Stress measurements a Scandinavian perspective. Jonny Sjöberg Itasca Consultants AB (Sweden)

Transcription

1 Stress measurements a Scandinavian perspective Jonny Sjöberg Itasca Consultants AB (Sweden)

2 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

3 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)

4 Ground surface The Borre overcoring probe Installation tool (adapter) Borehole Borre probe installed in borehole at large depth Borre probe with builtin data logger

5 Measurement procedure

6 Adapter retrieved Microstrain Drill string in place Core on surface Core retrieval start Temperature [ ] 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 :30 8:00 8:30 9:00 9:30 10:00 10:30 Time OC Start OC Stop Core Break 0.0

7 The Borre Probe Applications: More than 1000 measurements at more than 80 sites [ ] Mining, infrastructure, hydropower, underground storage, etc. SKB Site Investigations (nuclear waste disposal)

8 Borre overcoring measurements

9 Depth [m] Borre probe case record Number of measurement sites Maximum measurement depth (from ground surface) Maximum hole depth (from borehole collar)

10 The largest overcoring campaign! SKB Site Investigations Investigations for underground nuclear waste repository Target depth of repository: m Site investigations Stress measurements in deep surface boreholes (HF, OC) OC in 4 boreholes per site; m depth Forsmark

11 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!)

12 Resulting stress model Forsmark Overcoring Hydraulic fracturing Borehole breakouts Core disking

13 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

14 Microstrain Cell temperature ( degrees ) 500 Transient strain analysis Readings resetted Phase 1 Phase 3 10 A0 A A45 B45 B B90 C C90 C45 D E90 F 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 Hardening ( h ), Flushing (min ), Coring ( cm ), After coring (min ), Biaxial ( min ) 4

15 Anisotropic interpretation Case Olkiluoto KR10 measurement 1:8 - Transversely Isotropic Statistics for solution CL 95 % st_dev 3.27E-05 R Principal stresses average upper limit lower limit 1 magnitude 1.27E E E+01 trend plunge magnitude 1.01E E E+00 trend plunge magnitude 4.41E E E-01 trend plunge Orthogonal components average upper limit lower limit NN UU EE UE EN -8.22E E E+00 NU 1.33E E E-01 Direction cosines for principal stresses average upper limit lower limit 1 l m n l m n l m n

16 Anisotropic interpretation BORRE STRAIN GAUGE CONFIGURATION (LOOKING DOWNHOLE) R LOCAL BOREHOLE / CORE COORDINATE SYSTEM DEFINITION OF ANISOTROPY DIRECTION R2 0 Strike of anisotropy plane 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

17 Anisotropic interpretation

18 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

19 Temperature effects Microstrain Te mperature [ ] 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 :38 8:48 8:58 9:08 9:18 9:28 9:38 Time 0.0

20 Temperature effects Microstrain 1500 Te mperature [ ] 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 :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

21 Microstrain Temperature [ C] The Borre probe Laboratory study Styrofoam sheet (for insulation) T T Heater Strain gauges connected to probe Tripod T Plastic bucket with app. 5 l of water Magnetic stirrer T = thermometer Mean strain Water temperature Time [h]

22 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)

23 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)

24 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

25 A new LVDT cell

26 A new LVDT cell

27 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

28 Long-term instrument drift

29 Field tests at Äspö HRL TASS tunnel, 450 m depth

30 Field tests at Äspö HRL N

31 Field tests at Äspö HRL

32 Diametric deformation (mm) Temperature (C) Field tests at Äspö HRL Measurement locations: SC_Start Convergence orientations - looking from tunnel to the measurement hole SC_End, 40 cm :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

33 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 ± This study Deep, > 0.5 m

34 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)

35 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

36 Hydraulic fracturing in high stress

37 Quadruple packer & Mosnier tool

38

39 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

40 Case example: the Malmberget mine

41 Case example: the Malmberget mine

42 Case example: the Malmberget mine

43 Case example: the Malmberget mine

44 Case example: the Malmberget mine

45 Case example: the Malmberget mine Results from unit stress tensor calibration = [MPa/m], gradient x gradient y = [MPa/m], gradient xy = [MPa/m], H = z, (9) h = z, (10) Trend = 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

46 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?)