This paper proposes a method for estimating the surface area of a deformable fracture induced artificially in a hot dry rock. The method is based on a concept of a model consisting of some parallel plannar plates and uses an empirical permeability form including the fracture surface roughness. The fracture surface area is given in a simplified form by A=n·A
0, where A is the fracture surface area, A
0 the surface area of the planar plate, and n the number of the plate. n is determined by rounding off the value of t
2/12K to a whole number, where t is the average fracture aperture and K the fracture permeability. Fluid flow, energy and mass equations which involve the effect of A are solved numerically by the finite difference method. The results show that as A increases the dissolution and precipitation rates of quartz increase and that the computational error for accumulative silica production rate due to round off the value of t
2/12K is about 12% for 800 days after water injection.
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