The Gillis-English Bayou Field is a multi-pool retrograde gas condensate reservoir. Fourteen years of gas cycling has shown that elaborate engineering-management techniques can deter inter-pool flow, and increase liquid recovery. Recoveries prior to blowdown have exceeded seventy percant for swept portions of this reservoir.
Can multi-pool reservoirs with good communication between pools be gas cycled? The literature abounds with papers written about cycling retrograde condensate reservoirs. The vast majority of these papers deal with simple single zone accumulations or pools. The Gillis-English Bayou Field in Calcasieu Parish, Louisiana is not simple. It consists of four separate sands folded and faulted into nine distinct hydrocarbon pools, all underneath an overlying unconformity. PVT analysis indicated that severe retrograde condensation would occur below 7050 psia (48600 kPa), and gas cycling was initiated.
It soon became apparent the unconformity was a path for gas flow if a pressure difference developed between pools. This was recognized by Sun, and an elaborate reservoir management system was initiated and carried out over a fourteen year period. This has minimized inter-pool flow and resulted in highly successful liquid recoveries.
The driller's bit first penetrated the Upper Hackberry sands in 1961, at 9000' (2750 m) depths. More than one dry hole established the extremely erratic limits of production. A cross section of the reservoir is shown in figure 1. Four layered sands have an overlying unconformity and plunge down structure to water. The structure map for the Hackberry 1 sand, figure 2, shows separate pools or accumulations for each sand. Figure 3, a vertically, extended diagram, demonstrates the size and shape of nine pools, and the locations of all wells drilled. Some wells cut more than one accumulation, but no pool has over four wells.
Basic reservoir data is shown in table 1. The sands generally have non-uniform permeabilities ranging from 200 to 1800 md. Early PVT data indicated that eight of the nine accumulations were in the gas phase, with dew points ranging from 7050 psia (48600 kPa) to 6645 psia (45800 kPa). The last pool, 1NE, was in the oil phase with a 7290 psia (50300 kPa) bubble point. Initial production from all zones tested about 3500 CF/BBL (62 m3 gas/m3 liq.) gas-liquid ratio.
Further experimentation showed severe retrograde condensation took place when the reservoir pressure was allowed to decrease (table 2). Calculations using laboratory data indicated that only 16 percent of the condensate would be recovered by conventional depletion methods. Production through 1964 had decreased the average reservoir pressure from an initial 7290 psia (50300 kPa) to 7060 psia (48800 kPa), just above the dew point.