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Content archived on 2024-05-21

Faulf, fractures and fluids : gulf of corinth (3F-CORINTH)


Objectives and problems to be solved:
The goal of this project is to take advantage of the DG-Lab (Deep Geodynamic Laboratory) and the CORSEIS projects in the Gulf of Corinth to understand better fluid transfer in and around fault zones. DG-Lab will form a natural observatory in the Gulf of Corinth to drill and core an active normal fault. Such normal faults are of importance to the hydrocarbon industry because they form structural traps for oil & gas. The study of the fault will be completed by logging, mini-3D and VSP, but also by monitoring the wells to give a continuous record of stress and fluid flow. The fluids will be analysed, also on a continuous basis.
For the first time, this will provide a full set of data:
stress (versus depth, time and position to the fault),
strain (versus time and position to the fault),
fluid (flow rate + chemical analysis, paleo-fluid by cement and fluid inclusions data),
core and borehole imaging.
It will also enable understanding of the hydraulic and mechanical behaviour of the fault and calibration of tools for fractured reservoirs, especially by comparing the opening of the fractures with the stress field. The complete set of work planned in the Gulf of Corinth will result in the creation of the Corinth Rift Laboratory (CRL).
Description of the work:
In the framework of the DG-Lab project, two wells will be drilled using CORSEIS, the strain fields versus time will be recorded and permanent geophones will be installed (for seismicity).
In addition to this work, the following will be achieved:
(a) a complete record of stress, strain and fluid flow versus time in, above, below and around fault plane;
(b) an excellent description of the geometrical complexity and resulting anisotropy of the carbonate around the fault plane;
(c) a complete determination of the chemical composition of the current fluid and
(d) the integration of all this data within the description of the fracture network, the modelling of fault behaviour (hydraulic + mechanic), the modelling of fluid/rock interactions, the modelling of the fluid flow in this seismically active area, and the modelling of the micro-seismicity in terms of stress changes and fluid pulses. 3 new geophysical methods will be tested, VSP 4C with gas injection in the mud, Permanent electrode array in cemented well for fluid flow monitoring and Fibre optic captor for stress monitoring along the borehole.
Expected Results and Exploitation Plans:
3F-CORINTH will deliver technological and technical results. Testing of the new tools for monitoring, one for fluid flow base of an array of electrodes will be provided. The new measurement technique based on fibre-optic sensors for downhole long-term monitoring will be established. Technically a methodology to obtain a reliable imaging of fractures in the well bore, and to interpret the fracture response to the various geophysical tools in term of hydraulic properties will be developed. The use of permanent monitoring will also give the potential of testing the measurement of transient changes in the relevant parameters before or after any major tectonic stress change. It may also prove possible to calibrate the model for the determination of earthquake precursors. All data gathered on site will be accessible to a database with various levels of protection, rendered as easy as possible, especially through Internet. The build-up of the data bank will be organized by the co-ordinators of the 3 projects (CORSEIS, DG Lab and 3F-CORINTH) at the level of the CRL (Corinth Rift Laboratory).
Almost all the experiments planed in 3F-Corinth, captor installation and data acquisitions have been done with some little changes to adapt us to the local geology, to the eruptive hydraulic conditions of the AIG-10 well and to the global budget. WP1 were related to the fractures network geometry and hydraulic behaviour in the limestones around the Aigion fault. The AG-10 well was drilled mid 2002, the fault has been crossed at 760m as expected, and successfully cored (through the DG-Lab Project). The borehole images have been recorded between 700 and 1000m depth and few VSP's acquired. The data shows that the fault is a barrier for transverse fluid migration and its sealing capacity is large enough to maintain a pressure gap of 6 bars between the footwall and the hanging wall. The core shows "intact" carbonate, several cataclastic bands, a damaged zone and a breccia with an increase of the shale content. The most spectacular features are the open fractures, especially in the damage zone above the fault with a few mm of void in the main fractures. Sometimes calcite crystals are growing in these fractures, sometimes they remain open, creating a huge permeability. In term of data collection, by comparison with the planned outcomes, the only missing part is the stress measurement. It has been intended by IPGP during the summer 2003 unfortunately due to the eruptive condition in the well the experiment has not be completed. The data acquired through the geological field work and by analysis of the borehole image are now partially published and/or in press. WP2 concerned the monitoring of strain, temperatures and fluid in a second well which were drilled in the Trizonia island during the first part of 2003. The field work which has been done previously has confirmed the high activity of the faults which border this island. The partners involved in the monitoring have designed compatible tools (optical fibre glued on the casing and cemented electrodes in an electrically neutral cement outside the casing) for a common installation in the well. The tools have been manufactured and then installed during the summer 2003. The final competition of the tools, permament electricity, modem has been achieved in Dec 2003. Strainmeters have been installed across 7 segments of the various faults (Aigion, Helike, Lakka and Trizonia) and the monitoring has been continuous since August 2002. Repetitive gravity acquisitions have started, the first survey has taken place in November 2002, a second in November 2003.

The goal of WP3 is the description of the fault network and anisotropy of the seismic waves below the Gulf of Corinth through tomographic approach. The network has been installed for 6 months starting in January 2002. OBS's have been installed offshore during the 2002 spring. The data is now being processed. They highlighted the presence of pre-existing structure, likely related to the Hellenides compressive phase which localise the deformation. Thanks to WP4, fluids are now permanently recorded in two sources, the chemistry of the water, recorded every 10 mn is immediately available by modem in Rome where the main partner is based. The comparison between the seismic events and the flow and chemical anomalies of the waters show an increase of the C02 content related to the short distance seismic events. Two pumping tests have been done in the well AG10 and the samples have been also analysed. The fluid rock interaction is modelled using DIAPHOR to quantify the requested time to seal a fracture under the condition encountered in Aigion. Results show that in the range of hundred year the fractures above the fault can be sealed when below the fault, the chemistry of the water preclude depositions of calcite and favour dissolution. The integration of the data is done thanks to models in WP5. At small scale the rupture process has been analysed, especially the shale content and the temperature influence have been tested; at medium scale the fault spacing has been analysed versus the décollement property and at a regional scale a coupled model has been built. A new theory that links the spacing of the normal fault in a brittle layer with the friction at the brittle/ductile interface is proposed by one of the partners.

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Participants (15)