Information obtained from subsurface pressure and temperature measurementsin oil wells now is recognized as invaluable to geologists, engineers, andoperators of oil properties. Available only during the past few years, dataresulting from subsurface-pressure surveys in oil wells already have foundwidespread application in the estimation of reserves and in the, planning ofcomprehensive oil-field development programs. Subsurface pressure andtemperature data, together with measurements of fluid volumes produced byindividual wells and specific gravities of the fluids, also are included in thefundamental information essential in, studying the movement of oil, gas andwater through reservoir sands to wells. Heretofore, laboratory findings wererelied upon almost entirely for information, pertaining to the mechanics offlow of fluids through porous media, because there was no way to checklaboratory results with actual performance in underground formations untilsubsurface pressure and temperature instruments were developed. Laboratory andfield experiments now lend themselves to closer correlation-a fact that willaccelerate the interpretation of many phenomena that occur when oil and gasflow through reservoir rocks, thousands of feet below the surface of theearth.

This paper summarizes the results obtained thus far in a Bureau of Mines studyof certain data pertaining to subsurface pressures and production of wellsproducing oil and gas (and water). For wells that have been studied, flowingnaturally under volumetric or hydraulic control, a simple relation has beenfound between the mass rate of fluid (oil, gas and water) production., and thecorresponding drop in pressure in the reservoir sand. The equated relationembodies a productivity factor or ?index? expressed in terms of mass rate offluid production per pound drop in pressure through the sands. The equation asdeveloped is an expression of the ability of a well to produce, which permitscomparison on a uniform basis of the productivities of different wells.

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