(Expertise on) Drilling, stimulation and reservoir assessment June Jun - 01 Jul Stimulation of reservoir and induced microseismicity - Zurich, Switzerland, Workshop3 Stimulation of reservoir and induced microseismicity - Zurich, Switzerland, Workshop3 June Drilling cost effectiveness and feasibility of high-temperature drilling - Reykjavik, Iceland, Workshop4 Drilling cost effectiveness and feasibility of high-temperature drilling - Reykjavik, Iceland, Workshop4 WP4 Sverrir Thorhallsson WP7 Ernst Huenges

Stimulation of reservoir and microseismicity - Summary of the Ittingen workshop HUENGES 1, Ernst., Kohl² Thomas 1 GeoForschungsZentrum Potsdam (GFZ), ²GEOWATT

productive hydrothermal HotDry Rock Commercial Potentially commercial *Mechanical, chemical, or thermal stimulation, directional drilling etc. Suitable for reservoir enhancement* High Zero natural permeability mod.from USGS Groß Schönebeck Soultz-sous Forets Cooper Basin AU UnterhachingBasaltic rocks ISLandau

How can we enhance the energy recovery? (i)From dry rocks: Creating an artificial heat exchanger at depth using hydraulic- fracturing techniques; operating with surface water for heat extraction, e.g. Soultz-sous-Forêts (ii)From water-bearing rocks: Creating artificial pathways at depth to enhance the thermal water flow, operating with downhole pumps installed in a production well + reinjection well, e.g. Groß Schönebeck Mechanical stimulation

Mechanical stimulation (massive hydraulic injection): pore pressure Shear fracturingTensile fracture generation of microseismicity and transmissivity increases still a matter of trial and error Torsten Tischner

relocation seismic events GPK2, 2000 Asanuma et al Tohoku University, Japan MTC Project European HotDryRock Project Soultz-sous-Forêts

relocation seismic events GPK2, 2000 Asanuma et al Tohoku University, Japan MTC Project European HotDryRock Project Soultz-sous-Forêts

Hydraulic fracturing – Visualization of the process Processes in hydraulic fracturing Wellbore Elastic opening Pressure support of fracture walls Friction Leakoff Fracture Propagation Rock Strength Stress Intensity Factor Injection Peter Fokker

150 mm Proppant fracs successful propped hydraulic fracture requires: knowledge about the in-situ stresses, the reservoir permeability, the elastic parameters, and the fracture propagation criteria.

Geochemical stimulation Acid treatments successfully applied to enhance geothermal production rates to commercial levels developed to address similar problems in oil and gas production wells but applicability to a hot and fractured reservoir less well known temperatures dependence of the acid-rock reaction rate to clean (pre-existing) fractures by dissolving filling materials (secondary minerals or drilling mud) and mobilizing them for an efficient removal by flow transport. E.g. fractures partially filled with secondary carbonates (calcite and dolomite): dissolve these carbonates and enhance the productivity around the well with various amounts of hydrochloric acid.

Thermal (low T) stimulation Gudni Axelsson

Thermal stimulation cracking through thermal shocking (Low T) by intermittent cold water injection, with periods of thermal recovery in-between injection periods (High T) one of the most common methods used for high-temperature well stimulation in Iceland Gudni Axelsson

Lessons learned (1) Understanding Many parameters play a role in the failure mechanism that is expected: Fluid pressure development in the reservoirs, that depends on: –injection rate, time, and length –the setup of the test in the well –fluid type (water, heavy brine, gel of various viscosity, presence of proppants) –fluid temperature reservoir Stress field, that is different for each EGS site –Rock and fractures parameters, like cohesion and Mohr angle, may highly influence results obtained, and could be modified by the use of proppant agents or acid treatments –Shearing represents the relaxation of the natural shear stresses within the rock mass triggered by the weakening of the fracture by the elevated fluid pressure. –The pore pressure increase required to initiate shear failure on favourably oriented fractures is often very small. –Earth's crust is close to failure (critically stressed). Permeability/porosity creation is done by the natural stresses.

Lessons learned (2) To improve the basics it is recommended: to focus on understanding stimulation and to work on the predictability of stimulation to extend the knowledge about stress, artificial fractures, structures, fabrics (fracture propagation) to focus on understanding self propping to take proppant crushing into account to visualise stimulation in different geological environments to investigate scale dependence of fracture propagation and the validation of acidification treatments

The needs are: Extended experiences with 1:1 stimulation tests in boreholes Experimentally modelling of ongoing processes in laboratories Long-term circulation tests Developing modelling tools (microseismicity) and technical tools (seismic anisotropy) Reliable monitoring tools (temperature, pressure, chemistry) to go again to shallow borehole experiments Geology 3D (impact of geology on success of EGS/HDR) Lessons learned (3)

Lessons learned (4) Review HC-Stimulation and EGS-development and others implicate: More exchange hydrocarbon industry with geothermal industry to extend the application of gel injection to take proppant crashing into account to consider the combination of geothermal experiments and CO2- sequestration the exchange with nuclear industry /hydrogeology

Public acceptance Geothermal energy as one of the future energy sources for base load power and heat provision is almost accepted. Exploring (explosion seismics), accessing (drilling into) and engineering (stimulation) the reservoir needs to be explained to the public to hold the acceptance on a high level. Especially high rate stimulation recognisable seismological events must be prepared in a reliable publication management ensuring that every potential affected person is reached. In cases of violation of set thresholds the procedure most be stopped by the geothermal developer. This is much better than being stopped be local authorities! Lessons learned (5)

Thank you for attention Further informations: