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.

作为惰性气体²²² Rn可以用作天然放射性示踪剂，用于表征地热系统的水文地质特性。在裂缝性储层中，通过非导电岩石基质的扩散使反冲的²²² Rn进入较大的水力传导裂缝。在这项研究中，我们研究了从地热盐水中²²² Rn活度评估裂缝尺寸的可能性。地热流体中的²²² Rn活性主要受裂缝表面的²²² Rn通量和裂缝几何形状（孔径，长度，连通度等）控制。

通过²²² Rn的质量平衡方法，对Bruchsal（德国）的地热储层进行了现场特定裂缝长度和宽度的估算。Bruchsal油藏的整体水力参数是从经典水力测试中获得的。结果显示出4.2×10 ^-3原子s ^-1 cm ^-2的²²² Rn扩散通量，等效断裂宽度为10 mm。当量裂缝长度根据流速在几百米之间变化，其中约250 m对应于平均流速2.0×10 ^-6  m s ^-1。

由于裂缝的比表面积与裂缝宽度成反比，因此裂缝表面积取决于（1）裂缝系统的类型，即估算的等效裂缝宽度是表示单个大裂缝还是大裂缝网络小孔径裂缝的数量和（2）与平均等效裂缝宽度的差异程度。ra扩散模型与抽水试验结果和传热模型的联合反演有望大大降低解释性水力参数的歧义性。