Abstract

This paper presents the first part of an experimental study performed jointly by IFP and Saudi Aramco to characterize the stability of emulsion samples collected from different Saudi Aramco Gas/Oil Separation Plants (GOSP). Earlier studies1–2 have shown the importance of emulsion characteristics on the performance and optimization of oil-water separation at Saudi Aramco GOSPs. A strict and rigorous methodology was applied to understand the characteristics, behaviour and composition of these systems. Samples were first separated by simple gravity sedimentation in order to identify the amount of separated water, oil and residual emulsion. Each separated phase was then analyzed separately. This included the chemical composition and bulk properties of crude oil and the salinity of the separated water phase. Residual emulsions were characterized using several techniques including Karl Fisher analysis for water content, as well as optical microscopy, DSC (Differential Scanning Calorimetry) and cryo-SEM to assess the size and polydispersity of the dispersed droplets. The objective of the study is to highlight the main physico-chemical parameters responsible for the varying tightness of these emulsions, and provide insights to optimize their treatment costs and resolve emulsion issues in the GOSPs. The second part of this study still under progress will investigate the stability of residual emulsions in terms of chemical demulsification and interfacial properties.

Introduction

The formation of stable water-in-crude oil emulsions is unavoidable during oil production and their presence increases the technical problems related to oil-water separation in production surface facilities3,4. The occurrence of these emulsions is mainly due to high shear rates and zones of turbulence that are encountered during the production process and especially at the wellhead in the choke valve5.

These emulsions can be very stable due to the presence of polar compounds such as asphaltenes and resins that play the role of "natural emulsifiers" and also to the occurrence of many types of fine solids (crystallized waxes, clays, scales, etc)6–9 that help in the formation of resistant films at the crude oil/water interface 10–11.

Effective separation of crude oil and water is an essential operation to ensure not only the quality of crude oil but also the quality of separated water phase at the lowest cost. Crude-oil dehydration is generally performed in separators by classical physical treatments such as heating or electrocoalescence 4. Unfortunately, these physical means are generally not sufficient to break the emulsions, especially in offshore production. Chemical additives have to be added to disrupt the interfacial film and to enhance and speed up emulsion breaking. Chemical demulsification appears to be an essential step in crude-oil dehydration 12.

A strict and rigorous methodology was applied to gather information about the behaviour and composition of Saudi Aramco emulsion samples collected from several GOSPs. The samples were first separated to identify the amount of separated oil, water and residual emulsion. Each phase was then analyzed separately. Bulk properties (viscosity, density) as well as the chemical composition of crude oil were determined. Salinity of the separated water in terms of ionic composition was measured when possible or assessed by DSC in other cases. The residual emulsions were characterized using several techniques such as Karl Fisher titration to determine the water content, and DSC (Differential Scanning Calorimetry), optical microscopy and/or cryo-SEM to assess the size and polydispersity of the dispersed droplets.

Materials and Methods

Emulsion Samples from GOSPs

Ten samples were collected from different Saudi Aramco GOSPs representing varying degrees of emulsion tightness and watercuts. These samples were collected from the production header just before the first separator before any chemical injection. The samples were then sent to IFP. Table 1 shows the details of these samples. Note that the values in the table represent watercuts of the GOSPs emulsions during the sampling process.

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