Hysteresis Effects for Gas Condensate Wells Undergoing Buildup Tests below the Dew Point Pressure

Abstract

Abstract An increasing liquid saturation is observed in gas condensate reservoirs when they undergo drawdown. This liquid saturation is not reversible when the well is subjected to a pressure buildup thus resulting in a hysteresis effect. Hence, once reduced, the effective reservoir permeability does not recover and in a subsequent drawdown, the flow rate remains at largely the same level as before the shut in. This hysteresis effect is examined and quantified. An existing method to interpret the pressure behavior of gas condensate reservoirs is critiqued and is found appropriate for the time when "steady-state" saturation emerges and as long as relative permeability data are available. A new method allows the calculation of the "apparent" reservoir permeability at steady state and when employing a step-pressure test then relative permeability data may be obtained from the interpretation. Introduction The pressure and flow rate behavior of gas condensate wells is distinctly different from that of all other two-phase fluids. When the flowing bottomhole pressure in a gas condensate reservoir drops below the dewpoint pressure, significant decreases in productivity can be expected. This reduction is attributable to a decrease in the gas relative permeability caused by the accumulation of condensate as a pressure gradient is formed in the reservoir. Of particular interest is the irreversible hysteresis phenomenon that is exhibited in gas condensate reservoirs. A "hysteresis criterion" may be introduced here. Assuming that a pore volume, Vp, is occupied initially by gas then the mass of this gas may be given by: (1) If, after production, the pressure drops to p and the gas condensate saturation is S,, then the pore volume occupied by condensate and gas will be VPS, and Vp (1 - S,), respectively. The mass of condensate is then mc = PC Vp SC (2) and the mass of the gas is (3) If (4) or (5) then hysteresis will be observed. In other words, although a pressure buildup would indicate a revaporization based on PVT properties, the condensate mass accumulation and the reservoir pressure gradient precludes reverse fluid migration into the reservoir. This liquid accumulation results in permanent "damage" and the recovery of the flow rate will not be realized in a drawdown following a pressure buildup. This phenomenon was described in detail by Fussell. In pressure transient analysis this apparent permeability reduction has been "described" either by a pseudoskin factor or by an average reservoir permeability reduction. In both instances the problem lies with the ever expanding zone of condensate accumulation. As such, the concept of the pseudoskin is not appropriate, especially in view of the continuously changing liquid saturation. Instead, the concept of apparent reservoir permeability reduction is preferable. P. 75^