Improvements in Single-Phase Paraffin Deposition Modeling

Abstract

Summary Paraffin deposition under single-phase flow conditions was investigated to determine its dependence on shear stripping, deposit aging, flow regime, temperature gradient, and fluid properties. In this study, a new model for the prediction of single-phase wax deposition has been developed. Most of the models previously used assume that equilibrium exists at the deposit-fluid interface. A kinetic resistance of the fluid is considered in the new model. Therefore, the interfacial-wax concentration might be different from the equilibrium-wax concentration. The model also includes continuous diffusion of wax into the deposit. We believe that this enrichment of the deposit is responsible for the increasing hardness of the deposit with time - a process known as "aging." The effect of shear stripping may also be incorporated in the prediction. The model predictions are compared with predictions from previous models, as well as with experimental data gathered at the Tulsa U. Paraffin Deposition Projects, with two different oils: a black oil and a condensate. Even though some tuning is required for each type of oil, the new model is based on physical phenomena, reducing the empiricism of previous models. Introduction In oil production and transportation systems, when the fluid temperature drops below the wax-appearance temperature, the long-chain normal paraffins of the formula CnH2n+2 where n>20 will solidify and adhere to the pipe walls, if a radial heat flux to the surroundings exists. This phenomenon, known as "paraffin deposition," can cause a reduction in the effective flow area. Paraffin deposition can result in significant operational and remedial costs, reduced or deferred production, well shut-ins, and pipeline replacements and/or abandonment. It is imperative to properly identify the conditions for paraffin precipitation and to predict paraffin deposition rates for the design and optimization of oil- and gas-production systems, as well as to implement proper strategies for prevention and remediation. Understanding the paraffin deposition process under single-phase flow conditions is crucial to properly model the phenomena under both the single-phase and multiphase flow conditions encountered in oil-production systems. Model Enhancement One of the main limitations in the current Tulsa U. (TU) single-phase paraffin deposition model1 is the assumption of constant oil fraction in the deposit that the user is required to specify as an input parameter. It is also assumed that all the mass flux from the bulk fluid contributes to deposit growth, and no diffusion into the deposit is considered. The current model does not consider the aging effect on the deposition process. Singh et al.2 proposed a model that considers the diffusion of wax into the existing deposit. The boundary condition used at the deposit-fluid interface was that the diffusion flux at the interface is equal to the slope of the wax solubility curve in equilibrium with the deposit temperature gradient. In this thin film model, the wax fraction in the deposit changes with time, but it is uniform across the deposit. Also, Singh et al. did not consider any shear-stripping effects, as all of their tests were conducted under laminar flow conditions. The new model proposed in this paper is analogous to the Singh et al. model in the sense that it also considers that part of the bulk flux will contribute to new deposit growth, and the rest will be diffused into the existing deposit. The model considers a kinetic resistance for the diffusion into the deposit; therefore, the interfacial concentration might be different from the equilibrium concentration at the interface temperature. The kinetic resistance would be different for different oils. Also, the proposed model assumes that the deposit layer is immobile.

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