Analytical studies suggest that the thermal stresses produced by fluid circulating in a hot dry rock geothermal reservoir are likely to initiate and propagate cracks in the rock. Such thermally induced cracks will augment the power extracted from a reservoir if they cause a significant increase in effective heat transfer and flow areas. It is thus important to determine experimentally: (a) the conditions under which such cracking will occur, (b) the compatibility of these conditions with expected operating conditions of reservoirs, and (c) the extent to which energy extraction can be enhanced by cracking.

In order to begin a study of items (a) through (c), the behavior of granite samples subjected to thermal stressing is being investigated. In particular, the fracture strength and porosity of the samples is being explored for various combinations of rock temperature and quenching severity. In Hurphy's analytical model for thermal stress cracking under full-scale geothermal reservoir conditions, it is hypothesized that cracking will occur in those regions where tensile thermal stress exceeds the "effective" compressive earth stress, assuming that the tensile strength of the rock is negligible. compressive stress to be overcome is significantly reduced if the rock is or becomes sufficiently permeable to allow fluid infiltration such that the pore pressure is raised to the hydrostatic level. Also, changes in the porosity of the rock may influence reservoir heat transfer behavior.3 Thus information on the effect of thermal stressing on strength and porosity will be helpful in improving understanding of both the thermal fracturing and energy extraction characteristics of geothermal reservoirs.

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