Paraffin Deposition Analysis Of Waxy Crude Oil Under Turbulent Flow Condition

Abstract

In past few decades, paraffin (wax) deposition has created numerous production problems in many of the world's oil-producing regions. Often the problem is more severe for offshore fields where solutions to rectify wax precipitation can be very costly. Due to the various difficulties and expenses associated with operating waxy reservoirs, especially offshore, it has become necessary to understand the natures of petroleum waxes and the fundamental variables that affect their depositions during production and processing. In this study, we analyse the characteristics of reservoir fluids from two offshore fields as well as their propensity to paraffin deposition due to temperature and pressure variations.

Introduction

Waxes are mixtures of long-chain hydrocarbons with carbon chain lengths ranging from C15 to C75. They are crystalline in nature and tend to crystallize or precipitate from crude oils at and below their cloud points. Crystallization is the process in which an ordered solid structure is produced from a disordered phase, such as a dilute solution. It usually involves two distinct stages, namely nucleation and growth which may be considered separately. Different techniques, namely a cross polarized microscopy (CPM) and a laser-based solids detection system (SDS), have been used to measure the onsets of paraffin crystallization temperatures, also known as wax appearance temperatures (WAT) and/or cloud points. As the temperature of the liquid solution or melt is lowered to its wax appearance temperatures (WAT), the energy of molecular motion becomes increasingly hindered, and the randomly tangled molecules in the melt tend to accumulate together and form clusters of adjacently associated chains. This process continues and paraffin molecules tend to associate and dissociate to and from these ordered sites until the clusters reach a critical size and stability is attained. This process is termed nucleation and the clusters are called nuclei. These nuclei are only stable below the melting temperature of the wax as they are disrupted by thermal motion above this temperature. After the formation of nuclei and at a temperature below the WAT, additional molecules will get deposited on the nucleation sites and become part of the growing lamellar structure. This formation is called the growth process.

Nucleation can be either homogeneous, which means that the sample is pure and the nucleation sites are time dependent or heterogeneous, which implies that all nucleation sites are activated instantaneously. Heterogeneous nucleation is most common in crude oils where impurities such as asphaltenes, formation fines, clay and corrosion products act as nucleating materials for wax crystals.

Although WAT and pour point temperatures are specific thermodynamic properties for waxy crude oils, their relative positions/boundaries within P-T diagrams as well as the rate and amount of wax deposition are influenced by a number of factors that include:

• oil composition
• temperature or cooling rate
• pressure
• paraffin concentration
• molecular mass of paraffin molecules
• occurrence of nucleating materials such as clay, formation fines and corrosion products
• water-oil ratio.

Following are the conditions that must be fulfilled for wax deposition to occur.

1. Temperature of the pipeline wall should be below the wax appearance temperature (WAT).
2. The wall temperature should be lower than the center line temperature. In other words, radial temperature gradient in the flow should be negative.
3. There must be sufficient wall friction for the wax crystals to stick to the wall.

While transporting waxy oil through a cold pipeline, wax will be accumulated on the cold pipe wall through molecular diffusion and shear dispersion mechanisms. Many wax deposition models only apply molecular diffusion mechanism during modelling, but neglect shear effect. However, many studies show that the flow turbulence effect has a significant impact on wax deposition and cannot be neglected in wax deposition modelling. A semi-empirical technique proposed to scale up shear effect and then conventional modelling techniques can be applied to predict wax deposition for waxy crude production lines.

Scope

Paraffin deposition is a major issue in crude oil production and transportation. In the past few decades, number of studies have been conducted to rectify this issue economically. Paraffin inhibitors are widely being used to reduce the paraffin deposition and related problems. Successful completion of this study can be helpful in future research and oil production.

Expected results

Analyse the behaviour of crude oil deposition under turbulent flow condition and to study the influence of various factors on its deposition and accumulation.

Next steps

Initial literature survey has been conducted. More literature has to be studied and finally use an appropriate technique to predict wax deposition for waxy production lines under turbulent flow conditions.