Wax Deposition

The build-up of solid waxy layers onto cooled surfaces such as pipe walls is generally considered to be a temperature-dominated phenomenon. Several theories have been proposed to describe the effect and several commercial semi-empirical models have been developed to predict oil field deposition rates. However, it is generally agreed that to predict waxing and/or the performance of inhibitors it is important to measure deposition in a controlled series of tests.

 

KAT employs a series of stirred cold finger tests for rapid screening of inhibitors. The equipment comprises of stainless-steel U-tubes each immersed in separate stirred samples of test fluid, held in glass bottles at ambient pressure.

The tests would be allowed to run for a pre-determined optimum duration after which time the U-tubes are removed and digital images of any deposits formed recorded. A written description of the deposits, their tenacity, weight, and the levels of entrained fluid are also noted.

 

Having identified a suitable inhibitor its performance may be evaluated under more realistic temperature and flow conditions using the flow-loop methodology, described below.

 

KAT employs a laboratory scale laminar flow loop to determine wax deposition rates of uninhibited and inhibited test fluids. Here the deposition rates are determined at a series of discrete temperature conditions approximating to positions of specific interest or expected high deposition within the operating system.

 

The flow-loop system has several advantages for assessing deposition which include control of flow and temperature conditions; real time monitoring of wax build-up rates; recovery of deposit for analysis, and realistic simulation of single-phase flow regimes where the velocity profile over the deposit can be readily matched to those across the laminar boundary layer [where the deposition is taking place] within most full-scale lines under turbulent flow conditions.

2L of hot oil is circulated through the pump and heat exchanger to deliver the fluid into the test section at the desired temperature. The wall temperature of the test section is independently set or adjusted to provide a deposition surface. Deposition rates are determined by equating the change in pressure drop across the deposition test section to a reduction in internal pipe radius using Poiseulles’ equation for Newtonian fluids in laminar flow. The oil is then returned to the hot reservoir via another heat exchanger to recondition the sample.

 

Deposits can be collected at the end of a test for inspection, confirmation of mass and/or further analysis. Alternatively, if sample volumes are limited then the fluid temperature can be elevated to melt the wax and reconstitute the fluid for reuse.

 

Wax deposition flow loop testing, in conjunction with characterisation of the waxes in the fluid by HTGC, provides the data required for deriving the Diffusion Coefficient Multiplier. The DCM is required for detailed wax deposition modelling and understanding of the wax deposition characteristics. Determination of the DCM is performed by applying KAT / AFS IP methodology.