(Expertise on)Drilling, stimulation and reservoir assessment

June 2006 29 Jun - 01 Jul Stimulation of reservoir and induced microseismicity - Zurich, Switzerland, Workshop3

June 2007 28 - 29 Drilling cost effectiveness and feasibility of high-temperature drilling - Reykjavik, Iceland, Workshop4  

WP4 Sverrir ThorhallssonWP7 Ernst Huenges

Stimulation of reservoir and microseismicity - Summary of the Ittingen workshop

HUENGES1, Ernst., Kohl² Thomas1GeoForschungsZentrum Potsdam (GFZ),

²GEOWATT

productivehydrothermal

HotDryRock

CommercialPotentially commercial

*Mechanical, chemical, or thermal stimulation, directional drilling etc.

Suitable for reservoirenhancement*

High Zeronatural permeability

mod.from USGS

Gro

ß Sc

höne

beck

Soul

tz-s

ous

Fore

ts

Coo

per B

asin

AU

Unt

erha

chin

gB

asal

tic ro

cks

IS

Land

au

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 fracturing Tensile fracture

S2

S1

S2

S1

Wasserdruck

Öffnung, Scherung (Bruchgeräusche)

S2

S2

S1

S1

Abstützung

STIMULATION :

S2

S1

S2

S1

Kluft

generation of microseismicity and transmissivity increases

still a matter of trial and error

Torsten Tischner

relocation seismic events GPK2, 2000

Asanuma et al. 2002Tohoku University, JapanMTC Project

European HotDryRock Project Soultz-sous-Forêts

relocation seismic events GPK2, 2000

Asanuma et al. 2002Tohoku University, JapanMTC 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)

UnderstandingMany 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)

http://conferences-engine.brgm.fr/getFile.py/access?resId=0&materialId=14&confId=3

Thank you for attention

Further informations: