Numerical simulation of geothermal energy production from hydraulic fractures in tight sedimentary rock formations by cyclic-injection-production-schemes

摘要

In the geothermal research project GeneSys the Geozentrum Hannover is investigating new methods for exploiting heat from tight sedimentary rocks. Key technology is the waterfrac-technique, by which large artificial fractures are created in these formations. The fractures are used as underground heat exchangers. One of the concepts tested successfully in an abandoned gas well (Horstberg Z1) envisages extracting heat from large rock volumes by cyclic injection/production schemes. Based on the results of these tests a conceptual and numerical model was developed and calibrated. A large number of numerical simulations were performed by using the finite element code ROCKFLOW in order to determine the long-term performance of different versions of this one-well cyclic concept. The results of the numerical simulations show that geothermal energy production by cyclic loading and de-loading of hydraulically induced fractures in low permeable rock is a promising technique for space heating for consumers of medium size, such as hospitals, building complexes etc. with a thermal power consumption of some MW_(th). The weekly cyclic scheme, being tested already in borehole Horstberg Z1, is technically simple but its long-term thermal performance is only fair. The annual-cyclic-scheme has a much better thermal performance. Its technical implementation however is difficult. The choice between these schemes has to include the specific conditions at the site. The computations of multiple fracture systems showed that they have advantages in terms of pumping power consumption compared to singe-fracture systems.