| Title | Fluid-Flow Zones in a Geothermal Sandstone Reservoir: Localization From Thermal Conductivity and Temperature Logs, Borehole EPS1 (Soultz-sous-Forêts, France) |
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| Authors | Sébastien HAFFEN, Yves GERAUD, Marc DIRAISON, Chrystel DEZAYES |
| Year | 2013 |
| Conference | Stanford Geothermal Workshop |
| Keywords | geothermal, thermal gradient, thermal conductivity, permeability, sandstone reservoir, fluid circulation |
| Abstract | In order to determine the fluid flow pattern within the sandstone formation in the Rhine graben (Buntsandstein, Lower Triassic), we compare two temperature gradient logs: on the one hand, the observed gradient log derived from temperatures measured along a borehole, on the other hand, a calculated one based on thermal conductivities measured on borehole cores. If the heat fluxes are only due to conduction processes, calculated and observed temperature gradient logs are expected to be similar. If they are also governed by convection processes, resulting from fluid flows through the fault system connecting permeable layers; discrepancies should be observed between the two temperature gradient logs. Comparison of logs, by subtracting one from the other, allows locating flow path zones along a borehole. We applied this methodology to the sandstone formation of the Buntsandstein cored by borehole EPS1 (Soultz-sous-Forêts, Upper Rhine Graben, France), for which temperature logs and cores samples are available. Variations between the two temperature gradient curves revealed three main hot fluid flow intervals alternating with non-flow zones. The temperature gradient logs were then compared with the fracture distribution along the borehole and sedimentological and petrophysical analyses carried out on the borehole cores, in order to identify parameters explaining the presence of flow zones. Two types of flow zones can be distinguished: (1) those structurally controlled with two major fault zones providing a flow path for the deep hot fluids, and (2) those controlled by sedimentary or diagenetic processes within the Playa-lake and Fluvio-aeolian marginal erg facies. We can also determine that some high matrix permeability levels (braided river formations) do not support macroscopic fluid flows because macroscopic sedimentary structures of thick oblique clayey layers drastically reduce the matrix connectivity, hence the macroscopic permeability. |