GAPSS Geothermal Area Passive Seismic Sources Sismicità associata allo sfruttamento del campo geotermico di Larderello-Travale G. Saccorotti, D. Piccinini

Enhanced Geothermal Systems (EGS) ● Is a subsurface heat exchanger designed to improve and potentially expand the heat extraction operations so that they become more economic; ● Most commonly, an EGS is needed wherever the reservoir rocks are hot but their permeability is low. In such systems, permeability may be enhanced by hydraulic fracturing, high-rate water injection, and/or chemical stimulation; tensile failure occurs Hydrofracturing when fluid injection > rock fracture gradient → tensile failure occurs Failure should end when the pressure is < rock fracture gradient Injection pressure < rock fracture gradient → can also induce seismicity (low magnitude – 4.6 Geysers 1980's)

Mechanisms of induced seismicity in EGS ● Pore pressure increase: increase fluid pressure can decrease static friction → seismic slip. High number of MEQ occur as the pressure migrates away from the well.. ● Temperature decrease: Cool fluids can cause contraction of fracture surfaces (thermoelastic strain) → slight open of fracture reduce the static friction promoting slip. Alternatively, contraction can cause new fractures (non double-couple mechanism). ● Volume change due to fluid injection: cause a perturbation in local stress conditions close to the failure state → seismic slip ● Chemical alteration: injecting non-native fluids (or allowing “outside fluids” to flow into reservoir) may cause geochemical alteration of fracture surface changing the friction coefficient (barriers became asperities?) ● Stress condition (orientation and size of deviatoric stress in relation to faults) ● Extent of the faults ● Rock mechanical properties ● Hydrological factors ● Hystorical natural seismicity

Mechanisms of induced seismicity in EGS ● Pore pressure increase: increase fluid pressure can decrease static friction → seismic slip. High number of MEQ occur as the pressure migrates away from the well.. ● Temperature decrease: Cool fluids can cause contraction of fracture surfaces (thermoelastic strain) → slight open of fracture reduce the static friction promoting slip. Alternatively, contraction can cause new fractures (non double-couple mechanism). ● Volume change due to fluid injection: cause a perturbation in local stress conditions close to the failure state → seismic slip ● Chemical alteration: injecting non-native fluids (or allowing “outside fluids” to flow into reservoir) may cause geochemical alteration of fracture surface changing the friction coefficient (barriers became asperities?) ● Stress condition (orientation and size of deviatoric stress in relation to faults) ● Extent of the faults ● Rock mechanical properties ● Hydrological factors ● Hystorical natural seismicity

GAPSS is a passive seismic experiment which began as of early May, It is intended to last until summer Its main goal is to verify the resolving power of passive exploration techniques in an area where the subsurface geological structures are well known → i.e., the Larderello Travale Geothermal field (LTGF). GAPSS involves the cooperative efforts of INGV personnel from Pisa, CNT and RM1. Most of the Instruments have been provided by CNT THE GAPSS EXPERIMENT

WHERE GAPSS IS

LARDERELLO-TRAVALE GEOTHERMAL FIELD AND THE GAPSS GEOMETRY

SEISMICITY AT LTGF 'In the early 1970s, injection of cold condensate from the power plants was initiated in order to recharge the upper reservoir [...]. The area has a long history of seismicity, and therefore many, if not most of the events are likely to be natural' [Evans et al., Geothermics 41 (2012) 30–54] [Batini et al., 1985] EQ rate: 5-60 eqs / months

SEISMICITY & INJECTION 'The production history of the Larderello-Travale geothermal field is so complex that few quantitative data are available for the productive and reinjection wells'' [Batini, Console & Luongo, Geothermics (1985) ]. Nonetheless, it is clear that: - Seismicity rate correlates positively with amount of injected fluids; - Max magnitudes correlates negatively with amount of injected fluids; [Batini et al., 1985] EQ RATE MAX MAG INJECTION

SIGNAL DETECTION & ANALYSIS 1. STA / LTA detection algorithm (all stations) 2. TRIGGER with coincidence sum 3. VISUAL INSPECTION 4. MANUAL PICKING 5. LOCATION (LIN & NON-LIN)

THE GAPSS CATALOG 15 May 2012 – 30 Sept. 2012: 948 locations, ~ 460 class A & B - 1D Vp/Vs model suggest strong lateral heterogeneities; - High B-value as expected for geothermal fields - Seismicity rate 5.25 ev/day!!!

LOCATIONS

CROSS-SECTIONSL T K-Horizon K-Horizon follows the 450° isotherm. Fractured level filled by supercritical fluids (Batini et al., 1983) OR Fragile-Ductile transition level containing fluids overpressure (Brogi et al., 2003)

A TYPICAL DAY OF LTGF SEISMICITY 1000 s 100 s Conventional event detection algorithms obviously fail in grabbing most of the events 100 s 1 day

CLUSTER ANALYSIS ✔ MASTER STATION: LA12 (largest number of time readings) ✔ 4-s-long time window, start 1s before Tp ✔ 2-20 Hz frequency band ✔ EXCLUSIVE, 'CLOSED' CLUSTER algorithm w/ min 5 events ✔ CORRELATION 0.8 ✔ 10 clusters (max 23 events)

MATCHED FILTERING ● For each cluster: selection of the largest event as a template waveform; ● Slide the template along the continuous recording and compute cross-correlation; ● Store events for which max[xcorr] > 0.75

SAMPLE RESULTS: JUNE 2, 2012 Constant Rate Omori's Law Two closely- spaced clusters exhibit marked differences in the time recurrence of failure: Omori's law vs constant rate.

TEMPORAL EVOLUTIONS

CLUSTER AND ACTIVE WELLS LOCATIONS 10 km Location of the clusters (circles) and of the active wells (red dots). White arrows are wells where reinjection occurs (source: Ministry of Industry and Economical Development).

Naturally-Triggered Earthquakes

Kinematic of Rayleigh Waves Rayleigh wave polarisation is oriented perpendicularly to the main fault system of the area → maximisation of the triggering potential. Multichannel analysis serves to infer the seismic velocity of the medium to be used for stress calculation.

The dynamic stress field Stress fields as a function of time and depth for a 10 s Rayleigh wave with a max amplitude of 1 mm. (a) Vertical displacement seismogram. (b) Horizontal normal stress. (c) Vertical normal stress. (d) Shear stresses. Positive = extensional Negative = compressional.

The dynamic stress field Interpolation of seismic recordings allows deriving the actual ground motion at the triggered hypocenters, which is eventually transformed into stress values. ● Failure occurred with dynamic stresses as low as 5 Kpa ● Faults must have been already very close to failure ● High pore fluid pressure ● Triggered depths are consistent with location of the K-horizon !

Threshold on Triggering Gomberg and Davis, JGR, Mw= Mw=5.8 These 2 earthquakes were recorded with amplitudes differing by a factor of ~4. Is this enough to inherit triggering ? Or rather the time in between them (9 days) was shorter than the 'recharge time' of faults? Empirical Magnitude – distance triggering threshold relationship found at The Geysers Geothermal Field, California

Acknowledgements