This paper decrihes how Conoco planned and implemented downhole injection of oil contaminated drill cuttings on the Murdoch development project in sector 44/22 of the Southern North Sea. Five wells were predrilled through a template from December 1991 to March 1993. The first well, 44/22 D-1, was drilled as a producer and was also designed and used as the injection well for all subsequent wells. Injection was conducted via the 13-3/8"×9-5/8" annulus. The casings were designed to accomodate the injection loads. The injection interval was the Triassic Bunter, a sand/shale sequence between 6200'-7000'TVD, bounded by 200' of salt above and 1500 of shale below. Topside equipment consisted of a prototype Cuttings Processing Package (CPP), a screw conveyor, a slurry storage tank and a small triplex pump.
UKCS regulations prohibit the discharge of oily drill cuttings, but exemptions can be obtained which limit oil on cuttings to 100 gm/kg by weight. As of January 1994, all exploration and apraisal wells will have to meet a 10 gm/kg limit and effective January 1997 all wells will have to meet the 10 gm/kg limit.
For technical and economic reasons, Conoco chose to proceed with the use of oil based drilling fluids on certain projects. With the inevitable legislative constraints of the future the options for disposal of oily drill cuttings were evaluated. These included boxing and hauling the cuttings to shore for landfill disposal, cuttings wash systems, and cuttings reinjection. It was quickly decided that landfill disposal was not an option. This decision was based on safety (handling the boxes), weather (down time) and the increasing number of landfill closures. It was decided that bringing this environmental problem ashore was not environmentally or economically prudent.
Cuttings wash systems were evaluated and proven to be ineffective at reaching the ultimate legislative limits of 10 gm/kg. Cuttings wash systems could only provide a reduction to 40-50 gm/kg and handling the dirty wash fluid compounded the problem.
A process of slurrification and re-injection of oil base drill cuttings was examined and feasibility studies were initiated. It revealed that this option was economically as well as environmentally viable. The task at hand was then to identify the candidate or candidates for utilisation of cuttings reinjection.
It was recognised that this would be the first time this technology would be employed on the UKCS for environmental conservancy. With this in mind a site was selected that could benefit from this technology, but also the project had to be able to economically justify this technology.
In early 1991, Conoco evaluated the economic feasibility of the Murdoch development in the Southern sector of the North Sea (Fig 1). This project was planned as a 9 well development with 6 wells predrilled prior to jacket placement. Scheduling problems with completion of the drilling phase in time for first gas became of paramount importance. Depth, degree of difficulty, high angle wells (Fig 2) and the need for increased rates of penetration demanded that oil based fluids be used.