The activation of automatic alert system for seismic event with magnitude > 1.5 occurring near the ... In general, th
DESCRAMBLE - Final Conference Drilling in dEep, Super-CRitical AMBient of continentaL Europe
High resolution microseismic monitoring: first results Spinelli Rebecca, Cinci Marcello.
Pisa 28th March, 2018
High resolution microseismic monitoring Agenda
Agenda: • Aims of the microseismic monitoring • Seismographic coverage of the area • Seismometers set up • Recorded seismicity of the area • Possible correlation between seismicity and drilling operations
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High resolution microseismic monitoring Aims of the microseismic monitoring The high resolution microseismic monitoring of the VENELLE_2 area is aimed at:
- Improving the resolution and quality of the natural microseismicity monitoring of the area - The activation of automatic alert system for seismic event with magnitude > 1.5 occurring near the VENELLE_2 well during the deepening operation, in particular during the supercritical phase conditions - Checking any possible correlation between seismicity and drilling operations - Highlighting any possible change in seismicity by comparison of blank measurements and seismicity occurred during drilling.
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High resolution microseismic monitoring Seismographic coverage of the area - Larderello Travale Seismic Network (LTSN) The LTSN is in operation since 1976 26 Seismic Stations (SS), with spacing varying from 3 to 9 Km, cover an area of about 820 km2 LTSN ensures the detection of microseismic events with magnitude often also near 0, with a catalogue completeness of 0.7 in Magnitude. All SS are real time connected to the Larderello seismological datacenter. Here, the signals coming from the stations are scanned through the automatic acquisition system. When an event is detected, it is automatically processed and, in a few seconds, hypocentral and magnitude determination are provided.
Whenever an earthquake with given magnitude occurs near the geothermal plant or drilling site, an alert is being sent to the seismologist and to the plant operators. 4
High resolution microseismic monitoring Seismographic coverage of the area - VENELLE_2 High Resolution Microseismic Network (V2HRMN) With the aim of improving the LTSN detection capability for the VENELLE_2 area, four Additional Temporary Seismic Stations (ATSS) at 1.5 to 2 km distance from the VENELLE_2 well were installed. The four new ATSS are: CA1, VE1, VE2, and VE3 A dedicated microseismic network (V2HRMN) composed by the four ATSS and 16 out of 26 SS of the wider LTSN, was set up The V2HRMN was in operation from the beginning of 2016 On June 2016 the borehole and posthole sensors were installed
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High resolution microseismic monitoring Seismometer setup - Castiglioni_1 station (CA1) A Trillium Borehole Broadband 3C Seismometer was installed into the Castiglioni 1 (not used) geothermal well at depth of 136m from surface. The seismometer is equipped with a hole-lock mechanism to ensure the coupling of the instrument with casing. For this application, in collaboration with Nanometrics engineers, a special cable and a stainless steel pressure vessel for H2S hostile environment were designed.
136m is the maximum usable depth because of the high temperature 6
High resolution microseismic monitoring Seismometer setup - VE1, VE2, VE3 stations The Trillium Compact Posthole 3C seismometer was placed into a shallow well at depth of 10 m. The well was Cased with PVC casing In order to ensure a good coupling between sensor and rock, the bottom part of the well was filled with concrete poured in place down to 10 m depth The annulus was filled with sand.
The sensor is resting on the bottom hole concrete
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High resolution microseismic monitoring Seismometer Setup - Digital recorder for the four new stations The four ATSS were equipped with Nanometrics digital recorder
• High-resolution true 24-bit ADC in three or 6 channel configurations • GPS clock • Sample rates up to 5000 sps • Ultra-low noise floor for use with high performance broadband seismometers • Rugged field enclosure rated for continuous submersion
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High resolution microseismic monitoring Recorded seismicity of the area – Epicenters distribution The two years period of observation covers: • blank measurement, 15 months duration, before the start of VENELLE_2 deepening operation, • The drilling period from April 28, 2017 to November 12, 2017, thru the end of 2017. In the observation period, 373 events with magnitude ranging from -1.04 to 1.92 were detected. Seismic activity is present more or less over the entire area, In detail it is possible to observe: • The Terme del Bagnolo, Sasso Pisano and Lustignano clusters •
The NW-SE alignment that from Lagoni Rossi thru the well VENELLE_2, Lago Boracifero goes down to Poggio al Lupo.
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High resolution microseismic monitoring Seismicity of the area – Depth distribution Earthquakes take place almost entirely within 5 km depth, and predominantly at 2-4 km. In general, the hypocenters are located above the K horizon, with a depth ranging from 1.5 to 3.5 km. In the south eastern quadrant, the hypocenters deepen down to 3.5 - 4.5 km forming a SW- NE alignment in correspondence of a strong discontinuity of the K horizon. Here lack in the continuity of K horizon seismic signal is observed. This discontinuity acts as a clear-cut boundary for the seismic active zone.
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High resolution microseismic monitoring Seismicity of the area – Focal Mechanisms The focal mechanisms where evaluated by a seismic source model taking into account the slip along the fault and also the fault opening by tensional forces (Caputo and Console, 1989). The normal fault focal mechanism can be correlated with the NWSE trending normal fault system that extends downwards in listric surfaces (Brogi et alii, 2003).
The strike-slip focal mechanisms observed for the deepest earthquakes are aligned along a SW-NE K horizon discontinuity, and can be correlated with tectonic structures linked to the close structural lineament transverse to the thrust belt (Sani et al. 2016)
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High resolution microseismic monitoring Seismicity of the area – Frequency distribution On July 2016, a strong variation of the earthquakes occurrence is observed.
Monthly frequency histograms of the recorded seismic events
The apparent increase in seismicity is mainly due to the contribution of the events with magnitude < 0.5, and especially with magnitude < 0
This is not a real increase in seismicity, but it is the improvement of seismic network detection capability due to posthole and borehole sensors installation. 12
High resolution microseismic monitoring Seismicity of the area – Magnitude of completeness (Mc) The improvement of seismic network detection capability allows to obtain a catalog with Mc = 0.2
The Mc was estimated by comparison between GutenbergRichter (G-R) model and the observed frequencymagnitude distribution (FMD)
The Mc is the magnitude at which the lower end of the cumulative FMD departs from the G-R model
The Mc estimated for the VENELLE_2 area is less than the Mc estimated for the whole geothermal area, which is about 0.7 13
High resolution microseismic monitoring Possible correlation between seismicity and drilling operations Spatial and temporal relationship between seismic events occurred in close rock volume around VENELLE_2 well and drilling operations allow the evaluation of possible induced or triggered seismicity phenomena. • Partial or Total loss of circulation (PLC and TLC) • Leak-Off Tests (LOT)
Spatial-Temporal relationship between seismicity an PLC-LOT operations Temporal relationship between seismicity and PLC-LOT operations
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14 No temporal and spatial correlation seems to exist between the seismicity and drilling operations.
High resolution microseismic monitoring -
Thank you For your attention The research leading to these results has received funding from the European Union’s Horizon2020 Research and Innovation Program under grant agreement No 640573 (Project DESCRAMBLE) 15
