Design of a High-Rate, High-Volume Oil/Water Separator
Authors: Joe Stires 
Published: December 2007

At ultra-high water cuts (95% or more), the most difficult production problem may not be getting the oil out of the ground, but getting oil out of the water. While it is not uncommon to use large wash tanks to separate the oil from this type of flowstream, few designs can be tailored to a specific application. This paper presents one such design, including the equations used to analyze its performance. This type of analysis can be applied to any separator to predict what its performance should be.

Introduction

The increasing value of oil is making it economically attractive to install high-volume artificial lift equipment in an oil well, and produce it to a WOR of 125 or more. However, there are few methods for the design of an oil/water separator that will handle effectively liquid inputs of 20,000 B/D or more at these ultra-high water cuts. By developing and presenting the design equations for one such design, this paper describes a foundation for further wok.

Large-volume wash tanks often are used as oil/water separators or water polishers in high-rate, high-water-cut applications. Despite widespread use of wash tanks, there are no design equations available to aid in their design1. This paper presents several equations developed while adapting the API separator to oilfield use. It also evaluates design performance with field data.

A comparison of actual performance with performance predicted by reaction kinetics equations shows that, for the most part, overall performance approached or exceeded predicted performance. Although some flow characteristics failed to meet the modeled performance, we conclude that the design equations developed are valid for design of a high-water-cut, high-rate oil/water separator.

Discussion

To take advantage of rising oil prices, many oil companies have increased production from older leases over the past several years. In 1975, Continental Oil Co. (now Conoco Inc.) began a program to maximize production from the Big Lake field, Reagan County, TX. The field was the discovery field of the Permian Basin region and has been on production since 1923. Production has been primarily from the Grayburg limestone, at 3,000 ft (914 m). This reservoir has a strong water drive, and water cuts currently average 90% or more. Liquid productivities are usually more than 3000 B/D (477 m3/d) per well.

Most wells on our lease in this field were equipped with high-volume artificial lift in 1975-76 to take advantage of the high productivities of these wells. The lease-wide oil production increase from 290 to 525 B/D (46 to 83 m3/d) was accompanied by an 18,000-B/D (2862-m3/d) increase in water. Total water production, 40,000 B/D (6359 m3/d), exceeded the capacity of the treating facilities on this lease. As a result, oil carryover into the produced-water disposal system increased, going as high as 700 mg/L (0.7 g/dm3) in some tests. This meant losses of about 30 bbl (4.8 m3) oil into the disposal system daily and significant lost revenues. This unseparated oil also caused increased operating costs as water disposal wells began to plug.

After reviewing these problems, we decided to buid a satellite battery to handle half the produced liquid on the lease. The central battery was to be modified to increase its treating and disposal capacity to 32,000 B/D (5088 m3/d) by installing some type of water-polishing device between the existing free-water knockouts and the produced-water disposal system.

Although the capacity of this water-polishing device at ultra-high water cuts was the primary design criterion, several other factors had to be considered.