Cycling Plant

Key Takeaways

• Retrograde condensate loss economics justify the substantial capital investment of a cycling plant in reservoirs with high condensate-gas ratios (CGR) and high condensate content in the reservoir gas: the condensate yield from a retrograde condensate reservoir is typically expressed in barrels of condensate per million standard cubic feet of gas (bbl/MMSCF), with lean gas condensate reservoirs yielding 5-30 bbl/MMSCF and rich gas condensate reservoirs yielding 30-300 bbl/MMSCF; at high condensate prices (typically 80-95% of WTI crude price) and high CGRs, the value of the liquid condensate recovered during cycling is far higher per unit volume than the value of the dry gas sold to market, making the incremental condensate recovery from pressure maintenance the primary economic driver of the cycling plant; reservoir simulation studies of retrograde condensate fields consistently show that well-designed gas cycling (at injection rates sufficient to maintain reservoir pressure at or above the dew point throughout the cycling period) recovers 70-90% of the original condensate in place, compared to 40-60% recovery without cycling, with the difference representing the condensate that would have dropped out irreversibly within the reservoir under natural depletion; the incremental condensate recovery from cycling must exceed the net present value of the alternative use of the reinjected gas (either immediate sale or compression for another purpose) to justify the cycling plant capital cost, and the breakeven analysis typically favors cycling in rich gas condensate fields and does not favor it in lean gas fields where the condensate premium does not compensate for the deferred gas sales.
• Cycling plant process design involves gas compression, condensate extraction, and lean gas reinjection in a continuous loop that handles the total produced gas stream from the field and returns the lean gas (after condensate removal) to the reservoir at a pressure sufficient to maintain reservoir pressure at or above the cycling target: the wellstream produced from the condensate wells flows through the wellhead and gathering system to the cycling plant inlet, where it is processed through one or more stages of separation (high-pressure and low-pressure separators) that extract the condensate and produced water before the remaining gas enters the compression system; the lean gas is recompressed to reservoir injection pressure (which may require 3-5 stages of centrifugal or reciprocating compression for deep, high-pressure reservoirs with initial pressures of 5,000-15,000 psi) and injected through dedicated injection wells drilled into the crestal position of the reservoir above the producing wells (crestal injection maintains the gas pressure across the full reservoir extent and provides a favorable displacement geometry for the lean gas pushing the condensate-rich gas toward the producers); the condensate extracted by separation (containing C5+ natural gasoline plus C3-C4 liquid petroleum gas components) is stabilized (by removing the dissolved gas and light components to meet vapor pressure specifications), stored, and shipped to market via pipeline or truck; the cycling plant is designed for continuous operation with high reliability (typical availability targets of 97-99%) because any shutdown reduces the reservoir pressure maintenance, allowing retrograde condensation that directly and irreversibly reduces the condensate recovery.
• Lean gas breakthrough at producing wells marks the end of the effective cycling period (when the reinjected lean gas, which has swept through the reservoir and displaced the condensate-rich gas, begins to arrive at the producers and dilutes the produced gas stream with gas that has already had its condensate extracted): lean gas breakthrough is detected by a decline in the condensate-gas ratio of the produced gas stream (the CGR drops as lean gas dilutes the condensate-rich native gas), an increase in the dryness of the produced gas (the specific gravity and heating value decrease as the heavy components are replaced by lean methane-dominated reinjected gas), and in some cases by tracer breakthrough analysis using chemical tracers added to the reinjected gas to confirm the source of the breakthrough gas; the breakthrough geometry depends on the reservoir heterogeneity (high-permeability channels allow early breakthrough of the lean gas front while lower-permeability regions are bypassed), the displacement efficiency (related to the miscibility between the lean gas and the reservoir gas at reservoir conditions), and the injection-to-production well spacing and pattern; after lean gas breakthrough, the cycling plant continues to operate but achieves progressively less condensate recovery per unit of gas reinjected, and the economic evaluation must determine the optimal time to convert the cycling plant to gas blowdown (depressurization and production of all remaining gas for sale) by comparing the declining incremental condensate recovery against the operating cost of continued reinjection; many cycling plants convert to blowdown when the lean gas breakthrough has become extensive and the net condensate recovery gain from continued cycling no longer justifies the compression operating cost.
• Enhanced condensate recovery during cycling using enriched gas injection (adding propane, butane, or natural gasoline to the reinjected lean gas to create a miscible or near-miscible displacement with the reservoir condensate) can improve the displacement efficiency and delay lean gas breakthrough in heterogeneous reservoirs where the lean gas fingering through high-permeability channels reduces the volumetric sweep efficiency: enriched gas cycling works on the same miscible displacement principle as CO2 EOR in oil reservoirs, with the added intermediate molecular weight components in the injection gas achieving miscibility with the reservoir condensate at lower pressures than would be required with pure methane injection, potentially eliminating the interfacial tension between the injected and in-place gas and allowing displacement at very low residual condensate saturation; the economic benefit of enriched gas over lean gas cycling must weigh the cost of the hydrocarbon enrichment agent (propane, butane) against the incremental condensate recovery, and enriched gas cycling is most cost-effective in rich condensate reservoirs at relatively shallow depths where the miscibility pressure is achievable at reasonable compression costs; most cycling plant operations use lean gas (sales-quality gas after condensate removal) as the injection gas rather than enriched gas due to the simplicity and lower cost of lean gas injection, accepting some reduction in displacement efficiency relative to the theoretical optimum of miscible enriched gas cycling.
• Cycling plant blowdown and decommissioning after the end of the cycling period requires careful reservoir management to produce the remaining gas and associated condensate under declining reservoir pressure, potentially with supplemental condensate recovery strategies (solvent injection, well recompletion) to recover condensate that dropped out during the depletion phase: during blowdown, the reservoir pressure is allowed to decline below the dew point, and retrograde condensate dropout begins in the reservoir, reducing the relative permeability to gas and potentially causing condensate banking near the wellbore (the permeability reduction in the near-wellbore zone from condensate saturation above the critical condensate saturation impairs gas productivity, requiring periodic remediation by solvent injection through the producers to mobilize the near-wellbore condensate); methanol or solvent injection into producing wells can mobilize some of the retrograde condensate in the near-wellbore zone and restore productivity during blowdown, but the condensate deeper in the reservoir that dropped out and accumulated below the critical saturation cannot be recovered; the residual lean gas in the reservoir at the time of blowdown initiation must all be produced against the increasing backpressure of the compressor suction, requiring that the gathering and compression system be maintained in operation throughout the blowdown period until reservoir pressure declines to a level where compression costs exceed the value of the remaining gas; the timing of the transition from cycling to blowdown is one of the most important economic decisions in cycling plant operations, typically made through reservoir simulation that optimizes the combined present value of cycling operation benefits (ongoing condensate recovery) versus blowdown net present value (accelerating the deferred gas sales from the reinjected gas volumes).