Stress measurements a Scandinavian perspective Jonny Sjöberg Itasca Consultants AB (Sweden)
Scandinavian experiences Deep overcoring the Borre probe OC experiences & quality control A new LVDT overcoring cell New tool for hydraulic fracturing in high stress environments Stress calibration a mining case study What the future may bring
The Borre Probe Leeman-type, "soft", 3D (9 strain gauges) cell Built-in data logger (no wire connection needed) with high sampling frequency Developed for use in deep (600 m), water-filled boreholes Special glue enables bonding underwater and in low temperatures (+5 C) Featured as an ISRM Suggested Method (2003) Owned and used by Pöyry SwedPower (previously Vattenfall)
Ground surface The Borre overcoring probe Installation tool (adapter) Borehole Borre probe installed in borehole at large depth Borre probe with builtin data logger
Measurement procedure 1 2 3 4 5 6
Adapter retrieved Microstrain Drill string in place Core on surface Core retrieval start Temperature [ ] 14.0 1600 1400 1200 1000 800 600 400 200 0 12.0 10.0 8.0 6.0 4.0 2.0 Gauge 1 (L-1) Gauge 2 (T-1) Gauge 3 (45-1) Gauge 4 (L-2) Gauge 5 (T-2) Gauge 6 (45-2) Gauge 7 (L-3) Gauge 8 (T-3) Gauge 9 (45-3) OC Start OC 16 cm OC Stop Core Break Temperature -200 7:30 8:00 8:30 9:00 9:30 10:00 10:30 Time OC Start OC Stop Core Break 0.0
The Borre Probe Applications: More than 1000 measurements at more than 80 sites [1976 2011] Mining, infrastructure, hydropower, underground storage, etc. SKB Site Investigations (nuclear waste disposal)
Borre overcoring measurements
Depth [m] Borre probe case record Number of measurement sites 0 5 10 15 20 25 30 35 40 45 50 55 0-100 100-200 200-300 300-400 400-500 500-600 600-700 700-800 800-900 900-1000 1000-1100 1100-1200 Maximum measurement depth (from ground surface) Maximum hole depth (from borehole collar)
The largest overcoring campaign! SKB Site Investigations Investigations for underground nuclear waste repository Target depth of repository: 400 700 m Site investigations 2002 2007 Stress measurements in deep surface boreholes (HF, OC) OC in 4 boreholes per site; 50 640 m depth Forsmark
The largest overcoring campaign! SKB Results 300 pilot holes 180 measurement attempts 60 successful measurements 470 m depth at Forsmark 640 m depth at Oskarshamn (world record!)
Resulting stress model Forsmark Overcoring Hydraulic fracturing Borehole breakouts Core disking
OC Quality control & interpretation Transient strain analysis: Comparison of theoretical & measured strains Identification of debonding, heterogeneities, microcracking Stresses from early (pre-overcoring) strains (high accuracy on coring advance required) Anisotropic interpretation: Modification of Amadei-code for transversely isotropic materials Determination of elastic constants for transversely isotropic materials using biaxial testing (Nunes, 2002) Tools developed for Borre and CSIRO HI probes
Microstrain Cell temperature ( degrees ) 500 Transient strain analysis Readings resetted Phase 1 Phase 3 10 A0 A90 300 9 A45 B45 B135 100 8 B90 C0-100 7 C90 C45 D135-300 6 E90 F90-500 5 Temp -700 Flushing on Coring started End of coring Flushing off Core out Hardening Flushing Coring Flushing of borehole Biaxial testing 38 h 30 min 50 min 30 min 30 min 20 min -900 3-80 -40 0 40 80 120 160 Hardening ( h ), Flushing (min ), Coring ( cm ), After coring (min ), Biaxial ( min ) 4
