Treatment of Variations of Composition With Depth in Gas-Condensate Reservoirs

Abstract

Summary Considerable variations of hydrocarbon composition with depth have been reported in several North Sea gas-condensate fields. Within the gas phase, the mole fraction of methane present, for example, may change by several percent between the top and bottom of the field. These compositional gradients can cause dewpoint variations of as much as 4 psi/ft [90 kPa/m]. Neglect of these effects in simulation will lead to significant errors in the estimates of hydrocarbons initially in place. These errors can, in turn, cause errors of up to 20% in predicted cumulative oil production in some condensate fields. In this paper, we develop a theoretical treatment of variations of composition with depth in hydrocarbon reservoirs. Results of sample calculations show the type of results predicted by the theory and the extent of the errors that may occur if compositional gradients are neglected.