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To obtain waterflood performance with time for a five-spot pattern the approach of Craig, Geffen, and Morse for relating oil recovery and producing WOR to cumulative injected water is coupled with the correlation of Caudle and Witte for calculating five-spot water injection rates.
This method of predicting five-spot water injection performance is valid either with or without free gas present, provided that there is no trapped gas behind the flood front. The calculations, however, are not valid for floods in which there is bottom water present.
These calculations assume a vertical sweep efficiency of 100 percent in each layer, ie., the fluids are not segregated by gravity.
For stratified reservoirs with layers of different water-oil relative permeability characteristics, the performance of each layer must be calculated individually.
The performance of a waterflood can be divided into four stages.
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The period of radial flow out from the injectors from the start of injection until the oil banks, formed around adjacent injectors, meet. The meeting of adjacent oil banks is termed "interference". |
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The period from interference until fillup of the pre-existing gas space. Fillup is the start of oil production response. |
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The period from fillup to water breakthrough at the producing wells. Breakthrough marks the beginning of water production. |
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The period from water breakthrough to floodout. |
For a multi layered system, Craig proposed performing the calculations for one selected layer, identified as the base layer. The performance of each of the remaining layers is obtained by “sliding the timescale”, and summing the individual layer performance variables to calculate the composite multi layer performance. See Composite Layer Performance.
References:
Craig, Jr., F., "The Reservoir Engineering Aspects of Waterflooding", Society of Petroleum Engineers, 1971
Craig, F., Geffen, T., and Morse, R., “Oil Recovery Performance of Pattern Gas or Water Injection Operations from Model Tests” JPT, Jan. 1955
Caudle, B., and Witte, M., “Production Potential Changes During Sweep-out in a Five-Spot System” AIME, 1959, Vol. 216
Ahmed, T., “Reservoir Engineering Handbook”, Elsevier, 2006
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