Modelling the Shearing Effect of Flowing Fluid and Wax Ageing on Wax Deposition in Pipelines

Abstract

Wax precipitation and deposition in pipelines is one of the most important flow assurance challenges for the oil and gas industry. Much work has been carried out to model wax deposition as measured by flow loops and other experiments. Models are routinely scaled up to predict the behaviour of full-scale pipelines, but little information is available to verify if such predictions are reliable. A fully compositional wax deposition model has been developed that combines an accurate model for the thermodynamics of wax precipitation, a model for diffusive wax deposition based on heat and mass transfer analogy and a method for the shearing effect on the wax deposits caused by the turbulence of the flowing fluid. The wax is treated throughout as a fully compositional distribution of n-paraffins, because previous work has shown that simplifying the representation of the n-paraffins leads to results that appear to be physically unrealistic. The model has been shown to be in reasonable agreement with wax deposition rates reported from flow-loop experiments. This paper describes the application of the wax deposition model to real pipelines and the shearing effect on the deposited waxes. The shearing effect will be illustrated by reference to the real field cases, and the operating experience of a sub-sea pipeline. The deposition model is analysed to assess its agreement with the real field operating experience, and to assess under what conditions the pipeline could be considered at risk of blocking. The paper also considers the effect of wax ageing on wax deposits. In cases where fluid turbulence may limit wax build-up, the wax deposit may gradually harden with time due to ageing. A hardening deposit could lead to long-term remediation problems. A number of possible wax ageing mechanism are considered, and how they would modify the deposition process. It is found that the effective diffusivity of hydrocarbon molecules inside the waxy gel deposit is a major uncertainty affecting the predictions. Model sensitivities are investigated in the light of this uncertainty, and the possible impact of ageing on the pipeline is assessed.