Characterization of a penny‐shaped reservoir in a hot dry rock

The mechanical stability of a penny‐shaped reservoir is characterized by fracture mechanics including thermoelastic effects in connection with research into the extraction of geothermal energy from hot dry rocks. The condition for stability of a reservoir, which is not changing radius by propagating or closing, requires 0 < k < kc, where k and kc are the stress intensity factor and the fracture toughness of the rock. From this condition the upper and lower critical flow rates m* and m*, necessary for reservoir stability, are obtained in terms of the outlet fluid pressure on the earth surface, the fluid temperatures at the inlet and outlet, the geometry of the reservoir, and the physical constants of the fluid and the rock. The behavior of a reservoir when the inlet flow is changed also is studied. According to the value of the outlet flow rate m0, the following three cases are considered: case 1; m0 = m*; case 2; m* > m0 > m*; and case 3; m0 = m*.