The Integration of Oilfield, Mining and Geotechnical Exploration Technologies for a Deep Scientific Investigation to Depths up to 2000 m

Abstract

ABSTRACT A programme of boreholes to depths up to 2000 m for the investigation of potential sites for the UK national repository for the disposal of low and intermediate level radio active waste included the requirement for high quality continuous coring through a range of geologies, a 159 mm (6-1/4 in) minimum diameter hole for a comprehensive wireline logging programme and a suite of testing both during drilling and at the end of each borehole interval. These special requirements prompted the adoption of a blend of oilfield, mining and geotechnical exploration equipment and procedures to achieve the various objectives within the quality standards. Three rigs have been used at sites at Seilafield, Cumbria, England and Dounreay, Caithness, Scotland. At Seilafield, two intermediate depth standard diesel electric oilfield land drilling rigs were modified to include hydraulic top drives, wireline coring winches, small triplex coring pumps integrated with a secondary mud system dedicated to coring, electromagnetic flowmeters to monitor in and out flows and an improved, more sensitive automatic drilling system than is normally fitted to this class of rig. At Dounreay, a large hydraulic top drive mining exploration rig was used which included a pipe handling robotic device to improve trip speed. Continuous coring was carried out on all rigs with a 139.7 mm (5-1/2 in) heavy duty mining wireline drilling system with 159 mm (6-1/4 in) bit diameter and 100 mm (4 in) nominal core diameter with the use of uPVC corebarrel liners to protect the core and assist in surface handling to minimise disturbance. During drilling, environmental pressure measurements were taken at intervals of about 50 m to determine the in situ environmental pressure profile and on the completion of each interval of drilling, a suite of straddle and full sector tests were carried out after a comprehensive wireline logging programme. A comprehensive quality assurance programme is in operation. The boreholes were opened after each interval had been cored and tested to allow standard oilfield casing up to 508 mm (20 in) diameter to be installed and cemented. Hole opening was carried out using conventional hole openers and rock bits with mud flush and with down-the-hole hammers and air or foam flush in the stronger rocks. Five holes were completed by August 1991 representing over 7000 m of drilling and associated testing and logging. Further boreholes are in progress. Logging and particularly testing together with the associated tripping accounted for a significant element of the overall time to carry out these special boreholes, but the nature of the project is such that accurate, high quality data is essential for planning further investigations and assessing the suitability of the site for an underground repository. The highly successful integration of oilfield, mining and geotechnical exploration technologies for this special investigation programme includes several elements which are important in the application of slim hole methods for oil and gas exploration. Many of the technical issues associated with a slim hole approach have been addressed in the development and application of the drilling, coring and testing equipment and systems. The project has given an opportunity to evaluate the advantages and disadvantages, merits and limitations of applying different drilling and associated technologies for deep hole construction to safely and routinely provide high quality geoscientific information.