The Effect of Well Spacing and Drawdown on Recovery From Internal Gas Drive Reservoirs

Abstract

Abstract Theoretical calculations for the decline of pressure and the variation ofinstantaneous producing gas-oil ratio with increased cumulative production havebeen made for reservoir systems under various producing drawdown values and forvarious spacings. These variables have been included by a modification of previously published techniques. Thisconsists essentially of utilizing a radial system flowing gas and oil in asuccession of steady state calculations. The results of the calculations do notshow any variation in the predicted behavior of the internal gas drivereservoir when either the drawdown or the spacing are changed. Introduction The prediction of performance on an internal gas drive reservoir has beenpresented by various authors. These predictions are in the form of a pressuredecline and an instantaneous producing gas-oil ratio as a function ofcumulative oil withdrawal. In making these predictions a basic assumption hasbeen that the saturation of the reservoir is uniform throughout at any time, orthat the pressure differential imposed on the reservoir system is zero. Becauseof these assumptions the validity of such predictions has been questioned.Their application to actual systems, where saturations are not uniform andwhere producing differentials exist, has been a hazardous procedure. The authors cited have used different methods of approach to the derivation ofequations for making their predictions and for arriving at a solution of bothequations, but basically the same relationships are utilized in each case. Inall instances the instantaneous producing gas-oil ratio is calculated from theequation: (Equation 1) In utilizing this equation an average reservoir pressure and an averagereservoir saturation must be known. Predictions can be made, therefore, onlyfor the conditions of a uniform reservoir saturation and a uniform reservoirpressure. This restriction of requiring a uniform oil saturation can be resolved byapplying two-phase steady state flow for the radial system to the reservoir'shistory. Radial steady state flow is defined when the ratio of phases flowing, the pressures imposed, and the dimensions of the system are fixed. T.P. 2592