Case Study: Drill-Cuttings Treatment Alternatives—A Comparative Emissions Assessment

Abstract

The offshore oil and gas industry has always been cognizant of its impact on the marine environment. The choices that operators make in how they operate, including the disposal of drill cuttings, must address an increasing number of environmental and climate targets in addition to those related to health, safety, and cost. As a typical well will produce approximately 1000 metric tons of oil-based drill cuttings, quantifying greenhouse-gas (GHG) emissions associated with the disposal and treatment of drill cuttings has become an essential step to achieving net-zero ambitions. Since 1991 (1993 for fields in production), strict regulations relating to the discharge of oil-based drill cuttings have been in force under the OSPAR (Oslo/Paris) Convention. Those regulations banned the practice of discharge to sea of untreated oil-based drill cuttings and led to a situation where they were generally shipped to shore for treatment and disposal. In 2020, TWMA engaged DNV, the independent energy expert and assurance provider, to undertake a comparative study between the company’s offshore thermal drill-cuttings treatment solution and conventional alternatives including “skip and ship,” bulk transfer, and cuttings reinjection (CRI) used on the Norwegian Continental Shelf (NCS). It is the first paper to show a direct emissions comparison between offshore processing and alternative methods implemented. The study assessed the carbon dioxide (CO2) footprint and nitrogen dioxide (NOx) emissions for each of the different alternatives. The values were then used to create an interactive emissions calculator that can easily be applied to specific projects to clarify the actual potential for emissions reduction within the drilling waste management process. Background Technological improvements, as well as cost focus on existing solutions, have meant that offshore thermal drill-cuttings treatment has been widely adopted in many offshore basins as one of the safest and most cost-effective approaches. In Norway, which was the subject of the study, adoption of the technology has been slower than in other countries, with onshore thermal treatment of oil-based cuttings applied as the predominant technique, while some fields use offshore slurrification and injection into dedicated disposal wells. However, the cost of drilling new disposal wells and the track record of successful offshore thermal projects in other countries have improved the frame conditions for the use of offshore thermal treatment of cuttings on the NCS. It has also been demonstrated, in an earlier independent comparative study by Carbon Zero (SPE 207519), that the carbon footprint of skip and ship to shore of drill cuttings is 53% higher than that of drill-cuttings treatment at the wellsite (SPE 202639).