Gravitational Concentration Gradients in Static Columns of Hydrocarbon Fluids

Abstract

Variations in the composition of the liquid phase of natural reservoirswhich are continuous through significant ranges in elevation have been noted bypetroleum technologists. In general, there is a greater gas-oil ratio and thegravity of the oil is higher in production from the upper parts of theformation. In the present paper the possible changes in composition withelevation which would result from the attainment of thermodynamic equilibriumin a uniform gravitational field have been evaluated for a naturally occurringhydrocarbon liquid, a binary hydrocarbon liquid phase and a binary hydrocarbongas. The results indicate a progressive decrease in the mole fraction of thecomponents from methane through the butanes with increasing depth, accompaniedby a corresponding increase in the amount of the heavier components making upthe system. The magnitude of these variations is perhaps insufficient toexplain completely the variations in the properties of the liquid phase thatare encountered at various elevations within a particular producing horizon.However, the changes are significant and the treatment presents an example ofthe application of thermodynamic data to the evaluation of the properties ofhydrocarbon fluids under the conditions approaching those encountered innatural reservoirs. Theoretical Considerations Muskat has discussed the work of earlier authors relating to the equilibriumrelationships of multicomponent systems in a gravitational field. He alsopresented explicit solutions for several cases and discussed the physicalsignificance of gravitational concentration gradients. In engineering work it is customary to consider the pressure, the temperatureand the composition of a phase to be the independent variables sufficient toestablish its state. T.P. 1004