The practice of reservoir pressure maintenance by gas cycling has been adopted in many retrograde (wet) gas reservoirs to prevent condensation and ensuing loss of hydrocarbon liquids.
An investigation of the factors involved in condensate recovery by gas cycling was made for this paper. The principle factors are (1) the sweep patterns developed by the cycled gas and the resulting wet gas recovery and (2) the revaporization of the liquid condensate within the reservoir upon contact by dry injected gas.
It is concluded that cycling condensate reservoirs under conditions of declining pressure rather than constant pressure is advantageous both from a recovery and an economic standpoint. By operating at declining pressure, the wet gas displaced from the swept areas is recovered concurrently withwet gas recovered by gas expansion from the unswept portions of the reservoir. Any liquid condensed in the swept areas is revaporized by dry injection gas. and recovered as an enriched gas.
By this mode of operation, high condensate recovery is obtained, gas sales may be possible at an earlier stage of depletion, more flexibility in field and plant operations is feasible, and reduction in investment and operating costs is achieved.
Calculations of reservoir performance are presented for the Windfall Devonian Leduc reef reservoir, Alberta, Canada, exploited by this scheme.
Hydrocarbon accumulations in which the fluids exhibit retrograde condensation upon reduction of reservoir pressure comprise an important and ever increasing fraction of reservoirs exploited in North America. This type of accumulation is usually described as a gas condensate or wet gas reservoir. In 1965, approximately 15% of all the reservoirs exploited in North America were of this type(l). The recovery of natural gas liquids from these accumulations must be made in the vapor phase because the liquid saturation retrograded within the pore space of the reservoir at reduced pressures is usually below the critical level at which the liquid will form a continuous phase and thus will flow or could be displaced as a liquid.
When the retrograde gas reservoir is depleted solely by pressure reduction (blow-down) some or all of the following disadvantages are apparent:
Loss of valuable condensate in the reservoir.
Declining loading of the liquid facilities of the plant during the life of the project.
Declining well productivities, which must be offset by installation of compressors and/or by drilling of additional wells.
In order to prevent, or at least reduce, such a liquid loss due to retrograde condensation, the contents of gas-condensate reservoirs are often displaced through cycling operations. The wet gas is produced, stripped of liquefiable hydrocarbons and perhaps sour gases and the residual gas is reinjected into the reservoir for the purpose of displacing further wet gas. Frequently an additional make-up volume of gas is reinjected in order to maintain full reservoir pressure and prevent any retrograde condensation at all.
Although cycling appears to be an ideal solution to the retrograde condensation problem, there are a number of factors which affect adversely this method of operation: