In the study of gas reservoirs in the preceding section, it was implicitly assumed that the fluid in the reservoir at all pressures as well as on the surface was in a single (gas) phase. Most gas reservoirs, however, produce some hydrocarbon liquid, commonly called condensate, in the range of a few to a hundred or more barrels per million standard cubic feet. So long as the reservoir fluid remains in a single (gas) phase, the calculations of the previous sections may be used, provided the cumulative gas production Gp is modified to include the condensate liquid production. On the other hand, if a hydrocarbon liquid phase develops in the reservoir, the methods of the previous sections are not applicable, and these retrograde, gas-condensate reservoirs must be treated specially.
The reservoir gas production Gp used in the previous sections must include the separator gas production, the stock-tank gas production, and the stock-tank liquid production converted to its gas equivalent (GE). Figure 4.4 illustrates two common separation schemes, one of which, the two-stage system, shown in Fig. 4.4(b). Figure 4.4(a) shows a three-stage separation system with a primary separator, a secondary separator, and a stock tank. The well fluid is introduced into the primary separator where, like the two-stage system, most of the produced gas is obtained. The liquid from the primary separator is then sent to the secondary separator where an additional amount of gas is obtained. The liquid from the secondary separator is then flashed into the stock tank. The liquid from the stock tank, Np, and any gas from the stock tank are added to the primary and secondary gas to obtain the total produced surface gas, Gp(surf).
Figure 4.4 Schematic representation of surface separation systems.
The produced hydrocarbon liquid is converted to its gas equivalent, assuming it behaves as an ideal gas when vaporized in the produced gas. Taking 14.7 psia and 60°F as standard conditions, the gas equivalent of one stock-tank barrel of condensate liquid is
The gas equivalent of one barrel of condensate of specific gravity of 0.780 (water = 1.00) and molecular weight 138 is 752 SCF. The specific gravity may be calculated from the API gravity. If the molecular weight of the condensate is not measured, as by the freezing point depression method, it can be estimated using Eq. (4.20):
The total gas equivalent for Np STB of condensate production is GE(NP). The total reservoir gas production, Gp, is given by Eq. (4.21) for a three-stage separation system and by Eq. (4.22) for a two-stage separation system:
When water is produced on the surface as a condensate from the gas phase in the reservoir, it is freshwater and should be converted to a gas equivalent and added to the gas production. Since the specific gravity of water is 1.00 and its molecular weight is 18, its gas equivalent is
Studies by McCarthy, Boyd, and Reid indicate that the water vapor content of reservoir gases at usual reservoir temperatures and usual initial reservoir pressures is in the range of a fraction of one barrel per million standard cubic feet of gas.10 Production data from a Gulf Coast gas reservoir show a production of 0.64 barrel of water per million standard cubic feet compared with a reservoir content of about 1.00 bbl/MM SCF using the data of McCarthy, Boyd, and Reid. The difference is presumably that water remaining in the vapor state at separator temperature and pressure, most of which must be removed by dehydration to a level of about 6 pounds per million standard cubic feet. As reservoir pressure declines, the water content increases to as much as three barrels per million standard cubic feet. Since this additional content has come from vaporization of the connate water, it would appear that any freshwater produced in excess of the initial content should be treated as produced water and taken care of in the Wp term rather than the Gp term. If the water is saline, it definitely is produced water; however, it includes the fraction of a barrel per million cubic feet obtained from the gas phase. If the produced gas is based on the dehydrated gas volume, the gas volume should be increased by the gas equivalent of the water content at the initial reservoir pressure and temperature, regardless of the subsequent decline in reservoir pressure, and the water production should be diminished by the water content. This amounts to about a 0.05% increase in the produced gas volumes.
Leave a Reply