Reservoir Fluid Flow and Natural Drive Mechanisms

Condensate Reservoirs: Pressure Maintenance

Pressure Maintenance by Gas Cycling

Retrograde condensation may cause a significant fraction of the liquid content of the gas condensate to be left in the reservoir. Distillates are a very valuable part of the accumulation. Because of this, pres sure maintenance above the dewpoint during the exploitation of the reservoir is commonly practiced.

One way of maintaining pressure is by injecting the dry gas component of the produced wet gas. This is what is left of the produced wet gas after the liquid has been removed at the surface. This operation is called gas cycling. The injected dry gas partially maintains reservoir pressure and at the same time becomes miscible with oil and drives the wet gas toward the producing wells. However, in some cases, the volume of the dry gas component will represent only a fraction of the produced gas volume. In such a case, if the injection volume is not supplemented with additional gas, a gradual decline of the reservoir pressure may take place, and liquid loss may result. The degree of gas volume augmentation will depend on the pres sure level of the reservoir relative to the dewpoint value.

Recovery from cycling operations depends on the cycling efficiency of the operation. This value is the product of three efficiencies: the areal sweep, E_A; the vertical sweep, E_I; and the microscopic displacement, E_D. The areal sweep efficiency is a function of the location of the wells and their rate of production and injection, and the heterogeneity of the reservoir. It is the area swept by the injected gas divided by the total area of the reservoir. The invasion or vertical ef ficiency, E_I, is a strong function of stratification and the permeability variation among the reservoir layers. E_I is the portion of a vertical section of the reservoir contacted by the injected fluid, divided by the hydrocarbon area in all layers behind the injected fluid front. If a highly permeable layer (a “thief” zone) exists, dry gas will channel through it to the producing well, resulting in a low E_I value.

Use of Material Balances

Laboratory-obtained data from the constant-volume cell described previously may be used to predict gas condensate reservoir performance. The data give increments of gross gas produced as a function of pressure as well as the liquid in each increment of gas. Thus, total gross gas and liquid production in percent of initial gas in place as a function of pressure may be calculated. Also total liquid recovery can be obtained. If a volumetric estimate for the initial gas condensate in place is available, the laboratory data may be used to give incremental recovery of gas and liquid as a function of pressure. On the other hand, if cumulative gas production for a known average reservoir pressure is given, then the laboratory data may be used to calculate the initial gas in place and subsequent recovery. For example, assume that at an average reservoir pressure of 3000 psia, G_p MMSCF of gas had been produced. From laboratory depletion data at 3000 psi, the total incremental gas recovery is some percent (say, X) of the initial gas in place. Thus, initial gas in place in the reservoir may be calculated by

……….. (34)

Knowing G, and the percent gas recovery and liquid condensation with pressure from laboratory data, reservoir performance calculation is straightforward.

Pressure Maintenance by Water Drive

Some gas condensate reservoirs are under active water drive. The water influx may be sufficient to maintain the reservoir pressure above its dew-point value in some cases. If not, it may have to be augmented by water injection. Ultimate recovery is still controlled by the three recovery efficiencies described earlier. However, in the case of water displacing gas, E_D is considerably smaller. Water tends to trap gas, resulting in a poor displacement ef ficiency. Assuming E₁ and E_A are the same, recovery by water drive is normally lower than that by gas cycling by about 20%. It should also be noted that E₁ may not be the same as that for gas cycling because of the difference in density between water and dry gas. Water tends to segregate to the bottom of the perforated interval while gas will override the top. Permeability distribution in the various layers determines whether E_VS for a water drive is less than, equal to, or greater than E_VS for gas cycling. Again, reservoir simulators are the best available tools to accurately predict performance in complicated reservoir situations.