For measuring the rock strength and elasticity at high temperature a triaxial cell has been developed in which a sample (having a diameter of 40 mm) can be heated up to 1000 °C, with a confining stress of 150 bar and an axial stress up to 2500 bar. Measurements on a homogeneous sandstone showed a relatively small influence of the temperature on the strength for samples which were heated and then sheared. Pre-shearing of the samples (which had little influence without heating) resulted in a reduction of the strength at 800 °C to 30% of the value at room temperature. This was found to be related to a strong increase in micro-cracking at high temperature. With the aid of the finite element method the response of a cavity in rock to heating was computed. For the model test an ovaloid hole was drilled in a cube of homogenous sandstone (0.3 m on a side). The cube was loaded in a true-triaxial loading device and the convergence of the cavity was monitored during heating of the cavity wall by a heater inside the cavity. In this way the validity of the numerical model could be checked in a case with well defined boundary conditions, but in the presence of stress and temperature gradients and asymmetric stress conditions. Both in the model test and in the numerical simulation a significant influence of the thermal loading was found. This resulted in a strong convergence of the cavity. Although a large difference occurred between prediction and experimental result, the numerical model was considered successful in view of the crude approximation to both the material properties and the material behaviour. For instance the numerical model did not include large scale fracturing, which occurred in the model test.
Western-Europe has large unminable coal reserves at a depth of 1000–1500 m which may be recovered with the aid of in-situ gasification. However, gasification at great depth is not by far a technically proven process. In several field tests problems were encountered to obtain a link between injection and production well or to maintain the link during gasification. The combination of high rank coal and high overburden stress has hampered the development of a gasification channel as does occur in shallow gasification. Even if the first stage of linking, igniting and starting the gasification is accomplished, there remains the problem of lateral growth of the gasifier. Economic analyses indicate that a width of 30 to 100 m is required for 1 m seam thickness (Boswinkel, 1984). Such an extensive lateral development of the gasifier requires a balance between cavity failure and coal consumption such that a good gas quality is obtained. This is not mere speculation but can be supported by field data from the Pricetown experiment, which was conducted in a thin seam at moderate depth. In the post-burn study it was stated that the formation of slag (partly molten rock) caused overriding of already carbonized coal (Zielinski et al, 1981).
