Mechanical Stimulation and implications from microseismicity - PowerPoint PPT Presentation

ENGINE – ENhanced Geothermal Innovative Network for Europe Workshop 3 "Stimulation of reservoir and microseismicity" Mechanical Stimulation and implications from microseismicity Thomas Kohl, GEOWATT AG Clément Baujard, GEOWATT AG Zürich, Switzerland 29 June 2006 ENGINE - Kartause Ittingen

Mechanical (hydraulic) stimulation - Faulting (= shear fracturing): shearing of pre-existing fractures, Soultz (mechanism is stress field dependant) - Jointing (= hydrofrac, tensile fracturing, extensional fracturing): creation of new fractures, common in petroleum industry - Jacking : aperture enlargement of pre-existing fractures, Rosemanowes and Le Mayet-de-Montagne with proppant and gel injections 29 June 2006 ENGINE - Kartause Ittingen

Hydraulic Stimulation major parameter for failure in an EGS reservoir is the stress regime, i.e. relative vertical / horizontal stress 29 June 2006 ENGINE - Kartause Ittingen

Mechanical (hydraulic) Stimulation σ H Faulting (shear fracturing) � Increase of pore pressure σ h � Slip of pre-existing mechanical discontinuities � Generation of larger apertures / or new faults Mohr (-Coulomb) - Criterion ( ) τ = + Φ • σ c tan n Microseismicity � Prediction of Magnitudes (Gutenberg-Richter) Evans � Identification of large structures (e.g. multiplet analysis) � Identification of hydraulic diffusivity Stimulation of multiple fracture sets, mostly in crystalline rock 29 June 2006 ENGINE - Kartause Ittingen

Mechanical (hydraulic) Stimulation Jointing (tensile fracture) � Develops perpendicular to least principal stress Criterion > + σ P S f min > + σ + α ⋅ P S P f min p Applied mostly in sedimentary rocks Creation of single, far extending fractures 29 June 2006 ENGINE - Kartause Ittingen

Analysis of Microseismic Density from Stimulations in Soultz • GPK2 Stimulation (July 2000): 14'080 events • GPK3 Stimulation (July 2003): 21'600 events • GPK4 Stimulation (September 2004): 5'753 events • GPK4 Stimulation (February 2005): 2'966 events • GPK4 1st Step rate test (February 2005): 183 events • GPK4 Acidization test (March 2005): 304 events • GPK4 2nd Step rate test (March 2005): 256 events 29 June 2006 ENGINE - Kartause Ittingen

Analysis of Microseismic Density from Stimulations Total events Calculated cube volumes: 50x50x50m 3 29 June 2006 ENGINE - Kartause Ittingen

Analysis of Microseismic Density from Stimulations Calculated low-density structure N96p64W. 29 June 2006 ENGINE - Kartause Ittingen

Analysis of Microseismic Density from Stimulations Comparison of GPK4 Stimulations: September 2004 February 2005 29 June 2006 ENGINE - Kartause Ittingen

Analysis of Microseismic Density from Stimulations Comparison of GPK2/3/4 Stimulations Compare at 100m cube: 29 June 2006 ENGINE - Kartause Ittingen

Conclusion on possible hydraulic impact of low-seismic zone High conductive zone (draining High impedance zone fluid into a far field fault zone and (extreme low natural thus prevents any pressure increase) fracturization = possible no-flow � "Fingering" of microseismic boundary) density indicates flow into this � Orientation nearly zone perpendicular to S H ; � No increase of the density of � Long transients during microseismic events once zone has reached and injection GPK4 shut-in continues � Weak hydraulic connection � Weak hydraulic connection between GPK3 and GPK4 between GPK3 and GPK4 � Hardly no tracer recovery � Tracer diffusion into this between GPK3 and GPK4 "storage zone" can explain the � High seismic density small tracer recovery � Next to the intersection with between GPK4 and GPK4 depth, high fluid-losses aseismic zone were encountered during drilling 29 June 2006 ENGINE - Kartause Ittingen

Conclusion on possible hydraulic impact of low-seismic zone individual observations are non-unique (I.e. tracer breakthrough) ⇒ ambivalent characterization. Although orientation does not coincide with N-S pattern, such faults necessarily exist on Horst structures High impedance needs extreme low fracturization that is hardly to imagine for the general permeability pattern ⇒ aseismic zone corresponds to a subvertical structure that is well linked to N-S striking drainage systems ⇒ Due to its orientation, we can expect a low compliance for normal stress variations and especially little shearing. 29 June 2006 ENGINE - Kartause Ittingen

Complex tectonic regimes Interplay of different tectonic mechanisms can lead to faulting ~parallel to S h : • rotational bulk strain • Pull-apart • en-echelon structures 29 June 2006 ENGINE - Kartause Ittingen

Conclusion The hydraulic re-stimulation of GPK4 includes the risk of low efficiency and of higher seismicity. A proper hydraulic characterization of the aseismic zone between GPK3 / GPK4 is necessary for a successful GPK4 re-stimulation. Microseismicity favours structures parallel to S H � However perpendicular structures may exist � Visible only as low seismic activity 29 June 2006 ENGINE - Kartause Ittingen

