Induced Seismicity Potential in Energy Technologies (2013) / Chapter Skim
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2 Types and Causes of Induced Seismicity
2 Types and Causes of Induced Seismicity
Pages 37-58
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From page 37... ...
Understanding these components gives some confidence in being able to draw conclusions about what seismicity might be induced in the future, and under what conditions. The physical mechanisms1 r esponsible for inducing seismic events are discussed here with reference to specific energy technologies; detailed explanations of these technologies and their relationship to induced seismic events are presented in Chapter 3.
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From page 38... ...
Due to the weight of the overlying rock and other processes in the Earth's crust, rocks are usually under compression. The compressive normal stress acting on a rock at depth varies with direction; this variation of the normal stress with direction is linked to the shear stresses that are responsible for slip along a fault if the frictional resistance of the fault is overcome.
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From page 39... ...
This implies that the magnitude of the increase in pore pressure that will cause a known fault to slip cannot generally be calculated. Nonetheless, understanding how different factors contribute to slip initiation is valuable because it provides insight about whether fluid injection or withdrawal may be a stabilizing or a destabilizing factor for a fault (in other words, whether fluid inject on or withdrawal causes the difference between the driving shear stress and the i shear strength to increase or decrease)
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From page 40... ...
Within the context of slip on a fault, the normal and shear stresses acting across the fault, σ and τ, can be directly expressed in terms of the vertical stress (σv) , the horizontal stress (σh)
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From page 41... ...
Shearing of a jointed block subjected to normal force Fn and shear force Fs, with fluid inside the joint at pressure ρ. Slip along the joint is triggered when the shear stress τ is equal to the frictional strength μ(σ – ρ)
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From page 42... ...
A set of six quantities, the three principal stresses and their directions, thus represents the state of stress. Fortunately, vertical can often be considered as one of the principal directions, with the consequence that the vertical stress σv at depth h is then simply given by the weight of the overlying rock (i.e., σv =pgh, where p is the average density of the overlying rock and g is gravity)
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From page 43... ...
. The ratio of the mean horizontal stress to the vertical stress (Figure 2b)
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From page 44... ...
. Large variation of the pore pressure and/or temperature could also induce significant stress changes that have to be accounted for when assessing the potential for induced seismicity.
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From page 45... ...
Thus, for fluid injection to trigger a significant earthquake, a fault or faults of substantial size must be present that are properly oriented relative to the existing state of crustal stress, and these faults must be sufficiently close to points of fluid injection to have the rocks surrounding them experience a net pore pressure increase.
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From page 46... ...
The complexities associated with geological settings -- in particular, the actual shape of the reservoir, its size, as well as the nonuniformity of the pore pressure field -- affect the nature of the stress perturbation. The horizontal and vertical stress variations within most geological reservoirs are rarely identical; inside a tabular reservoir of large lateral extent compared to its thickness, only the horizontal stress is affected by the pore pressure change.
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From page 47... ...
Injection of a finite volume of fluid inside the porous elastic sphere embedded in a large impermeable elastic body induces a pore pressure increase Δρ inside the sphere as well as a stress perturbation Δσ inside and outside the sphere, caused by the expansion ΔV��������������������������������������������������������������������� of the sphere.
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From page 48... ...
The magnitude of the induced pore pressure increase and the extent of the region of pore pressure change depend on the rate of fluid injection and total volume injected, as well as on two hydraulic properties of the rock, its intrinsic permeability (k) and its storage coefficient (S)
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From page 49... ...
. The dependence of the magnitude of induced pore pressure and of the size of the perturbed pore pressure region on the injection rate, the volume of fluid injected, and the rock hydraulic properties (permeability and storage coefficient)
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From page 50... ...
A larger pore pressure increase brings the system closer to the conditions for initiating slip on a suitably oriented fault, if such a fault exists; a larger region of disturbed pore pressure will increase the risk of intersecting and activating a fault. Inducing a significant seismic event requires an increase of the pore pressure above levels that have existed prior to fluid injection and over a region large enough to encompass a fault area consistent with the magnitude of the earthquake.
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From page 51... ...
estimates that volume contraction of reservoirs from fluid withdrawal can cause earthquakes up to M 5.0. Several examples of induced seismicity associated with fluid withdrawal and associated pore pressure decrease have been reported, notably at the Lacq gas field in France (Box 2.5)
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From page 52... ...
from Rangely, began operation and detected numerous small seismic events M ≥ 0.5 in the vicinity of Rangely. With sustained fluid injection and elevated pore pressures the seismic events continued and the largest, M 3.4, occurred on August 5, 1964.
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From page 53... ...
, including in situ stress measurements, monitoring and modeling of changes of reservoir pore pressures, laboratory measurement of the sliding resistance between rock surfaces in the reservoir formation where seismic events were occurring, and detailed seismic monitoring to precisely locate the events and determine the fault orientation with respect to the stress field. Together these measurements, when used with the Coulomb criterion expressed in terms of the effective stress, predicted that a critical reservoir pressure of 257 bars was required to induce earthquakes at an injection site within the cluster of earthquakes -- a result that agreed with the observed and modeled pore pressures.
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From page 54... ...
. By 1983, the pressure had dropped by 500 bars, and 800 seismic events with magnitude up to M 4.2 had been recorded Figure 1 Location of seismic events compared to the size of the gas field (contours indicate depth to the top of the gas reservoir)
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From page 55... ...
. An analysis of the stress changes above and below the reservoir indicates that the induced seismicity is consistent with a thrust fault regime where the least compressive stress is vertical.
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From page 56... ...
, while the magnitude of the seismic event is related to the area of the fault undergoing slip. Inducing a seismic event requires a triggering event that will either increase the shear stress or reduce the normal effective stress on the fault and/ or reduce the fault frictional resistance, for example, an increase of the pore pressure that reduces the frictional strength to a level at which it is overcome by the driving shear stress.
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From page 57... ...
For example, in the well-documented Lacq gas field (France) the increase of the maximum shear stress was estimated to be about 0.1 MPa (1 bar)
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From page 58... ...
1994. Poroelastic stressing and induced seismicity near the Lacq gas field, south western France.
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