Variations in Reservoir Pressure in the East Texas Field

Abstract

The nature of the forces that are important in producing the oil in the EastTexas field and the manner in which these forces are affected by the rate ofproduction have always been problems of outstanding interest. These questionsare of great interest to the industry as a whole on account of the economicimportance of the East Texas field and to the engineers in particular onaccount of the unique character and behavior of the field. Subsurface pressuredata throughout the field have been collected systematically, so that a quitecomplete history of the reservoir pressures has been kept as the field has beendeveloped and produced. Subsurface samples of the oil and gas have been taken and these have beenexamined to obtain data on the physical properties of the reservoir fluidsunder the actual conditions of temperature and pressure. All of these data havebeen valuable because they have afforded the basis for a careful study of the means by which the oilis produced. It is the purpose of this discussion first to point out several ofthe important facts revealed by the data mentioned and then to describe studiesthat have been made concerning the interpretation of these facts. Reservoir Pressure Data and Properties of Oil The reservoir pressure data on the East Texas field have shown conclusivelythat the pressures are readily affected by changes in the rate at which thefield is produced. This can well be seen in Fig. 1, where the average reservoirpressures obtained from time to time are plotted with the daily productionrates and the cumulative production of the field. It is noted that the averagereservoir pressure has declined irregularly, depending upon the rate ofproduction, from the estimated original value of 1625 lb. per sq. in. to thevalue of 1228 lb. per sq. in. observed for Aug. 11, 1934. The decline inpressure has been more rapid at the higher rates of production than at thelower rates. In contrast, the average reservoir pressure has increased, onseveral occasions, upon reduction of the withdrawal rate from a high value to alower value.