Oily drilling waste is a specific environmental hazard. It comprises small rock fragments or cuttings contaminated by drilling fluids, which are carried to the surface with the returning mud flow and sieved out on a shale shaker.
In the early days of offshore production, oily cuttings were simply dumped into the sea.
Nowadays, official regulations demand that they should either be injected back into sub-surface formations or transported to land for cleaning.
Since both these measures are expensive, the obvious answer is to minimise drilling waste at source.
Until that is possible, however, StatoilHydro has decided to investigate the conversion of oily cuttings into commercial products.
Flow chart showing various options for the disposal of drilling waste.
Fertiliser
One option is worm farming (vermicomposting), where worms and bacteria degrade the oil content of untreated cuttings and reduce total extractable hydrocarbons to almost background levels.
The process if further enhanced by using a purpose-designed drilling fluid.
During bio-remediation, the mixture is gradually transformed into a type of high-quality fertiliser.
Although this has been done in New Zealand, StatoilHydro wanted to see whether the treatment complies with Norwegian compost and soil improvement regulations.
After two years of experimental work, the results show that the process is entirely acceptable. What is more, it is a low-cost solution and leaves no environmentally compromising residues behind.
Asphalt
A slightly more expensive option involves cleaning cuttings in a land-based or offshore treatment plant and pulverising them into a clay-rich powder with the aid of a thermo-mechanical continuous conversion hammer mill principle.
One-third of the resulting powder is then combined with two-thirds bitumen to form asphalt for use as a pipeline-coating medium.
The asphalt is sprayed onto the line pipes before they are encased in concrete to add weight and counteract buoyancy.
A major application is planned for the Ormen Lange field in the Norwegian Sea.
