The initial gas and oil (condensate) for gas-condensate reservoirs, both retrograde and nonretrograde, may be calculated from generally available field data by recombining the produced gas and oil in the correct ratio to find the average specific gravity (air = 1.00) of the total well fluid, which is presumably being produced initially from a one-phase reservoir. Consider the two-stage separation system shown in Fig. 1.3. The average specific gravity of the total well fluid is given by Eq. (5.1):
where
R1, R3 = producing gas-oil ratios from the separator (1) and stock tank (3)
γ1, γ3 = specific gravities of separator and stock-tank gases
γo = specific gravity of the stock-tank oil (water = 1.00), given by
Mwo = molecular weight of the stock-tank oil that is given by Eq. (4.20):
Example 5.1 shows the use of Eq. (5.1) to calculate the initial gas and oil in place per acre-foot of a gas-condensate reservoir from the usual production data. The three example problems in this represent the type of calculations that an engineer would perform on data generated from laboratory tests on reservoir fluid samples from gas-condensate systems. Sample reports containing additional example calculations may be obtained from commercial laboratories that conduct PVT studies. The engineer dealing with gas-condensate reservoirs should obtain these sample reports to supplement the material. The gas deviation factor at initial reservoir temperature and pressure is estimated from the gas gravity of the recombined oil and gas, as shown. From the estimated gas deviation factor and the reservoir temperature, pressure, porosity, and connate water, the moles of hydrocarbons per acre-foot can be calculated, and from this, the initial gas and oil in place.
Example 5.1 Calculating the Initial Oil and Gas in Place per Acre-Foot for a Gas-Condensate Reservoir
Given
Initial pressure = 2740 psia
Reservoir temperature = 215°F
Average porosity = 25%
Average connate water = 30%
Daily tank oil = 242 STB
Oil gravity, 60°F = 48.0 °API
Daily separator gas = 3100 MCF
Separator gas specific gravity = 0.650
Daily tank gas = 120 MCF
Tank gas specific gravity = 1.20
Solution
From Eqs. (2.11) and (2.12), pc = 636 psia and Tc = 430°R. Also, Tr = 1.57 and pr = 4.30, from which, using Fig. 2.2, the gas deviation factor is 0.815 at the initial conditions. Thus the total initial gas in place per acre-foot of bulk reservoir is
Because the volume fraction equals the mole fraction in the gas state, the fraction of the total produced on the surface as gas is
Thus
Because the gas production is 95.1% of the total moles produced, the total daily gas-condensate production in MCF is
The total daily reservoir voidage by the gas law is
The gas deviation factor of the total well fluid at reservoir temperature and pressure can also be calculated from its composition. The composition of the total well fluid is calculated from the analyses of the produced gas(es) and liquid by recombining them in the ratio in which they are produced. When the composition of the stock-tank liquid is known, a unit of this liquid must be combined with the proper amounts of gas(es) from the separator(s) and the stock tank, each of which has its own composition. When the compositions of the gas and liquid in the first or high-pressure separator are known, the shrinkage the separator liquid undergoes in passing to the stock tank must be measured or calculated in order to know the proper proportions in which the separator gas and liquid must be combined. For example, if the volume factor of the separator liquid is 1.20 separator bbl per stock-tank barrel and the measured gas-oil ratio is 20,000 SCF of high-pressure gas per bbl of stock-tank liquid, then the separator gas and liquid samples should be recombined in the proportions of 20,000 SCF of gas to 1.20 bbl of separator liquid, since 1.20 bbl of separator liquid shrinks to 1.00 bbl in the stock tank.
Example 5.2 shows the calculation of initial gas and oil in place for a gas-condensate reservoir from the analyses of the high-pressure gas and liquid, assuming the well fluid to be the same as the reservoir fluid. The calculation is the same as that shown in Example 5.1, except that the gas deviation factor of the reservoir fluid is found from the pseudoreduced temperature and pressure, which are determined from the composition of the total well fluid rather than from its specific gravity. Figure 5.3 presents charts for estimating the pseudocritical temperature and pressure of the heptanes-plus fraction from its molecular weight and specific gravity.
