An Analysis of Material-balance Calculations

Abstract

AbstractA least-square analysis procedure has been developed and applied for the studyof the deviations in estimations of oil in place as given by thematerial-balance equations. The data used were those obtained from fieldobservations on the Jones sand in the Schuler pool, Arkansas, and the Monroedolomite in the Reed City pool, Michigan. Possible effects of water intrusionwere taken into account by expressing the cumulative water influx in terms ofvarious functions previously proposed in the literature. Formulas weredeveloped and values calculated for the minimum deviations and minimalpercentage deviations in the calculated volumes of oil in place from theaverage values, as could be obtained by suitable choice of the waterencroachment and initial gas-cap volume parameters.The results show that the fluctuations in these calculated volumes are veryinsensitive to the values chosen for the rate of water intrusion and initialgas-cap volume, and hence cannot be safely used in discriminating betweenspurious values for these parameters and such as actually pertain to theproducing reservoirs. Moreover, the exact form of the water-intrusion functionseems to have but little effect on the fluctuations in the computed oil inplace.Accordingly, it is concluded that the material-balance method does not ofitself provide a satisfactory criterion for determining the basic physicalunknowns of producing reservoirs or in making conclusive decisions regardingthe production mechanism, unless independently established geological controldata are available for eliminating unreasonable values for the reservoirparameters, even though they may still satisfy the material-balance formulas.If, however, from core or logging data the initial oil in place and the gas-capvolume can be established in advance, the material-balance formulas can beinverted to find the water intrusion. Moreover, by determining by suchcalculations the water-intrusion coefficient, the future production performancecan then be predicted by the aid of the material-balance formulas for specifiedoperating conditions. This type of application may be of particular value inpredicting the results of pressure maintenance operations.IntroductionThe evaluation of the forces acting upon an oil reservoir is a problem ofconsiderable interest to the operator as an aid in determining the propermethod of operation for the reservoir to obtain an optimum recovery of oil. Inmany pools, the nature and magnitude of the forces acting upon the reservoirremain unknown until an appreciable amount of ultimate recovery is obtained. Tobe of value to the operator, a reliable interpretation of reservoir performanceshould be available in the early life of the pool to permit an optimum plan ofoperation. Progress has been made in an understanding of reservoir behaviorand. the importance of the different factors involved. The material-balancemethod of estimating oil in place has been proposed as a tool in analyzingreservoir-performance problems.T.P. 1780