The Wax Deposition Envelope of Gas Condensates
Authors: Leontaritis Kosta J. 
Published: October 2008

Naturally occurring hydrocarbons in the earth's crust contain varying amounts of certain molecules generally called asphaltenes and waxes. Asphaltenes exhibit a behavior .at some thermodynamic states called flocculation. That IS, asphaltene particles or micelles present in crude oils aggregate or flocculate into larger aggregates or flocs. The locus of .all thermodynamic points in a P-T-x phase diagram at which flocculation occurs is called the Asphaltene Deposition Envelope (ADE) paraffin waxes present in crude oils, on the other hand, at some thermodynamic states, exhibit the phenomenon of crystallization. The locus of all thermodynamic points in a P-T-x phase diagram at which wax crystallization occurs is called the Wax Deposition Envelope (WOE). Paraffin waxes are rather a polar molecules and their interactions are expected to be van der Waals or London dispersion type, whereas asphaltene molecules are polar in nature and, as a result, are expected to have polar, hydrogen bond type, or associative type molecular interactions. Asphaltene flocculation and wax crystallization are two mechanisms via which organic deposition takes place from crude oils. In crude oils, the probable shapes of the thermodynamic envelopes that are manifestations of these two mechanisms of organic deposition have been defined and proposed by the author recently. Experimental and modeling data are presented in this paper which show that some gas condensates also exhibit a wax deposition envelope. The shapes of the WOEs of several gas condensates in the Gulf of Mexico are presented in this paper. The shapes of the above WOEs indicate potential waxdeposition and near-well formation damage in those cases where the Gas condensate contains high carbon number waxes that precipitate in solid state at reservoir temperature. In other words, the temperature of the reservoir may not be high enough to keep the precipitating waxes in liquid state. Hence, the gas condensate, which is a supercritical fluid, enters the WOE at the "Dew Point" pressure. This casts new insight into the conventional explanation that the productivity loss in gas condensate reservoirs, when the pressure near the wellbore reaches the dew point, is only due to relative permeability effects. It is evident then that near-well bore formation damage due to wax deposition from gas condensates is a very likely possibility.

Wax Crystallization and Deposition-Background

Some crude oils, when their thermodynamic conditions are changed (especially composition and temperature), precipitate organic solids usually in the form of crystals which are generally called waxes. The wax crystals change the flow behavior of crude oil from Newtonian to non-Newtoman which leads to higher viscosity, with increased energy consumption for pumping and a decreased pumping capacity. The other effect is to reduce the effective cross sectional area of the pipe via wax deposition. Wax deposition also increases the pipeline roughness which results in an increase in pressure drop. The deposits also cause. Subsurface and surface equipment plugging and malfunction, especially when od mixtures are transported across Arctic regions or cold oceans. Wax deposition leads to more frequent and risky pigging requirements. If the wax deposits get too thick, they often reduce the capacity of the pipeline and cause the pigs to get stuck. Wax deposition in well tubings and process equipment may lead to more frequent shutdowns and operational problems. Also, wax deposition can result in formation damage.\ If the temperature of the fluid in the formation falls below the cloud point, wax precipitates and may deposit in the formation pores, partially blocking or plugging the fl