Naturally fractured reservoirs are considered a prime target for the development of geothermal power production in deep sediments and basement rock. For a successful subsurface heat exchanger it is important to assess if an injection and production well can be connected to a natural fracture network interconnecting the wells and whether the fracture network provides sufficient transmissivity. This study demonstrates that analysis of thermal convection and fracture flow properties from oil and gas wells can be jointly used for the assessment of natural fracture permeability and its volumetric upscaling. We applied this approach to a Dinantian carbonate platform encountered at the Luttelgeest-01 Well (LTG-01) in the Netherlands. Based on publicly available data for the LTG-01 Well we made an estimate of the fracture permeability of the carbonates. Well logs, wireline measurements, measurements on samples and mud loss analysis were examined. Subsequently the minimum required permeability was obtained from a Rayleigh number analysis, consistent with thermal convection inferred from the conspicuously low temperature gradient observed in the well at the fractured reservoir depth level. Subsequently the observed and Rayleigh permeability were compared and the effect of the inferred permeability on reservoir performance was evaluated. The results show that the fracture permeability found at the LTG-01 Well is limited to no more than 3-9 transmissive zones over a depth interval of 600m. Individual fracture transmissivities range from 5 to 10 Darcymeter. It follows from the Rayleigh number analysis that the minimum required cumulative transmissivity of the depth section is approximately 20 Darcymeter. Comparing these results shows that several fractures are sufficient to explain the thermal anomaly by means of thermal convection, provided that the fractures have a good volumetric connectivity. Tensile fracturing of the reservoir, parallel to the horizontal injection and production well can result in significant flow rates as the fractures can well connect to the inferred natural fractures. The flow performance for a reference case with a reservoir temperature of 190°C, at 5 km depth and with a default permeability of 4 mDarcy can range up to 100 l/s. The Levelised Cost of Energy (LCOE) is estimate dto be 17 €ct/kWh, depending on subsurface conditions and cost for hydraulic stimulation.

Press the Back button in your browser, or search again.

Copyright 2014, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.