GAS RELATIVE PERMEABILITY AND FLOW BEHAVI- OUR IN A RETROGRADE GAS/CONDENSATE RESER- VOIR UNDER DEPLETION

摘要

The dropout of retrograde liquid when depleting a gaslcondensate reservoir below the dew point may lead to alterations of the gas flow behaviour. Near the wellbore, both gas and liquid condensate are expected to flow, resulting in a trapped gas saturation, whilst further out in the reservoir, the. hydrocarbon liquid may be immobile until a critical con- densate saturation is reached. Because of its importance in field development, gas relative permeabilities at various hy- drocarbon liquid saturations, together with critical condensate and trapped gas satura- tions, were determined for a North Sea gas/condensate field. Specially designed equip- ment was constructed for this purpose, and measurements were conducted under con- stant composition expansion on a long vertical composite core. Synthetic fluids and then real reservoir fluids at the appropriate conditions of pressure and temperature were used. To aid in the interpretation of the experiments, compositional simulations, were also car- ried out. For the case of the system with real reservoir fluids, the balance between gravitational and capillary forces was found to be of the greatest importance determining the disti- bution of the liquid condensate in the composite core. The experiments, combined with numerical simulation, showed that, when the pressure has dropped sufficiently below the dew point, capillarity replaces gravity as the dominant force governing the distribution of liquid in the composite core. This explains the some- what unusual shape of the measured gas relative permeability curve vs. liquid saturation. The force dependence on the flow behaviow is enhanced by the vertical flow in the het- erogeneous core. The study also yielded realistic values of critical condensate and trapped gas saturations.