Depletion of two retrograde gas reservoirs is analyzed with a compositional simulation model, focusing on changes in well performance caused by liquid dropout. In one case presented, a low-permeability reservoir at its dewpoint, transient effects are shown to contribute more to well productivity decline than liquid dropout. This is demonstrated by comparing the well productivity with that of an otherwise identical dry gas well. The evolution of the near-well liquid bank is interpreted in terms of compositional changes occurring in the fluid once the flowing bottomhole pressure falls below the dewpoint. The thermodynamic path of depletion is shown to be a function of radial distance (i.e., the pressure regime), and to be considerably more complex than implied by constant volume depletion (CVD) experiments. Near-well fluids undergo a transition from a retrograde gas to a volatile oil early in the depletion, passing through a critical composition in the process. This brings about large changes in phase properties and their saturations, and thus flow behavior, with even a small pressure change. This has implications for interpretation of pressure buildup and falloff tests on these wells. At a later stage in the field life, near-well fluids are converted from a volatile oil to a dry gas (passing through a critical composition again). With time, fluids in the reservoir interior undergo similar qualitative changes. Only the fluids near the reservoir edge follow a CVD path. The described phenomena are shown to occur in a higher-permeability reservoir also as soon as the bottomhole flowing pressure (BHFP) falls below the saturation pressure.
Many workers have investigated depletion of gas-condensate reservoirs, specifically the gas flow impairment once the BHFP falls below the fluid dewpoint.Even though direct evidence is lacking, general consensus seems to be that the near-well liquid dropout causes a much more severe productivity decline if the formation permeability is low. Several recent papers also addressed the interpretation of well tests in gas-condensate wells in presence of the condensate bank. While this work is not concerned with the latter topic, some of the phenomena reported here impact well test interpretation.