Modeling Tool HEX-S: Prognosis GPK4 stimulation 04SEP13 Forecast Measurement -3000 Z -4500 x3 X -4000 Y Depth -5000 -5000 1000 x1 0 Easting -6000 -1000 -5500 500 1000 0 -1000 -2000 x2 -1000 0 -1500 -2000 Northing 2.5E+07 20 100 90 2E+07 80 II modl : 6.9-7.4 II modl : 9.7-10.7 15 Ref.pressure 45.0 MPa 70 ∆ Pdh [MPa] Qin Omega 60 1.5E+07 ∆ P dh [Pa] 10 50 40 30 1E+07 5 20 10 0 0 5E+06 100000 200000 300000 time [s] 0 0 100000 200000 29 June 2006 ENGINE - Kartause Ittingen time [s]

Modeling Tool HEX-S: Stimulation Model Forecast model did not include aseismic zone New fault model of the 5km reservoir at Soultz � deterministic fractures intersecting the GPK3 and GPK4 borehole � faults derived from the seismic distribution using the density analysis � aseismic zone with high hydraulic conductivity, i.e. flow injected to GPK4 will be drained through this zone into a nearby N-S extending Soultz fault. 29 June 2006 ENGINE - Kartause Ittingen

Modeling Tool HEX-S: New Stimulation Model Determination of fault planes from microseismic distribution 29 June 2006 ENGINE - Kartause Ittingen

Modeling Tool HEX-S: New Stimulation Model ection 30 l/s ⏐ 26 Apr 2006 ⏐ Three Scenarios: GPK4 GPK3 1.2E+07 1.1E+07 1E+07 9E+06 1. Single injection in GPK4 with 8E+06 7E+06 6E+06 30 l/s during 3 days and 5E+06 4E+06 3E+06 2E+06 increase to 45 l/s (i.e. injection 1E+06 000 -2000 -1000 0 1000 2000 scenario from Sep. 2004) Northing [m] ction GPK3 & GPK4: 2 x 30 l/s ⏐ 26 Apr 2006 ⏐ GPK4 GPK3 2. Dual injection in GPK3 / GPK4 each with 30 l/s during 3 days and increase to 45 l/s 000 -2000 -1000 0 1000 2000 Northing [m] ction GPK4: 60 l/s ⏐ 26 Apr 2006 ⏐ GPK4 GPK3 3. Doubling injection in GPK4 with 60 l/s during 3 days and increase to 90 l/s (i.e. doubled flow scenario 1) 000 -2000 -1000 0 1000 2000 Northing [m] Not yet fully calibrated! 29 June 2006 ENGINE - Kartause Ittingen

Modeling Tool HEX-S: New Stimulation Model 10'000sec / 1 day injection: 30 l/s ⏐ 26 Apr 2006 ⏐ Three Scenarios: 1.6E+07 GPK4 GPK3 1.4E+07 Injection GPK4: 30 l s-1 10'000 sec 1 day Injection GPK4: 30 l s-1 1.2E+07 Injection GPK3 & GPK4: 2 x 30 l s-1 � first injection step at Injection GPK3 & GPK4: 2 x 30 l s-1 Pressure [Pa] 1E+07 Injection GPK4: 60 l s-1 Injection GPK4: 60 l s-1 8E+06 t=10'000 s and 1 day 6E+06 4E+06 2E+06 0 -2000 -1500 -1000 -500 0 500 1000 Northing [m] 1 day injection 45 l/s ⏐ 26 Apr 2006 ⏐ � second injection step 1.6E+07 GPK4 GPK3 1.4E+07 Injection GPK4: 45 l s-1 at t=1 day after step Injection GPK3 & GPK4: 2 x 45 l s-1 1.2E+07 Injection GPK4: 90 l s-1 Pressure [Pa] 1E+07 change 8E+06 6E+06 4E+06 2E+06 0 -2000 -1500 -1000 -500 0 500 1000 Northing [m] Pressure History GPK4 ⏐ 26 Apr 2006 ⏐ � pressure history in 1.6E+07 1.4E+07 GPK4 1.2E+07 Pressure [Pa] 1E+07 8E+06 6E+06 Injection GPK4 30/45 l s-1 Injection GPK3 & GPK4: 30/45 l s-1 4E+06 Injection GPK4: 60/90 l s-1 2E+06 0 0 50000 100000 150000 time [s] 29 June 2006 ENGINE - Kartause Ittingen

Recommendations for Mechanical Stimulation Modeling indicates � Transmissivities are created mostly in the vicinity of the boreholes � Little success in far field stimulation Short-term injections (1-2 days): � prevents pressure build up in secondary flow zones (pore pressure) � limits the size of the affected area. � Successive short-term injections more efficient than long re-stimulations � Dual injection would yield shorter transients in matrix / larger volumes. When reaching maximum pressure: � avoiding long-term shut-in. � venting of boreholes as fast as possible Chemical stimulation not considered. � several successive chemical / mechanical stimulation � they are complementary in nature: • acidization with HCl rather affects the nearest borehole vicinity • mechanical stimulation will influence the natural fracture network 29 June 2006 ENGINE - Kartause Ittingen