common oilfield practice. In the 1920s, the practice began of reinjecting the produced water into porous and permeable subsurface formations, including the reservoir interval from which the oil and water originally had come. ![]() This water typically was disposed of by dumping it into nearby streams or rivers. In the early days of the oil industry, saline water or brine frequently was produced from a well along with oil, and as the oil-production rate declined, the water-production rate often would increase. Over the past 40 years, SPE has published three significant and in-depth books written by Craig, Willhite, and Rose et al. ![]() In these cases, the high porosity of the unconsolidated sandstones of the Wilmington oil field’s reservoirs and of the soft chalk reservoir rock in the Ekofisk oil field had compacted significantly when the reservoir pressure was drawn down during primary production. In oil fields such as Wilmington (California, US) and Ekofisk (North Sea), voidage replacement also has been used to mitigate additional surface subsidence. The water displaces oil from the pore spaces, but the efficiency of such displacement depends on many factors (e.g., oil viscosity and rock characteristics). This is accomplished by "voidage replacement"-injection of water to increase the reservoir pressure to its initial level and maintain it near that pressure. The principal reason for waterflooding an oil reservoir is to increase the oil-production rate and, ultimately, the oil recovery.
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