Geodynamics Measuring stress and strain Lecture 4.2 - How is stress measured? Lecturer: David Whipp david.whipp@helsinki.fi Geodynamics www.helsinki.fi/yliopisto 1
Goals of this lecture Introduce several methods for measuring stress Distinguish between rock strength measurements and in situ stress measurements in the crust 2
Measuring stress Rock yield stress (or strength) can be measured in a laboratory, providing insight into stresses rock can support before failure An alternative is to measure stress in the lithosphere in situ, where the focus is the stress field at the sample site, rather than the yield stress of the sample 3
Laboratory rock stress measurement Laboratory rock strength measurements are common example of direct stress measurement A rock sample is loaded in a press and squeezed until it fails http://www.controls-group.com This measures rock strength When the sample transitions from elastic to brittle or plastic deformation, the yield strength (stress) has been reached Fig. 5.2 in Stüwe, 2007 Geodynamics www.helsinki.fi/yliopisto 4
In situ stress measurement: Overcoring View down hole Overcoring is an in situ stress measurement made at the bottom of a drill hole The hole is drilled and strain gauges are installed on the base and sides in 3 orthogonal positions An outer annular hole is drilled around the original hole Cross section of bottom of hole Fig. 2.17, Turcotte and Schubert, 2014 This is thought to completely relieve the stress in the original hole, allowing stress to be calculated from the relaxation measured by the strain gauges Limitation: Max hole length: ~1 m (!) 5
In situ stress measurement: Overcoring View down hole Overcoring is an in situ stress measurement made at the bottom of a drill hole The hole is drilled and strain gauges are installed on the base and sides in 3 orthogonal positions An outer annular hole is drilled around the original hole Cross section of bottom of hole Fig. 2.17, Turcotte and Schubert, 2014 This is thought to completely relieve the stress in the original hole, allowing stress to be calculated from the relaxation measured by the strain gauges Limitation: Max hole length: ~1 m (!) 6
In situ stress measurement: Overcoring View down hole Overcoring is an in situ stress measurement made at the bottom of a drill hole The hole is drilled and strain gauges are installed on the base and sides in 3 orthogonal positions An outer annular hole is drilled around the original hole Cross section of bottom of hole Fig. 2.17, Turcotte and Schubert, 2014 This is thought to completely relieve the stress in the original hole, allowing stress to be calculated from the relaxation measured by the strain gauges Limitation: Max hole length: ~1 m (!) 7
In situ stress measurement: Hydrofracturing b Headframe Winch Drilling rods Packer Packer Zhao et al., 2013 Flowmeter Control panel High-pressure pump Data recorder Push-pull valve for packer Test interval Borehole Stress measurement by hydrofracturing involves isolating a section of a drill hole that is free of fractures and other porosities Fluid is pumped into the isolated segment, continually monitoring the pressure Pressure is increased until fracturing occurs The pressure when fracturing occurs is the breakdown pressure, pb If the pump is immediately turned off after fracture and the circuit is kept closed, the instantaneous shut-in pressure (ISIP) is recorded 8
In situ stress measurement: Hydrofracturing b Headframe Winch Drilling rods Packer Packer Zhao et al., 2013 Flowmeter Control panel High-pressure pump Data recorder Push-pull valve for packer Test interval Borehole Stress measurement by hydrofracturing involves isolating a section of a drill hole that is free of fractures and other porosities Fluid is pumped into the isolated segment, continually monitoring the pressure Pressure is increased until fracturing occurs The pressure when fracturing occurs is the breakdown pressure, pb If the pump is immediately turned off after fracture and the circuit is kept closed, the instantaneous shut-in pressure (ISIP) is recorded 9
In situ stress measurement: Hydrofracturing b Headframe Winch Drilling rods Packer Packer Zhao et al., 2013 Flowmeter Control panel High-pressure pump Data recorder Push-pull valve for packer Test interval Borehole Stress measurement by hydrofracturing involves isolating a section of a drill hole that is free of fractures and other porosities Fluid is pumped into the isolated segment, continually monitoring the pressure Pressure is increased until fracturing occurs The pressure when fracturing occurs is the breakdown pressure, pb If the pump is immediately turned off after fracture and the circuit is kept closed, the instantaneous shut-in pressure (ISIP) is recorded 10
In situ stress measurement: Hydrofracturing Hydrofracturing pressure log The ISIP value is the minimum pressure required to keep the fractures open If we assume vertical fracture orientations and fracture in pure tension, then The minimum horizontal principal stress is equal to the ISIP value Fig. 2.18, Turcotte and Schubert, 2014 The maximum horizontal principal stress can be deduced from pb and ISIP, but less accurately 11
The hydrofracturing (fracking) controversy FRACKING FOR FUEL Hydraulic fracturing is used to access oil and gas resources that are locked in non-porous rocks. Blowouts are possible. Methane gas escapes during the mining process. Water recovery tanks Polluted flowback water may be injected into a deep storage well, recycled or sent to a treatment plant. Hydrofracturing to measure stress is a bit different from the controversial hydrofracturing used in the oil and gas industry Water table Well Cement casing Steel pipe Possible flow of methane. Leakage of fracking fluids from the pipe has not been seen. Holes in the well casing allow fluid to exit and gas to enter. Poorly treated flowback water has leaked into drinking water. Leakage of fracking fluid from the fracture zone is highly unlikely. The general concept is the same, with injected fluids being used to fracture rock, but the extraction of oil or gas requires a much larger network of fractures to liberate trapped deposits High-pressure fracturing fluid Shale High-pressure fracking fluid opens networks of fractures in the shale. Sand props the fractures open. Fissures Gas flows from the fractures into the pipe. Some of the potential risks in this process are shown on the left Howarth et al., 2011 12
Let s see what you ve learned If you re watching this lecture in Moodle, you will now be automatically directed to the quiz! References: Howarth, R. W., Ingraffea, A., & Engelder, T. (2011). Natural gas: Should fracking stop? Nature, 477(7364), 271 275. doi:10.1038/477271a Zhao, X. G., Wang, J., Cai, M., Ma, L. K., Zong, Z. H., Wang, X. Y., et al. (2013). In-situ stress measurements and regional stress field assessment of the Beishan area, China. Engineering Geology, 163(0), 26 40. 13