Oil/Water-Separation Experience From a Large Oil Field
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 93386, “Oil-Water Separation Experience From a Large Oil Field,” by S. Kokal, SPE, and A. Al-Ghamdi, Saudi Aramco, prepared for the 2005 14th SPE Middle East Oil & Gas Show and Conference, Bahrain, 12–16 March.

The full-length paper discusses challenges related to emulsions in a large Saudi Arabian field. The field produces from several different reservoirs with a range of fluid properties. These properties provide an interesting case of operational challenges in oil/water separation including increased incidents of shorting in the separator and tripping of equipment and increased demulsifier consumption. The paper presents the results of a comprehensive study to understand the main causes of emulsion formation and ways to optimize oil/water separation.

Introduction

Crude oil has to be separated from produced water at wet-crude handling facilities or gas/oil-separation plants (GOSPs). The GOSP produces dry crude, gas, and water. The incoming oil and water can form an emulsion and may be difficult to separate. When emulsions are tight, they can create a number of operational challenges such as tripping of equipment in a GOSP, plant instability, increased demulsifier consumption, production of off-specification crude, and high pressure drops in flowlines. These emulsions have to be treated to meet crude specifications for transportation, storage, and export and to reduce corrosion and catalyst poisoning in downstream processing.

This field produces from seven different reservoirs including the more-viscous crude from the Arab reservoirs. Viscosities of these oils range from 2 to more than 10 cp, and gravities range from 28 to 40°API. The n-pentane asphaltene content varies from 0.2 to more than 7%, and water cut for producing wells ranges from dry wells to more than 70%. Most of the wells are offshore, and the crude is delivered to onshore GOSPs by undersea trunklines.

The onshore GOSP includes a high-pressure production trap (HPPT) or separator, an intermediate-pressure production trap (IPPT), and a two-stage dehydrator/desalter train. The dry oil from the desalter has to contain less than 0.2% water and less than 10 lbm/1,000 bbl of salt. The separated water is processed further in a water/oil separator (WOS) and injected back into the reservoir through disposal wells. The water from the WOS generally has to meet a specification of less than 100 ppm oil. The GOSP was designed to handle approximately 30% water cut. The water cuts in the field have been increasing steadily over the years and are expected to increase further in the coming years. Soon the water cuts will be higher than the design capacity of the GOSP. To handle the extra volume of water, and to avoid overloading the dehydrator/desalter vessels, one option was to convert the two-phase HPPTs into three-phase separators.

A comprehensive study was undertaken earlier to understand the main causes of emulsion formation in the field. The factors investigated were water cut, temperature, shear, asphaltene content, demulsifier dosage, and crude mixtures. These factors were quantified with an emulsion separation index developed to measure emulsion stability. The results showed a strong correlation of asphaltene content in the crude oil with emulsion tightness. Several recommendations were made and implemented for optimizing oil/water separation and reducing demulsifier usage in the field.