publisert 29.11.1999

Drill cuttings returned downhole

Ten years ago Statoil of Norway developed a concept for the grinding of drill cuttings from the drilling process to a consistence suitable for reinjection into subsea formations. This SMACCC process was further developed by Procon Engineering and is now available as a complete Procut slurrification system ready for use.

The process is recognised as highly environmentally friendly and has also proved itself to be more economic than the disposal of drilling cuttings on shore. The first Procut system was delivered to the Gullfaks A field in 1991. From then on the concept has been constantly improved and is today utilised on both fixed and floating installations.

A total of some 10 systems have been delivered and experience in use has been very positive. From the start of the oil industry in the North Sea thirty years ago, chemicals and drill cuttings have simply been dumped around the drilling areas. The consequence of this is that the areas around the old drill holes have been covered by layers of drill cuttings up to several metres thick.

The grit and remains of these drill cuttings may often contain toxic substances such as barytes and traces of oil and heavy metals. Although the piles of drilling waste in the area around the oldest Ekofisk platforms are less than a metre thick, in other areas these piles can be up to five metres thick where more recent drilling activities have taken place.

Even though the dumping of oil based cuttings was prohibited at the beginning of the 90s and resulted in the development of less environmentally damaging drilling fluids, it is only a matter of time before the dumping of these substances is also prohibited. And it is expected that new regulations on the removal of drilling waste round old drill holes will be introduced.

As far as is known there is as yet no accepted method of purifying the cuttings to a sufficient degree for dumping overboard without damage to the environment. The only alternative to reinjection is to send the waste cuttings ashore in large sacks or containers.

This is the procedure today but it is a time-consuming process involving the use of supply ships, while suitable disposal sites on shore must be set aside. These sites are steadily becoming scarcer. On the basis of this Statoil has worked out a concept which opens the way for a more environmentally friendly method of the disposal of the drill waste.

The Procut system converts the cuttings into a homogenous slurry which can reinjected downhole. In the slurrification process extra fluids must be added. This is usually seawater while other fluids can also be added such as drainage water and production water.

Background

With the benefits of the patented SMACCC technology, the PROCUT slurrification system offers unique advonhges far exceeding the qualities of other similar systems.

The main purpose of the PROCUT slurrificution system is to grind oil-based cuttings to a homogeneous slurry suitable for re-injection into a sub-sea formation. This is done using an automated and continuous process to transport the cuttings from the shale-shaker outlet to a process unit, grind them to a slurry and transport the finished product to a slurry- holding tank.

The slurry has the optimum density, viscosity and particle size for reinjection using a high-pressure pump. The PRCOCUT slurrification process is based on the Patented SMACCC process. When producing slurry for injection, fluid has to be added to the cuttings. The most common Fluid is sea-water, but the system can also take fluids from the drainage system, production water, etc..

The PROCUT system can also be utilised for slurrification of water-based cuttings for disposal directly into the sea. Using this method, the cuttings (now slurry) can be spread over an extensive area by the sea currents instead of building up piles on the seabed around the templates and other sub-sea structures.