Stability of a disc-shaped geothermal reservoir subjected to hydraulic and thermal loadings

Abstract

Description of a disc-shaped geothermal reservoir, pressurized and subject to thermal stresses, is given in terms of notions employed in mechanics of ductile fracture. The reservoir is assumed to extend in an infinite isotropic and impermeable medium. Range of the analysis, however, is extended to include a possibility of quasi-static stable cracking occurring along the reservoir circumference prior to spontaneous fracture. Extensive microcracking occuring in the region adjacent to the crack front prior to the onset of crack growth (and in the presence of high tectonic pressure encountered in a geothermal system) provides a stabilizing effect, similar in its nature to ductility exhibited in metal fracture. Numerical example given here shows that the theoretical estimate of the upper critical flow rate of the fluid pumped through the reservoir, at which fracture becomes spontaneous, can be considerably increased (as compared with the values obtained within the LEFM framework) when the nonlinear inelastic behavior of rocks containing the reservoir is accounted for. Available data indicate that the rate of crack extension during the early stage of crack growth can be reduced by several orders of magnitude depending on physical and geometrical factors controlling the microcracking, process. Such result implies widening of the safety margin for a hot dry rock geothermal system. The results given here may also be of use in designing the hydraulic fracturing experiments and in testing various fracturing concepts.