Understanding hydrothermal processes during production is critical to optimal geothermal reservoir management and sustainable utilization. This study addresses the hydrothermal (HT) processes in a geothermal research doublet consisting of the injection well E GrSk3/90 and production well Gt GrSk4/05 at the deep geothermal reservoir of Groß Schö nebeck (north of Berlin, Germany) during geothermal power production. The reservoir is located between )4050 to )4250 m depth in the Lower Permian of the Northeast German Basin. Operational activities such as hydraulic stimulation, production () change the HT conditions of the geothermal reservoir. The most significant changes affect temperature, mass concentration and pore pressure. These changes influence fluid density and viscosity as well as rock properties such as porosity, permeability, thermal conductivity and heat capacity. In addition, the geometry and hydraulic properties of hydraulically induced fractures vary during the lifetime of the reservoir. A three-dimensional reservoir model was developed based on a structural geological model to simulate and understand the complex interaction of such processes. This model includes a full HT coupling of various petrophysical parameters. Specifically, temperature-dependent thermal conductivity and heat capacity as well as the pressure-, temperature-and mass concentration-dependent fluid density and viscosity are considered. These parameters were determined by laboratory and field experiments. The effective pressure dependence of matrix permeability is less than 2.3% at our reservoir conditions and therefore can be neglected. The results of a threedimensional thermohaline finite-element simulation of the life cycle performance of this geothermal well doublet indicate the beginning of thermal breakthrough after 3.6 years of utilization. This result is crucial for optimizing reservoir management.
The static and poroelastic moduli of a porous rock, e.g. the drained bulk modulus, can be derived from stress-strain curves in rock mechanical tests and the dynamic moduli, e.g. dynamic Poisson's ratio, can be determined by acoustic velocity and bulk density measurements. As static and dynamic elastic moduli are different a correlation is often required to populate geomechanical models. A novel poroelastic approach is introduced to correlate static and dynamic bulk moduli of outcrop analogues samples, representative of Upper-Malm reservoir rock in the Molasse basin, southwestern Germany. Drained and unjacketed poroelastic experiments were performed at two different temperature levels (30 and 60• C). For correlating the static and dynamic elastic moduli, a drained acoustic velocity ratio is introduced, corresponding to the drained Poisson's ratio in poroelasticity. The strength of poroelastic coupling, i.e. the product of Biot and Skempton coefficients here, was the key parameter. The value of this parameter decreased with increasing effective pressure by about 56% from 0.51 at 3 MPa to 0.22 at 73 MPa. In contrast, the maximum change in Pand S-wave velocities was only 3% in this pressure range. This correlation approach can be used in characterizing underground reservoirs, and can be employed to relate seismicity and geomechanics (seismo-mechanics).
The pore pressure changes, due to injection and production of water into a geothermal reservoir, result in changes of stress acting on reservoir rock and consequently changes in mechanical and transport properties of the rock. The bulk modulus and permeability were mea- The crack porosity was calculated at various temperatures. While the pore volume changes of the cracks are not significant but control fluid flow pathways and consequently the permeability of the rock. A new model was derived which relates the microstructure of porosity, stressstrain curve and permeability. The porosity change was described by the first derivative of stress-strain curve and permeability evolution was ascribed to crack closure and was related to the second derivative of strain-stress curve. The porosity and permeability of Flechtinger sandstone decreased by increasing the effective pressure and after each pressure cycle.
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