Relation Between Pressure and Recovery in Long Core Water Floods

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Abstract
Conclusions drawn by previous research workers with respect to the relationbetween pressure gradients and/or velocity and oil recovery obtained bylaboratory water flood tests have been in disagreement probably due to variableprocedures and unnatural conditions and materials. The Bradford Laboratory ofthe Pennsylvania Grade Crude Oil Association as part of its secondary recoveryresearch program has conducted nineteen water floods on two long cores ofwidely differing characteristics in an attempt to clarify this relationship andmake it an aid in predicting flooding pressures in the field. Unlike previousresearch procedures the present experiments were conducted with the aim ofduplicating field conditions as closely as possible by using long unextractedconsolidated cores, a live crude, and natural brines for both flooding andconnate water content. Also, the pressure gradients and flooding velocitieswere representative of field conditions where similar sands were being flooded.Eleven floods on one core and eight floods on the other core showed increasedrecoveries and lower residual oil saturation with increased flood pressuregradients and flood velocities. A marked decrease in recovery was obtained fromboth cores at very low flood velocities. This pressure versus recoveryrelationship is shown to hold up to the point of water breakthrough and also upto the 100 and 1 produced water to oil ratio point. Introduction The possibility of water flooding oil sands was suggested by Carll of thePennsylvania Geological Survey in 1880. It is not known when the practice wastried intentionally for the first time, but its beneficial effects were notedin the annual production rate of the Bradford field as early as 1907. Thepractice was illegal in Pennsylvania until 1921. Early water floods in theBradford field usually consisted of shooting or splitting the casing secretlyto permit subsurface waters to enter the producing sands under hydrostatichead. As it was noted that the benefits of water flooding seemed to beproportional to the quantity of water dumped into the well many also began toutilize surface sources after the practice became legal. It was probably duringthe middle 20's before many producers realized that the pressurehead of thewater upon the producing sand determined the rate and quantity of water thatwould enter the sand. Hence, rate and quantity of production appeared to be adirect function of input pressures. T.P. 2591

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