UK Field Benefits From Reduced-Salinity Enhanced-Oil-Recovery Implementation
Authors: Wilson Adam 
Published: February 2015

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 161750, ’Low-Salinity Enhanced Oil Recovery, Laboratory to Day-1 Field Implementation—LoSal EOR Into the Clair Ridge Project,’ by Enis Robbana, SPE, Todd Buikema, Chris Mair, SPE, Dale Williams, Dave Mercer, Kevin Webb, SPE, Aubrey Hewson, and Chris Reddick, SPE, BP, prepared for the 2012 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed.

BP has shown, by use of its reduced-salinity (RS) water-injection technology, that incremental increases in oil recovery can be achieved across length scales associated with coreflood experiments (inches), field-based single-well chemical-tracer tests (feet), and field trials (interwell distances). This paper discusses the process undertaken by the Clair Ridge project in getting RS enhanced oil recovery (EOR) adopted as a Day-1 secondary waterflood.

Introduction

Clair Ridge Field Overview. Containing more than 6 billion bbl of oil in place, Clair is the largest oil accumulation in the UK continental shelf, and it lies 142 miles north of the Scottish mainland and 35 miles west of the Shetland Islands in 132–155 m of water.

The Clair A platform is a lightweight-steel- jacket development that was designed to be economic across the range of expected reservoir outcomes. The main risk at startup was whether waterflood of the fractured reservoir would work.

More than 5 years of production data indicate that the reservoir production mechanism in Clair Phase 1 is working. There is evidence of displacement of oil from the matrix by a combination of viscous sweep, gravity drainage, and imbibition of water from the fracture network into the matrix.

Success at Clair Phase 1 has paved the way for a much larger Clair Phase 2 development. The next phase of development will target roughly twice the oil in place of Clair Phase 1 in an area that has generally shallower and thicker reservoir, known as Clair Ridge. During development, various EOR schemes were studied. Gasflooding of the reservoir would be hampered by adverse relative permeability factors. These can be improved by enriching the gas with heavier components such as propane and butane; however, there is not a large source of these materials in the basin. Modeling has shown that the open fractures are expected to provide a shortcut between gas-injection and production wells. Alternatives such as CO2 flooding and polymer flooding were also considered and rejected. Clair is a waterflood reservoir, and recovery depends on sweeping the large areas of matrix rock between the conductive fractures.

What Is RS EOR? RS EOR can be defined as waterflooding of a sandstone oil reservoir using water with total-dissolved-solids (TDS) content of less than 2,000–8,000 ppm. The RS-flood TDS threshold shows some variation across different reservoir/fluid systems, with consistent enhanced recovery below the lower end of the range and varied effect within the range.

Full-reservoir-condition RS EOR corefloods have been performed on many reservoir systems across the world. All tests have shown similar characteristics, with increased oil production at water breakthrough and a decrease in remaining-oil saturation at the end of the waterflood tests. RS EOR recovers additional oil after both secondary (RS water injection from Day 1) and tertiary (RS injection following higher-salinity- water injection) waterflood applications. Figs. 1 and 2 show typical secondary and tertiary responses to RS EOR, respectively.