Experimental Study on Wax Deposition Characteristics of a Waxy Crude Oil Under Single Phase Turbulent Flow Conditions

Abstract

Abstract Crude oil, having a paraffin nature, has been studied extensively in thesmall-scale flow loop at Tulsa University Paraffin Deposition Projects (TUPDP). The effects of turbulence/shear and thermal driving force on wax depositioncharacteristics were experimentally studied using a waxy crude oil from theGulf of Mexico. The test matrix consisted of a total of 15 experiments whichinclude 12 short term tests and 3 long term tests. The tests were conductedunder different operating conditions with a wide range of Reynolds numbers from3,700 to 20,500. The shear stress ranged from 5.4 to 53.9 Pa. It was observed that paraffin deposition is highly dependent on the thermaleffective driving force which is the temperature difference between oil bulkand initial inner pipe wall and also on turbulence effects. The depositthickness obtained using both the pressure drop method and a direct measurementwas found to decrease with increasing shear stress and decreasing thermaldriving force. The wax content showed a gradual increase with an increase inflow rate. For the short term tests, the deposit mass with no entrained oilseemed to increase and then decrease with an increase in initial shear stressand decrease in effective thermal driving force whereas the total deposit masswas found to decrease with an increase in initial shear stress or decrease ineffective thermal driving force. Introduction When crude oil flows through the subsea and production pipelines, the oiltemperature drops due to the colder surroundings. It has been observed throughexperiments and postulated in literature that when the bulk temperature of thecrude oil in pipes is higher than the wall temperature, there exists adissolved wax concentration gradient between the bulk oil and the pipe wall. The n-paraffin components are considered to be mainly responsible for waxdeposition (Benallal et al. 2008). When the pipe wall temperature goes belowthe wax appearance temperature (WAT) of the oil, the liquid wax diffusestowards the wall. The liquid wax diffused towards the pipe wall crystallizes on the wall surface orat the interface between the bulk and deposit. The wax deposit may eventuallyblock the pipes. Hence, it is imperative to accurately predict wax depositionunder different operating conditions. The main problems associated with waxdeposition are the increased pressure drop in the pipeline, reducedproductivity and the risk of getting a pig stuck during regular maintenanceoperations. The current paraffin deposition models can predict wax depositioncharacteristics with high confidence under zero or low shear rate conditionswhile they can grossly over predict and under predict paraffin deposition underturbulent flow or high shear conditions. Understanding of the physics behindparaffin deposition in single phase can help accurately model the behavioraiding in the prediction of paraffin deposition and in deciding the piggingfrequency.