A natural gas contains a variety of hydrocarbon molecules ranging from methane (CH4) to very heavy hydrocarbons, all of which are in the gaseous state at initial reservoir conditions. Produced intermediate to heavier weight hydrocarbons liquidfy at the surface and become a major source of income. Calculations are performed to characterize the gas-liquid relationship in order to set separation conditions and predict performance of the two phases.
Gas reservoir performance histories are governed by any repressuring assistance exerted by natural water influx that may vary from a complete absence to a case where a strong water drive dominates performance. Ultimate recovery is greatest in the pressure depletion case due to gas compressibility effects exhausting more residual gas.
Gas compressibility is the predominant influence on production history on low to moderate gas reservoirs. However the effects of formation and water compressibiliry must be included in studies describing overpressured gas reservoirs.