A New Plug and Abandon Well Operation to Avoid Discharge
Authors: Sola Bengt , Daulton Dan 
Published: December 2007

A new process has been developed that radically changes the way in which cementing operations are conducted offshore. A conventional cementing operation performed in the Norwegian North Sea area is usually conducted with the use of large cement bulk tanks and a batch mixer. Often the cement mix water is prepared in a mud pit due to the quantity and number of different additives in addition to the base water requirement for the slurry design.

A cementing system that does not rely on offshore mixing, is available to the industry – a storable oilwell cement slurry that can be kept in a liquid state for more than 6 months and made to set as and when required (1). Operators in the North Sea are now benefiting from this technology. Slurries are mixed onshore and laboratory tested to ensure they meet the specific design criteria well before they are required on site.

Liquid Cement Premix (LCP) technology can be used to reduce the necessity of needing cement mixing facilities for plugging and abandonment from offshore rigs. This LCP process also helps reduce waste as well as minimise the environmental, human health and safety impact of the cementing operation.

The technique offers radical changes to conventional cementing operations and equipment requirements, it also leads to direct savings in the cost of personnel, equipment mobilisation and material wastage.

During recent years increased focus has been put on the reduction of chemical discharges to sea from offshore oil and gas exploration and production operations. Several hundred oil and gas wells in the North sea are scheduled to be abandoned in the next year or so. All of these wells will require the use of cement at some stage of the plugging operation. The mixing of dry powdered Portland cement and water has been used since the first application of cement in the oilfield and is still used as the primary plugging material. The manufacturing process of the Portland cement can yield slight variations in the quality of the cement slurries used. The cement is also often transferred from one bulk tank onboard to another-one in order to stabilize the rig ballast, this process will increase the H2O content in the dry cement and change the properties of the final slurry composition.

The new technique with use of LCP will improve the quality of the slurry and reduce the discharge from the cementing operations to zero.

In order to achieve the zero discharge goals for cementing operations significant technical improvements have been developed and put into effect. It must be taken into consideration that for installations not equipped with any cement mixing systems or older installations where the cementing mixing systems have not been upgraded in recent times, the cost of bringing in mobile equipment or alternatively installing new equipment that meets the requirements for zero discharges can be significant and in most cases uneconomic.

The following paper will discuss and present a new method for P&A cementing operations with a zero discharge philosophy. This is based on the successful experience of using a premixed cementing system in the Norwegian North Sea field for Liner cementing, plug cementing and external casing packer cementing with zero discharge.

The LCP is transported offshore in suitable transport tanks with air driven agitators and pumped in to the well to ensure a homogeneous slurry from start to finish.

Utilisation of LCP can negate problems with well cementing in environmentally sensitive areas.