Techniques for Measuring Wax Thickness During Single and Multiphase Flow

Abstract

Abstract Accurate measurement of wax thickness deposited on a cold pipe wall is of vital importance to the development of predictive models for paraffin deposition in flowlines and wellbores. Existing wax thickness measurement techniques, including pigging, spool piece, pressure drop, and heat transfer methods, are critically reviewed. An on-line wax thickness measurement technique, named the liquid displacement-level detection method (LD-LD), is presented. This new method is capable of giving both longitudinal and circumferential wax thickness distributions as well as the total wax deposit volume for single-phase and multiphase flow in pipes. Introduction Paraffin deposition in crude oil and gas condensate production systems is a major challenge for the economic development of oil and gas fields producing from cold environments. The paraffin deposition problem can cause a loss of millions of dollars per year worldwide through the enormous cost of prevention and remediation, reduced or deferred production, well shut-in, pipeline replacement and/or abandonment, equipment failures, extra horsepower requirements, and increased manpower needs. As oil and gas production moves to deeper and colder water, it becomes increasingly imperative to adequately identify the conditions for paraffin precipitation and predict paraffin deposition rates to optimize the design and operation of subsea multiphase production systems. A Joint Industry Project titled "Paraffin Deposition Prediction in Multiphase Flowlines and Wellbores" (Wax JIP) is underway at The University of Tulsa, and is sponsored by 30 worldwide oil and gas related companies, the U.S. Department of Energy (DOE), the Gas Research Institute (GRI) and the U.S. Department of Interior's Minerals Management Service (MMS). The objective of this JIP is to develop an integrated model for the prediction of paraffin deposition in multiphase flowlines and wellbores To achieve this goal, extensive experimental tests are underway for measuring wax deposition during single-phase waxy crude oil flow in a near-horizontal flowline (2) and natural gas-waxy crude oil two-phase flow in horizontal, near-horizontal, and vertical pipes. Clearly, accurate measurement of wax thickness is of vital importance to the success of this JIP. Since the initiation of the JIP, an extensive search for existing wax thickness measurement techniques and instruments has been ongoing. Four methods used by previous investigators for measuring wax thickness during single-phase flow have been identified, including pigging, spool piece, pressure drop, and heat transfer methods. A new liquid displacement-level detection (LD-LD) method has been developed for measuring wax thickness during multiphase flow in pipes. In this paper, the basic requirements for wax measurement techniques to yield accurate data are first established. Existing wax measurement techniques are then reviewed. Finally, the newly-developed wax measurement technique is presented. Basic Requirements When waxy crude oil flows in wellbores and pipelines, with or without the simultaneous flow of natural gas, wax precipitation and deposition will take place if the pipe wall temperature becomes lower than the paraffinic hydrocarbon liquid cloud point (or wax appearance temperature) and lower than the bulk oil temperature. It has been confirmed by a number of investigators that molecular diffusion of paraffin molecules dissolved in crude oil is the governing mechanism of wax deposition during the flow of waxy crude oil in pipes. This mass transfer due to molecular diffusion is in turn controlled by the fluid radial temperature gradient (or heat flux) at the fluid/wax (or wall) interface. The wax deposition rate inside a pipe will change significantly if the heat transfer condition is altered. Hence, any effects on the heat transfer from a pipe to the environment should not be imposed by a measurement device when measuring the wax thickness in pipes. It has been found that operating pressure has an apparent effect on the wax appearance temperature (WAT) or cloud point of a waxy crude oil. P. 249^