Material Balance

The material balance problem addressed in the current version of GEMINI is that of a reservoir producing above bubble point under an additive bottom water drive. Water influx will be calculated by the Carter-Tracy formulation (constant terminal rate solution to the diffusivity equation). In an ideal situation, the user will input the history of oil and water production volumes and average reservoir pressure in a spreadsheet like format along with the PVT parameters relevant to the reservoir fluids and formation. The module will calculate and plot volume of fluids withdrawn from the reservoir (F/E) against the volume of water influx (We/E) at regular time steps. Havlena-Odeh's method of aquifer-fitting will be used to describe (approximate) and aquifer that coupled with the reservoir selected, the F/E versus We/E plot will be a straight line with unit slope. The intercept of this straight line will provide the estimate of the material-balance OOIP (volume of original oil in place). In many field studies, the average reservoir pressure history is unavailable. In such cases, the user will have to assume that the volumetric OOIP is equal to the material balance OOIP. The user has to vary the average reservoir pressures (at different time- steps), The spreadsheet layout of the module and its dynamic link to the plot will make the process of iteration less cumbersome. The average reservoir pressure history, thus back-calculated, can be validated by comparing available DST data that have been recorded over time in the field.