Effect of Data Errors on Typical Reservoir Engineering Calculations

Abstract

Abstract In reservoir analysis important quantities such as reservoir pressure, in-place oil, and well damage are calculated from directly measured quantities and each measurement error has an effect on the calculated result. This paper employs statistical methods in the quantitative consideration of the effect of these measurement errors on answers calculated by specific formulas which may be taken as typical of reservoir engineering calculations. Detailed studies have been made concerning reservoir pressure, reservoir pressure decline, initial in-place gas in distillate fields, initial in-place oil in unsaturated fields, initial in-place oil in solution gas drive fields, and well damage. A wide range of examples is presented so that the conclusions may be generally applicable. Various intangible factors which cannot be included in the equations limit the quantitative applicability of the calculated error, but these factors reinforce rather than detract from the conclusions. Particular attention is given to the effect of bottom hole pressure measurements errors. The overall conclusion from this study is that in nearly all cases the errors in bottom hole pressure measurements make the major error in reservoir engineering calculations and that in many instances an increased accuracy of the pressure instruments to ±1 psi is highly desirable. Introduction The question of what accuracy is required in the data assembled for the purpose of making reservoir analyses has been the subject of much discussion in the past. Extremes of opinion have been encountered, ranging from a belief that reservoir analysis is approximate at best and that very approximate data are sufficient to an insistence that all factors be measured to the utmost possible precision.