Deep Offshore Slim Hole Drilling
Authors: Duhen T. , Sagot A. , Kerbart Y. 
Published: October 2008

Abstract
This paper describes means of porting the Slim Hole Drilling concept to deep offshore applications. Slim Hole technology is now routinely used onshore. Positive results have been proven in terms of cost reduction, minimisation of the environmental impact of drilling activity, drilling efficiency and safety and quality of geological evaluation.
Presently in conventional drilling, offshore units are mostly operating at water depths less than 1,500m. Increasing the water depths for these units is costly and difficult. One alternative is to consider slim Hole drilling. In fact the technological barriers initially raised by Slim Hole drilling for kick detection and kick control, mechanical behaviour of the drillstring and rheological behaviour of the mud are now completely mastered. It is now proven that technology is available today for electric logging and testing in slim hole wells. In addition, onshore experience for more than two years in various countries has shown the clear economical advantages of Slim Hole drilling.
The challenge of porting the Slim Hole concept to deep offshore applications is to adapt to floating support, and solve specific problems such as heave compensation, to allow an accurate control of the weight on bit as well as designing a new riser system.
This paper reviews different alternatives for drilling equipment to achieve the above challenge, allowing a reduction of the size of the vessels suitable for Slim hole drilling. The ability to adapt Slim Hole drilling to deep offshore drilling will allow oil exploration to 2,000m and beyond, with a significant reduction of costs compared to conventional exploration drilling.

Introduction
Forasol has gained experience in Slim Hole Drilling since the commissioning of the rig Foraslim-1 in 1994 dedicated to Slim Hole Drilling. Since then this rig has been operating for Total and Elf in Gabon and Shell in Romania. During these two drilling campaigns described in reference 1 and reference 2, the cost reduction as well as the minimisation of environmental impact provided by slim hole drilling has been fully validated. The efficiency and safety of the drilling process is also insured, through a refined measurement and control of all the drilling parameters, as well as the quality of the possible geological evaluation.
Taking into account this onshore experience, three factors are in favor of the development of this technology for exploration and appraisal activities on floating rigs. Firstly the high cost for operating in water depths greater than 1,500m with conventional offshore systems induces a reduction of costs with offshore slim holes still more important than onshore. Secondly the environmental impact of the oil industry will become a more and more important factor in operations as legislation puts more stress on it.
This paper presents the technical challenges to be solved as well as several suggestions which can be envisaged in order to transform the potential benefits of the deep slim hole concept offshore into an effective and efficient technology. These benefits will come firstly from the floating support.