In his classic text, Physical Principles of Oil Production, Morris Muskat devotes a Chapter to “Secondary Recovery,” a term that for decades would refer to operations following the natural or “primary” recovery associated with depletion of an oil field’s initial energy. Pressure maintenance referred to operations whose objective was primarily to abate the depletion of reservoir energy but not necessarily increasing volumetric sweep. Muskat states that in the case of waterflooding “…the rate of fluid injection…generally exceeds the volumetric withdrawals virtually through the whole course of the operations.” Muskat’s illustrations include technologies for modeling fluid injection and displacement long abandoned in favor of numerical computation; however, the fundamental technical issues in waterflooding were clearly established by 1949. His examples included the Bradford field discovered in 1871 which produced 80% of the United States crude oil in 1881. Deliberate waterflooding commenced in this field in 1921 and by 1946 over 235 million barrels of waterflood recovery had been obtained.
Because crude oil prices fluctuated between $2 and $4 from 1947 to 1972, waterflooding was largely applied in circumstances where there was a great deal of unrecovered crude oil at reasonably shallow depths, with relatively light crudes and reasonably high permeabilities. The massive East Texas field was a classic example of such a success. Advanced techniques for designing waterfloods along with predicting and optimizing their behavior form a mainstay of petroleum reservoir engineering.
The two essential factors in waterflood recovery are volumetric and microscopic displacement efficiency. Microscopic displacement efficiency is the reduction in oil saturation in those rock volumes that have been swept by injected water. The relevant saturations are the oil saturations prior to waterflooding and the residual oil saturation to waterflooding (Sorw). In the cases of certain enhanced recovery processes (e.g. miscible displacement and surfactant flooding) the residual oil saturation may be reduced below Sorw. Volumetric sweep efficiency is the fraction of the reservoir that is swept by injected water. Obviously if the entire reservoir volume was swept and the oil saturations reduced all the way to Sorw, the theoretical maximum waterflood recovery will have been achieved. This recovery can be referred to as the “technical limit” of recovery although there are inconsistent definitions for the technical limit in use.
A litany of technical factors obviate achieving such high recovery factors. These include:
• Reservoir heterogeneities (with injected fluids following high conductivity paths and paths that have been previously swept with water —thus having higher effective permeability to water),
• High oil viscosities (increasing the mobility ratio and increasing viscous fingering),
• Layering of the reservoir,
• Gravity segregation of injected water,
• Low permeabilites (slowing down injected fluids hurting economics as well as generally having unfavorable volumes of mobile oil)
(Next blog: the role of the asset team in waterflood optimization)
Oil well image courtesy US DOE