Figure 5.3 Correlation charts for estimation of the pseudocritical temperature and pressure of heptanes plus fractions from molecular weight and specific gravity (after Mathews, Roland, and Katz, proc. NGAA).7
Example 5.2 Calculating the Initial Gas and Oil in Place from the Compositions of the Gas and Liquid from the High-Pressure Separator
Given
Reservoir pressure = 4350 psia
Reservoir temperature = 217°F
Hydrocarbon porosity = 17.4%
Standard conditions = 15.025 psia, 60°F
Separator gas = 842,600 SCF/day
Stock-tank oil = 31.1 STB/day
Molecular weight 
in separator liquid = 185.0
Specific gravity in separator liquid = 0.8343
Specific gravity separator liquid at 880 psig and 60°F = 0.7675
Separator liquid volume factor = 1.235 bbl/STB at 880 psia, both at 60°F
Compositions of high-pressure gas and liquid = Table 5.2, columns 2 and 3
Table 5.2 Calculations for Example 5.2 on Gas-Condensate Fluid
Molar volume at 15.025 psia and 60°F = 371.2 ft3/mol
Solution
Note that column numbers refer to Table 5.2:
1. Columns 1, 2, and 3 are given. This information typically comes from a lab test performed on a sample taken from the separator. Column 4 is additional information that can also be found in Table 2.1. Using this information, calculate the mole proportions in which to recombine the separator gas and liquid. Multiply the mole fraction of each component in the liquid (column 3) by its molecular weight (column 4) and enter the products in column 5. The sum of column 5 is the molecular weight of the separator liquid, 127.48. Next, the ratio of liquid barrel per mole is needed for each component. This information is also found in Table 2.1. The last column of Table 2.1 is the estimated gal/lb-mol—these data will need to be converted to bbl/mol. The next several steps are used to match the quantity of produced liquid to produced gas and determine the composition of the entire well fluid rather than just the liquid or gas. Because the specific gravity of the separator liquid is 0.7675 at 880 psig and 60°F, the moles per barrel is
The separator liquid rate is 31.1 STB/day × 1.235 sep. bbl/STB so that the separator gas-oil ratio is
Because the 21,940 SCF is 21,940/371.2, or 59.11 mols, the separator gas and liquid must be recombined in the ratio of 59.11 mols of gas to 2.107 mols of liquid.
If the specific gravity of the separator liquid is not available, the mole per barrel figure may be calculated as follows. Multiply the mole fraction of each component in the liquid, column 3, by its barrel per mole figure, column 6, obtained from data in Table 2.1, and enter the product in column 7. The sum of column 7, 0.46706, is the number of barrels of separator liquid per mole of separator liquid, and the reciprocal is 2.141 mols/bbl (versus 2.107 measured).
2. Now that the ratio of the gas to liquid produced is known, recombine 59.11 mols of gas and 2.107 mols of liquid. Multiply the mole fraction of each component in the gas, column 2, by 59.11 mols, and enter in column 8. Multiply the mole fraction of each component in the liquid, column 3, by 2.107 mols, and enter the solution in column 9. Enter the sum of the moles of each component in the gas and liquid, column 8, plus column 9, in column 10. Divide each figure in column 10 by the sum of column 10, 61.217, and enter the quotients in column 11, which is the mole composition of the total well fluid. Column 12 is the critical pressure for each component; it is also found in Table 2.1. With that information, the partial critical pressure (column 13) can be found. The same will be done for columns 14 and 15 for temperature. Calculate the pseudocritical temperature 379.23°R and pressure 668.23 psia from the composition by summing the partial temperature and partial pressure values for each component. From the pseudocriticals, find the pseudoreduced criticals and then the deviation factor at 4350 psia and 217°F, which is 0.963.
3. Find the gas and oil (condensate) in place per acre-foot of net reservoir rock. From the gas law, the initial moles per acre-foot at 17.4% hydrocarbon porosity is
Because the high-pressure gas is 96.6% of the total mole production, the daily gas-condensate production expressed in standard cubic feet is
The daily reservoir voidage at 4350 psia is
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