Integrated approach into the characterization of the fracture network of a geothermal reservoir

Abstract

As an inert gas 222Rn can be employed as a natural radiotracer for the characterization of hydrogeological properties of geothermal systems. In fractured reservoirs, diffusion through non-conductive rock matrices allows recoiled 222Rn to enter larger, hydraulically conductive fractures. In this study, we examine the possibility of assessing fracture dimensions from 222Rn activity in geothermal brines. 222Rn activity in the geothermal fluid is mainly controlled by the 222Rn flux from fracture surfaces and the fracture geometry (aperture, length, degree of connectivity, etc.).Estimates of site-specific fracture length and width were derived by a 222Rn mass balance approach for the geothermal reservoir at Bruchsal (Germany). Integral hydraulic parameters of the Bruchsal reservoir were obtained from classical hydraulic tests. The results show a 222Rn diffusion flux of 4.2 × 10−3 atoms s−1 cm−2 resulting in an equivalent fracture width of 10 mm. The equivalent fracture length varies between a few hundred meters as a function of the flow velocity where about 250 m correspond to a mean flow velocity of 2.0 × 10−6 m s−1.Since the specific surface area of the fracture is inversely proportional to the fracture width, the fracture surface area depends on (1) the type of fracture system, i.e. whether the estimated equivalent fracture width represents a single large fracture or a fracture network of a large number of small aperture fractures and (2) the extent of the variance from the averaged equivalent fracture width. The joint inversion of the radon diffusion model with the pumping test results and a heat transport model is expected to considerably reduce interpretative hydraulic parameter ambiguity.