A Review of Downhole Separation Technology
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 94276, “A Review of Downhole Separation Technology,” by O.O. Ogunsina, SPE, and M.L. Wiggins, SPE, U. of Oklahoma, prepared for the 2005 SPE Production and Operations Symposium, Oklahoma City, Oklahoma, 17–19 April.

Downhole oil/water separation (DOWS) allows water to be separated in the wellbore and injected into a suitable injection zone downhole while oil, with traces of water, is produced to the surface. After introduction of DOWS in the 1990s, several trial applications were undertaken to test the technology. These trials allowed information to be collected on DOWS feasibility. The full-length paper reviews the status of downhole separation technology and presents an extensive list of references.

Introduction

Produced water is water produced to the surface from hydrocarbon-bearing formations during oil and gas extraction and can include formation water, injection water, and any chemicals added downhole. Conventional production processes involve producing both oil and water to the surface and then separating them at the surface. Surface separation is accomplished by use of separation and dehydration equipment including skimmer vessels, plate coalescence, hydrocyclones, and, in some cases, crossflow membrane filters to reduce the oil content in the water phase and improve water quality before disposal. As a reservoir matures and oil and gas production peaks, often there is an associated increase in water cut and a corresponding increase in both lifting and water-disposal costs. The increased water cut also necessitates additional maintenance for production equipment and downhole treatment for corrosion, bacteria, and scale.

Although producers still have a variety of choices in either disposing of the water or reusing it, there is a growing concern from the public related to the handling of this waste product. Public concern about the environmental impact of produced-water disposal has become a major issue, especially related to surface damage caused by spillage or subsurface contamination of drinking water caused by poor injection practices. Environmental regulations pertaining to produced-water management are expected to become more stringent in the future, requiring new practices and techniques to manage produced water.

DOWS technology was introduced to the oil and gas industry in the 1990s, and work to assess its feasibility was sponsored by the U.S. Dept. of Energy in 1999. DOWS, unlike conventional separation processes, separates oil and gas from produced water at the bottom of the well and injects the separated produced water into another formation, usually deeper than the producing formation, while the oil and gas are pumped to the surface.

Depending on DOWS type, typical DOWS project costs (procurement and installation) range from U.S. $120,000 to$300,000. Although reports show that some DOWS projects have had payback periods as short as 2 months, the relatively high project costs suggest that the design life or time to failure is a critical factor for the success of a DOWS project. The degree and duration of benefit that can be realized with DOWS technology depend on the characteristics of the producing well, reservoir rock and fluid properties, and the robustness of the separator/pump assembly.