Re-establishing Reservoir Performance by Revaporization of Condensate in the Nubian Sandstone Reservoir

The Nubian Formation is a significant hydrocarbon reservoir in the NC 98 Concession of Libya. It consists of a low to medium permeability, fine-grained sandstone. Reservoir fluids in the Nubian are either very rich retrograde condensate gas or volatile oil. In either case, the reservoir pressure is at or slightly above the saturation pressure. Because the gas-containing reservoirs are very rich in condensate, small decreases in pressure often result in high volumes of condensate accumulation. Similarly, for the volatile oil case, small decreases in pressure result in large fractions of the volatile oil turning into the vapor phase. Both of these phenomena can result in adverse relative permeability effects which can reduce the permeability to the primary hydrocarbon being exploited.

Laboratory experiments were conducted to characterize the phase behavior of the reservoir fluids. Conventional core analysis provided good coverage of the petrophysical description of the reservoir rock. Subsequent core flow experiments determined baseline flow to reservoir gas followed by the impairment to permeability at a series of pressure decrements. The pressures modeled in the laboratory were chosen to closely approximate anticipated field conditions. Laboratory results indicate a moderate to severe effect on permeability, caused primarily by the high critical saturation of condensate necessary for flow of condensate to occur. To mitigate the effect of condensate on permeability, laboratory experiments were performed to determine the viability of condensate removal through revaporization by gas injection.