Modelling of Solid Precipitation From Reservoir Fluid
Authors: Zhou X. , Thomas F.B. , Moore R.G. 
Published: March 2010

Abstract
A solid precipitation model is proposed for liquid-solid equilibrium based on the Flory-Huggins polymer solution theory along with a modification to account for the colloidal suspension effect of partial soluble solid compounds in the fluid mixture. The colloidal effect is dependent on the estimation of the critical resins concentration. A linear variation of free volume is assumed in terms of the component molecular weight for the evaluation of volume fraction in the polymer solution formulation. The binary interaction energy coefficient is introduced to allow for positive or negative deviation from the ideal solution. The effect of heat capacity difference between the liquid and solid phases and the effect of pressure correction are included in the model. New correlations have also been developed for the model.
A multimodel multiphase flash algorithm is formulated using an equation of state for vapour-liquid equilibrium and the proposed model for liquid-solid equilibrium. A detailed discussion of the model performance and the effect of the model parameters on the fitting to experimental results are also presented.

Introduction
Solid precipitation from reservoir crudes has been a serious problem in the oil industry. In reservoir engineering, the original reservoir fluid is well-stabilized after it has formed and migrated geologically to the reserve area where it is found. The production of the reservoir fluid alters the storage conditions which may cause solid deposition. These deposits can cause formation damage and in situ plugging which result in the loss of reservoir permeability and degradation of the porous media. The deposition of the heterocompounds on the surface of the media may reverse its wettability and decrease the transmissivity(9). In miscible gas flooding, solvents are injected into the reservoir in order to reduce the interfacial tension of the reservoir fluids and to recover more crude oil from the formation. This process can significantly change the fluid phase behaviour and result in asphaltenes, or other solid components, instability in the solution, thus increasing the potential for solid deposition in the reservoir.
The solid problem is frequently observed in the wellbore and well tubing. It may plug wells, increasing the fluid pressure gradient which can result in significant reduction in well productivity. It can also cause operation problems in the subsurface and surface equipment. In these cases, costly well workovers or other production treatments may be required.

Review
Many investigators have done considerable research on the nature of heavy organic compounds and the mechanisms of the solid deposition(10,14,27,28,31,32,34). The complexity of the solid compounds has impeded progress toward accurate prediction and efficient treatment of solid deposition during oil recovery. The precipitated solid usually constitutes the heavy organic compounds of waxes, asphaltenes and resins.
Waxes-are defined as normal paraffins and other molecules containing long chain alkyl groups usually ranging from C18 to about C60. Upon cooling below their cloud point temperature, the solid waxes crystallize and precipitate. Waxes can dissolve in many kinds of hydrocarbon solvents to form a homogeneous solution.