Published in Hart Energy E&P, December 2021
By Gareth Innes, TWMA
The industry’s focus on cutting greenhouse-gas emissions and adopting more sustainable drilling waste management solutions is not only tackling climate change targets but also improving safety, reducing cost and driving efficiencies. With water-based and non-aqueous drilling fluids (NAF) and drill cuttings accounting for the largest waste streams generated from global offshore drilling operations, E&P operators continue to look for ways to turn this waste stream into value.
The main options for oil-based drill cuttings treatment are offshore thermal processing or cuttings reinjection, both at source, and skip and ship or bulk transfer to shore.
With net-zero ambitions at the fore, TWMA recently engaged with DNV, the independent energy expert and assurance provider, to undertake the first comparative emissions study(1) between the company’s offshore thermal drill cuttings treatment solution and conventional alternatives including skip and ship to shore, 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 all other alternative methods.
Skip and ship to shore is one of the most commonly applied methods to manage drilling waste. Here, skips are utilized at the rig site to contain oil-based drill cuttings for transportation onshore. A well typically produces 1,000 MT oil-based drill cuttings, which could require more than 200 skips, each involving more than 12 crane lifts before reaching its final destination, amounting to more than 2,500 lifts per well. This activity also increases the likelihood of safety and environmental incidents.
In general, bulk transfer of drilling cuttings is similar to skip and ship. The main differences are oil-based drill cuttings are pumped between storage tanks on the rig, vessel and quayside rather than contained and lifted in individual skips.
CRI is based on drill cuttings, slurrified with seawater, then injected under pressure into a dedicated drill cuttings disposal well. Hence, one (or more) dedicated disposal well(s) is required.
Safe and sustainable solution
An alternative, and both financially and environmentally optimized solution, is thermal processing of drill cuttings at the rig site. TWMA’s TCC RotoMill® wellsite processing technology allows drilling waste to be processed at source, eliminating the requirement to ship drilling waste long distances for treatment.
The TCC RotoMill uses a process of thermal desorption to separate drill cuttings and associated materials into their three constituent parts of oil, water and solids for recycling and reuse. The process creates heat through friction, which evaporates the liquids (oil and water) from the solids.
As the remaining hydrocarbon content is generally very low, the treated cuttings powder and water can be discharged to sea, depending on local requirements. The recovered oil is reused in the drilling mud system, returning significant commercial savings to operators.
This solution reduces lifting operations by up to 95%, significantly improving project safety performance, whilst delivering significant cost savings of up to 40% per well compared to skip and ship by eliminating both the logistics associated and operational downtime.
Cutting carbon emissions
During the independent study by DNV, each alternative drill cuttings treatment method was assessed based on its CO₂ footprint and nitrogen dioxide (NOx) emissions using an interactive emissions calculator.
The alternative methods involving onshore treatment of drill cuttings were highly influenced by the distance to an onshore treatment facility, as well as power source for onshore treatment facility and distances regarding road transport. The study further showed that the CRI alternative ranked highest for CO₂ emissions per metric ton, double that of the thermal processing unit. For all scenarios, onshore treatment activity contributes a significant amount of the emissions for these alternatives.
Use of offshore thermal processing technology showed the same level of emissions in all three scenarios, with 95kg CO₂ and 0.09kg NOx emitted per metric ton of cuttings treated. This had the smallest emissions footprint and is based on the offshore power source as the alternatives have a fixed power demand per metric ton of cuttings treated.
The study showed that by processing drill cuttings offshore at the source with the thermal treatment technology, operators could reduce the carbon footprint of their operation by 50% compared to the skip and ship to shore method. In addition, operators can improve the safety performance of their operation and lower cost by eliminating the logistics associated with the onshore alternatives.
(1) Innes, G, Eia, J and Nesse, S. ‘A Comparative Emissions Assessment – Drill Cuttings Treatment Alternatives’. SPE-207519-MS. Presented at ADIPEC, Abu Dhabi, UAE, November 2021.
Link to published article: E&P Case Study: A Wellsite Processing Solution Proven to Lower Carbon Emissions | Hart Energy