ABSTRACT

Great difficulties - unexpected rock ruptures and excessive deformation, sudden dynamic loading, delayed ruptures, influx of solids - are encountered when working at great depth, due to very high level of stress, temperature, pore pressure and fluid or temperature gradients around excavations. Accesses to workings are sometimes impossible and observations difficult as in oil or gas production. Consequences can be considerable in terms of safety and cost. Very severe conditions met at great depth involve failure mechanisms not perfectly known, identified, and conditions of onset not correctly appraised. Moreover failure mechanisms may combine, making the analyses of the failure causes more difficult, and the remedies more or less efficient and uncertain. While excavations for mining and civil engineering are accessible (except passes for safety reasons in mines, and pressure tunnels for dams), pressure and temperature may be very variable in oil/gas wells, and observations have to be done by means of logging tools. It is sometimes claimed that conditions and problems met in mining and civil engineering are too different from those met for oil/gas production to be compared. We believe instead that these very different conditions and geometry of both types of excavation permits to collect a lot of information profitable to all the specialists. This is particularly true at great depth where rock mechanics environment is determinant. An exchange of information, observations, between specialists involved in projects of excavations or operations made with totally different purposes is highly fruitful. This presentation is a very preliminary comparison and basis for a further exchange of experience between specialists and responsible persons involved in mining, civil engineering and oil or gas production at great depth. After comparison of similarities and differences between requirements of each industry and effects of rock parameters, the impact of the Rock mechanics approach in terms of safety and economy is recalled. Some remaining difficulties and still unsolved issues are reviewed and some recommendations - or more modestly suggestions to help solving these problems - are made.

1 INTRODUCTION

Rock failures (or excessive deformation) - more generally unexpected behaviour - occurring at great depth are of paramount interest, as a compensation for their negative impact on the operations. The high level of stresses generated around the excavations for instance results in clear ruptures which can be compared to the existing criteria of failure. These criteria can in return be validated or contradicted, or some limitations can be recognised before a more general utilisation. Moreover, rock failure comparison between those observed in various industrial activities are of utmost interest. Excavations made for mining or civil engineering are most of the time at atmospheric pressure with ruptures due to stress concentration which can be directly observed and monitored before any permanent lining is installed (water pressure tunnel for instance). Excavations made in the oil industry (exploration or production wells) are submitted to various internal conditions (pressure, temperature, fluid and temperature gradients). They are much smaller and exhibit more complex rupture modes and failure mechanisms.

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