The concept of drive indices, first introduced to the reservoir engineering literature by Pirson, was developed.2 To illustrate the use of these drive indices, calculations are performed on the Conroe Field, Texas. Figure 7.1 shows the pressure and production history of the Conroe Field, and Fig. 7.2 gives the gas and two-phase oil formation volume factor for the reservoir fluids. Table 7.1 contains other reservoir and production data and summarizes the calculations in column form for three different periods.
Figure 7.1 Reservoir pressure and production data, the Conroe Field (after Schilthuis, trans. AlME).3
Figure 7.2 Pressure volume relations for the Conroe Field oil and original complement of dissolved gas (after Schilthuis, trans. AlME).3
Table 7.1 Material Balance Calculation of Water Influx or Oil in Place for Oil Reservoirs below the Bubble-Point Pressure
The use of such tabular forms is common in many calculations of reservoir engineering in the interest of standardizing and summarizing calculations that may not be reviewed or repeated for intervals of months or sometimes longer. The use of spreadsheets makes these calculations much easier and maintains the tabular form. They also enable an engineer to take over the work of a predecessor with a minimum of briefing and study. Tabular forms also have the advantage of providing at a glance the component parts of a calculation, many of which have significance themselves. The more important factors can be readily distinguished from the less important ones, and trends in some of the component parts often provide insight into the reservoir behavior. For example, the values of line 11 in Table 7.1 show the expansion of the gas cap of the Conroe Field as the pressure declines. Line 17 shows the values of the initial oil in place calculated at three production intervals. These values and others calculated elsewhere are plotted versus cumulative production in Fig. 7.3, which also includes the recovery at each period, expressed as the percentage of cumulative oil in the initial oil in place, as calculated at that period. The increasing values of the initial oil during the early life of the field may be explained by some of the limitations of the material balance equation particularly the average reservoir pressure. Lower values of the average reservoir pressure in the more permeable and in the developed portion of the reservoir cause the calculated values of the initial oil to be low, through the effect on the oil and gas volume factors. The indications of Fig. 7.3 are that the reservoir contains approximately 600 MM STB of initial oil and that reliable values of the initial oil are not obtained until about 5% of the oil has been produced. This is not a universal figure but depends on a number of factors, particularly the amount of pressure decline. For the Conroe Field, the drive indices have been calculated at each of three periods, as given in lines 18, 19, and 20 of Table 7.1. For example, at the end of 12 months, the calculated initial oil in place is 415 MM STB, and the value of Np[Bt + (Rp – Rsoi)Bg] given in line 14 is 131 MM ft3. Then, from Eq. (3.11),
Figure 7.3 Active oil, the Conroe Field (after Schilthuis, trans. AlME).3
These figures indicate that, during the first 12 months, 39.5% of the production was by water drive, 32.0% by gas-cap expansion, and 28.5% by depletion drive. At the end of 36 months, as the pressure stabilized, the current mechanism was essentially 100% water drive and the cumulative mechanism increased to 64.6% water drive. If figures for recovery by each of the three mechanisms could be obtained, the overall recovery could be estimated using the drive indices. An increase in the depletion drive and gas-drive indices would be reflected by declining pressures and increasing gas-oil ratios and might indicate the need for water injection to supplement the natural water influx and to turn the recovery mechanism more toward water drive.
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