Wax Deposition And Gel Strength Of Waxy Live Crudes
Authors: Hsu J.J.C. , Santamaria M.M. , Brubaker J.P. , Hawker Philip 
Published: October 2008

Laboratory test methods have been developed to measure wax deposition and gel strength of waxy live oils. The objectives are to investigate the impact of (1) oil bubble point, (2) flow rate, and (3) paraffin inhibitor on wax deposition, and the impact of (1) oil bubble point and (2) pipe size on gel strength of waxy crudes including stock tank oils and live oils. The effects of the studied parameters on wax properties are also investigated.

The results of wax deposition tests show that the wax deposition is reduced significantly as the bubble point of the fluid increases. The shear rate effect is not significant under laminar flow conditions. Paraffin inhibitor may work as expected at high temperatures, but not at low temperatures.

The break-away yield stress of a gelled oil, calculated from the gel strength data. is slightly dependent on pipe size greater than ¼ " Therefore, the gel strength of an oil in a pipeline can be scaled up by the measured yield stress. The break-away yield stress decreases exponentially with increasing bubble point pressure of the oil.

INTRODUCTION

A waxy crude flowing through a cold pipeline may cause problems in oil transportation and restart processes. Understanding characteristics of wax deposition of crudes flowing through a pipeline and gel strength during shut-in is crucial in saving capital investments and operation costs.

Problems caused by waxy crudes in production wells and transport pipelines are well known. The problems have been widely studied for many years in laboratories or pilot plants using stock tank oil. However, there are only a few published studies on waxy live crudes.

In this paper, laboratory apparatus to study wax deposition and gel strength of waxy live crudes are demonstrated. The effect of bubble point of the live oils on wax deposition and gel strength is presented.

EXPERIMENTAL EQUIPMENT and PROCEDURE

Live Oil Wax Deposition Measurement System

The schematic flow diagram of the live oil wax deposition measurement system (LOWDMS) is shown in Figure 1. The LOWDMS includes oven, chilling circulator, test tube unit, heating unit, density meter, flow meter, circulation pump, oil reservoirs, two differential pressure gauges and data acquisition / control unit and a computer. The system operating conditions range from 32 to 240 °F and up to 1500 psia.

The test tube unit consists of 16-sections of tube-shell type heat exchangers (Figure 2). Each tube of the test tube unit is a stainless tubing of about 27.5" in length and 0.25 OD- 0.21"ID (OD = Outside Diameter ID = Inside Diameter). At least one end of each tube is sealed with a teflon ferrule so that each tube can be pulled out to measure the amount of wax deposited. Seven l/16"-OD thermocouples are installed in the 16-tube unit. The test tube unit is well insulated and installed in the air bath. Installing the test tube unit inside the air bath eliminates insulating tubing and valves between the two baths. A differential pressure gauge is connected to both ends of the test tube unit, After each run, the test tube unit is disassembled and the amount of wax deposited in the tubes is measured.