1 INTRODUCTION
To investigate the effects of temperature, stress and pore water on creep of rock, we have conducted a series of triaxial tests on sandstone and siltstone rock. The samples were cylindrical with a diameter of 0.04 m and a length of 0.08 m. The sandstone samples were tested with pore water at a temperature range of 150-250º C and the siltstone samples were tested at a range of 250-900º C. Microscopic studies and XRD-analysis were used for identification of the creep mechanisms. The observed creep of the sandstone samples could be quantified, but the creep of the siltstone samples was controlled by a combination of thermo-mechanical and chemical deformation processes. Results will be used for numerical calculations to describe the stability of an underground gasification cavity, but the results on sandstone samples can also be used to predict the subsidence of oil reservoirs subjected to steam injection.
Western Europe has vast coal reserves (over 400,000 million tonnes), which lie mainly at a depth of 1000 m or more. Many of these coal deposits are therefore technically unminable or uneconomic to mine by means of conventional mining methods. The process of underground gasification has the potential to exploit many of these coal resources. The system consists of a pair of bore holes involving an injection well, through which the gasification agents (oxygen, water) are injected into the coal seam where gasification reactions produce a combustible gas. A production well leads the gas to the surface where it can be used as a fuel. Gasification at great depth is, however as yet not a technically proven process. Gasification at shallow depths has been used on a commercial scale in the 1950's in the Soviet Union. In the USA a field testing programme supported by much research was carried out from 1972 onward. However, the circumstances at which these gasification processes have taken place are not comparable to the European situation. The gasified coal seams were relatively thick (2 - 11 m), shallow situated (40 -270 m) and sometimes steeply dipping, while European seams are thin (mostly<3 m), at great depth (600 - 1500 m) and nearly horizontal. Commonly the European coal is also of a higher (and thus less reactive) rank than the sub-bituminous coals used in the USA. The main rock mechanical aspect of the gasification of deep, thin horizontal coal seams is the stability of the cavity formed by the consumption of coal. An extensive lateral developed gasifier (necessary from an economic point of view) requires a balance between coal consumption and rock failure such that a good gas quality is obtained. Two mechanisms which are important to roof rock failure at high temperature and stress conditions are thermo-mechanical spalling and creep. In this paper we present results of triaxial creep tests for two types of rock under various circumstances. First the mineralogy and origin of the samples are described, followed by the description of the apparatus and the procedure of testing.