Anisotropic interpretation Case Olkiluoto KR10 measurement 1:8 - Transversely Isotropic Statistics for solution CL 95 % st_dev 3.27E-05 R 2 0.9846 Principal stresses average upper limit lower limit 1 magnitude 1.27E+01 1.53E+01 1.19E+01 trend 135.13 182.82 120.25 plunge 22.6 29.9 22.4 2 magnitude 1.01E+01 1.24E+01 8.73E+00 trend 40.79 88.73 20.21 plunge 10.3 7.1 22.9 3 magnitude 4.41E+00 6.89E+00-6.67E-01 trend 287.86 346.73 249.6 plunge 64.95 59.14 56.99 Orthogonal components average upper limit lower limit NN 11.1 13.2 9.1 UU 5.8 9.1 2.6 EE 10.3 12.3 8.3 UE -2.7340-0.4925-4.9750 EN -8.22E-01 4.29E-01-2.07E+00 NU 1.33E+00 3.60E+00-9.43E-01 Direction cosines for principal stresses average upper limit lower limit 1 l 0.6545 0.8661-0.4657 m 0.3836 0.4980-0.3809 n -0.6515 0.0426 0.7988 2 l 0.7448 0.0221 0.8643 m -0.1791-0.1233-0.3895 n 0.6428 0.9921 0.3182 3 l -0.1299-0.4993 0.1899 m 0.9059 0.8584 0.8386 n 0.403 0.1177 0.5106
Anisotropic interpretation BORRE STRAIN GAUGE CONFIGURATION (LOOKING DOWNHOLE) R2 0 90 LOCAL BOREHOLE / CORE COORDINATE SYSTEM DEFINITION OF ANISOTROPY DIRECTION R2 0 Strike of anisotropy plane 120 120 120 R1 R1 DD =Dip Direction of anisotropy plane R3 R3 0 Strain gauge rosette (no. 1) seen from center of borehole DD Gauge 3 (6, 9) (45 ) Gauge 1 (4, 7) (longitudinal) Gauge 2 (5, 8) (tangential) Dip Dip of anisotropy plane Hole axis
Anisotropic interpretation
Remaining uncertainties Microcracking under high stress control drilling to reduce damage potential reduce maximum pressure in biaxial testing Temperature effects re-activation of glue during drilling-induced temperature increase temperature increase cause additional curing and shrinkage of glue bond
Temperature effects Microstrain Te mperature [ ] 12.0 1600 1400 1200 1000 800 600 400 200 0 10.0 8.0 6.0 4.0 2.0 Gauge 1 (L-1) Gauge 2 (T-1) Gauge 3 (45-1) Gauge 4 (L-2) Gauge 5 (T-2) Gauge 6 (45-2) Gauge 7 (L-3) Gauge 8 (T-3) Gauge 9 (45-3) OC Start OC 16 cm OC Stop Core Break Temperature -200 8:38 8:48 8:58 9:08 9:18 9:28 9:38 Time 0.0
Temperature effects Microstrain 1500 Te mperature [ ] 12.0 1300 1100 900 700 500 300 100-100 -300 10.0 8.0 6.0 4.0 2.0 Gauge 1 (L-1) Gauge 2 (T-1) Gauge 3 (45-1) Gauge 4 (L-2) Gauge 5 (T-2) Gauge 6 (45-2) Gauge 7 (L-3) Gauge 8 (T-3) Gauge 9 (45-3) OC Start OC 16 cm OC Stop Core Break Temperature -500 8:35 8:40 8:45 8:50 8:55 9:00 9:05 9:10 9:15 9:20 9:25 9:30 Time 0.0
Microstrain Temperature [ C] The Borre probe Laboratory study Styrofoam sheet (for insulation) T T Heater Strain gauges connected to probe 600 400 70 60 50 Tripod T Plastic bucket with app. 5 l of water 200 0 40 30 20 Magnetic stirrer T = thermometer -200-400 -600 Mean strain Water temperature 08 09 10 11 12 13 14 Time [h] 10 0-10 -20
Future improvements Quality control: Cooling during flushing; heating during glue curing (temperature, cleaning) Alter glue composition lower glass transition temperature Larger overcoring diameter (temperature, micro-cracking) Borre IV (new version) features: 4 strain gauges per rosette Integrated temperature gauge Improved orientation measurement of probe installation Upgraded logger unit (sampling and capacity)
A new LVDT cell Courtesy of Matti Hakala, KMS Hakala Oy, Finland Financial support / testing, etc. by Posiva & SKB 2D overcoring device developed for Posiva (Finland) Developed to overcome glue and scale problems associated with standard overcoring Developed to measure excavation-induced stresses near a tunnel surface (<0.7m)
