The two principal horizontal drilling techniques employed within the British Coal Corporation for the exploration of underground coal reserves and faulting are examined. The techniques employed are in-seam guided drilling and reverse-circulation continuous coring. The respective developments and some case histories are presented in detail. The future of these systems and trends in their application are also discussed. The guided longhole drilling technique (GLHD), which generally employs down-hole motor drilling and a specially developed electronic survey package, was conceived in 1979 and has been successfully applied since 1984. The longest in-seam hole is currently 635 m, although the system and equipment have the capability to drive to at least 1000 m. Problems are experienced in drilling certain coal seams with a weak structure, but steps taken to mitigate these conditions are described, including drilling techniques and parameters and specially formulated muds. Hardwire measurement-while-drilling surveying systems developed at British Coal's Technical Services and Research Executive (TSRE) (formerly Headquarters Technical Department (HQTD)) are also described, together with the results of testing on surface. The TSRE will continue to appraise and extend the scope of applications of GLHD (including the use of longholes for 'pre-' and 'post-drainage' of methane from underground workings). The reverse-circulation coring technique (RCC) effectively employs a wireline drill string without a core barrel; the core is flushed through the rods by the reverse fluid circulation. The first hole was drilled in 1980 and, to date, 66 holes with an aggregate length of more than 13 km have been drilled. Drilling techniques and parameters are reviewed. Future developments will relate to extension of the system's range beyond the current capability of 1000 m. The use of muds to control weak formations is also being considered.