A new LVDT cell ) 6) ) ) cm lc. Measurement of induced diametrical change Measurements through eight LVDT-gauges (in four diametrical directions) Accuracy < 1 mm 0.19 Internal logger and on-line cable connection In situ stress solved through numerical inversion Looking from tunnel to the measurement hole
A new LVDT cell
A new LVDT cell
A new LVDT cell Testing and verification Thermal heating => reversable change in LVDTs Mechanical impact => small (< 10 mm) change Long-term instrument drift very small (< 4 mm) Field trials at Äspö HRL and comparison with results from other stress measurement methods
Long-term instrument drift
Field tests at Äspö HRL TASS tunnel, 450 m depth
Field tests at Äspö HRL N
Field tests at Äspö HRL
Diametric deformation (mm) Temperature (C) 0.1 0.075 Field tests at Äspö HRL Measurement locations: SC_Start 50 45 40 0.05 35 0.025 30 25 0-0.025 Convergence orientations - looking from tunnel to the measurement hole 20 15 10-0.05 SC_End, 40 cm 5-0.075 0 13:00 13:15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30 15:45 16:00
Field tests at Äspö HRL Table 1. Comparison of test results to earlier best estimate of state of stress at the 450-m level. σ H σ H trend σ h σ v MPa (RT90) MPa MPa Christiansson & Jansson (2003) 24 ±5 136 10-13 12 This study Deep, > 0.5 m 23-24 136-139 12-13 10-11
Conclusions: A new LVDT cell LVDT-cell and numerical inversion technique developed Glue bonding problems avoided Faster measurements (no curing time) Ease of use; short boreholes & compact drill rig Excellent agreement with traditional borehole measurements (overcoring, hydraulic fracturing) Possible local effects of EDZ of drill-and-blast tunnel Additional testing in TBM-tunnel at Äspö HRL underway (EDZ-influences removed)
Hydraulic fracturing in high stress Hydraulic measurements at depth (1000 m) New tool developed for deep mining and high-stress environments Vattenfall Quadruple Packer Tool (VQPT) (now owned by Pöyry SwedPower): for use in 76 mm water-filled boreholes allows 70 MPa over-pressure suitable for SF, HF, HTPF lab record: 62 MPa; field record: 42 MPa Fracture orientation data determined with Mosnier tool
Hydraulic fracturing in high stress
Quadruple packer & Mosnier tool
Stress calibration of virgin stress Assumptions: Horizontal-vertical stress field Vertical stress assumed gravitational (model check) Constant and gravitational stress components total constant gradient grav ij ij ij z ij Unit stress tensor approach (McKinnon, 2001) unit stress (for each tensor component) applied to numerical model => unit stress response calculated superposition of results to obtain arbitrary stress state calibration by comparison with measured stresses and solving for factors expressing the relative contribution from each unit stress response
Case example: the Malmberget mine
Case example: the Malmberget mine
Case example: the Malmberget mine
Case example: the Malmberget mine
Case example: the Malmberget mine
Case example: the Malmberget mine Results from unit stress tensor calibration = 0.02784 [MPa/m], gradient x gradient y = 0.02501 [MPa/m], gradient xy = -0.00907 [MPa/m], H = 0.0356z, (9) h = 0.0172z, (10) Trend = 130.6 z FLAC = - z Mine y FLAC = x Mine Trend of H x FLAC = y Mine Calibrated boundary stresses different than measured influences from mining, density & topography! H
The future New / potential research projects: Project on mining-induced seismicity & understanding stress effects (funded): Stress inversion stress tomography overcoring (absolute) stress monitoring Deep drilling hydrofracturing (3 km) & borehole breakouts (funding?)