Seismicity | EPFL Graph Search

Seismicity is a measure encompassing earthquake occurrences, mechanisms, and magnitude at a given geographical location. As such, it summarizes a region's seismic activity. The term was coined by Beno Gutenberg and Charles Francis Richter in 1941. Seismicity is studied by geophysicists. Calculation of seismicity Seismicity is quantitatively computed. Generally, the region under study is divided in equally sized areas defined by latitude and longitude, and the Earth's interior is divided into various depth intervals on account of Earth's layering: Up to depth, , and > . The usual formula to calculate seismicity is: S = \frac{\sum_{i} {E_{s0}}_i}{ {\delta \phi}_0 {\delta \lambda}_0 {\delta h}_0 {\delta t}_0 } where : {E_{s0}}_i: is the energy of a single seismic event (i.e., earthquake); : {\delta \phi}_0: interval of latitude; : {\delta \lambda}_0: interval of longitude : {\delta h}_0: interval of the hypo

Stress management in the context of induced seismicity in subsurface reservoirs

Barnaby Padraig Fryer

The stress state of the subsurface has been shown to have an influence on a number of key processes. For example, the criticality of the stress state indicates how large stress changes need to be before a fault begins to slip, the mean effective stress controls compaction and permeability loss in hydrocarbon reservoirs, the differential stress has been seen in seismology to have a strong influence on the size distribution of earthquake catalogues, with stress state further influencing the affinity of a fault to slip either seismically or aseismically, and the stress state strongly influences the propagation of a hydraulic fracture, with stress jumps being capable of completely halting a fracture's propagation. For reasons such as these, it has long been recognized that the state of stress is an important parameter for subsurface industrial operations such as hydrocarbon production, Enhanced Geothermal System (EGS) stimulation, carbon storage, and hydraulic fracturing. Generally, however, the state of stress does not remain constant during many of these operations. Decreasing effective stresses have led to induced seismicity during injection, fluid production has led to total stress changes which have induced seismicity, even in areas where pore pressure has decreased, temperature-induced stress changes have led to shear stimulation, and changes in total stress have led to so-called "frac-hits" where hydraulic fractures propagate towards depleted zones. These examples demonstrate an operator's ability to alter the state of stress, yet deliberate attempts to alter the stress state for the benefit of future operations have only been suggested a handful of times. Here, this idea of altering the state of stress is extended through a series of investigations. It is shown that, because production-induced seismicity is caused by total stress changes associated with the gradient of the produced fluid pressure, a hydraulic fracture which reduces the gradient required for production will also reduce the resulting seismicity. Conversely, compaction which leads to permeability loss will result in higher pore pressure gradients and therefore more induced seismicity. Both of these studies, which also have implications for optimal horizontal wellbore orientation, are investigated through the use of a poroelastic reservoir simulator developed during the thesis. In a following study, it is suggested that high stress path reservoirs are attractive targets for fluid injection, as total stress changes will result in an increased stability despite increasing pore pressure. Further, the concept of stress preconditioning is introduced, such that operators alter the stress state of an EGS reservoir to promote more favorable earthquake distributions. These concepts, which make up the first three chapters of the thesis, all are forms of reservoir management that reduce the risk associated with induced seismicity. Following this, in a numerical study, the idea of stress preconditioning is extended to allow for directed stimulation treatments in EGS's. Finally, the idea of inducing stress jumps through production such that hydraulic fractures do not propagate vertically is evaluated by means of scaling analyses and numerical simulation, with implications for carbon storage. More than the individual propositions, this thesis promotes the mindset that, where appropriate, attempts should be made to alter the state of stress for the benefit of future operations.

EPFL2020

Reservoir stimulation’s effect on depletion-induced seismicity

Barnaby Padraig Fryer, Lyesse Laloui, Gunter Siddiqi

Depletion-induced seismicity can pose a problem in some fluid-producing subsurface reservoirs, in some cases requiring production rate control in order to limit the seismicity. This study investigates the use of reservoir stimulation to reduce the depletion-induced seismicity rate. Depletion-induced stress and pore pressure changes are evaluated in a shale cap rock, sandstone reservoir, and shale underburden system, which contains a horizontal well, all modeled in plane strain conditions. The seismicity rate is then predicted based on an existing seismicity model and is found to be dependent on the direction the well is drilled in with respect to the principal stresses. The case where the reservoir has first been stimulated is compared to the case where stimulation has not been performed (using the same production rates) for normal, reverse, and strike-slip faulting stress regimes. Seismicity is reduced in the case of reservoir stimulation for both reverse and strike-slip faulting stress regimes. The seismicity rate is only slightly reduced for the normal faulting stress regime. Stimulation also increases the distance that changes in pore pressure dominate over poroelastic stress changes in the reservoir. Further, it is found that the optimal orientation of a horizontal well, in terms of induced seismicity, is parallel to the minimum principal stress in normal faulting stress regimes and parallel to the maximum principal stress in reverse faulting stress regimes. The orientation of a horizontal well determines where the seismicity is located in a strike-slip faulting stress regime.

2018

On the effects of fluid pressure variations on rock-mass and fault mechanical behaviour

Corentin Jean-Marie Rémi Noël

In the past 20 years, the growing interest in deep geo-reservoirs for purposes such as carbon storage, waste water disposal, or geothermal energy exploitation have led to large-volume fluid injections into the upper continental crust. These fluid injections have caused a massive increase in the seismicity rate in some normally âquietâ regions. Recent observations suggested that different injection strategies could produce different mechanical responses in these reservoirs. In this vein, cyclic fluid injections have been proposed as an alternative to conventional monotonic injections to mitigate induced earthquakes. Additionally, recent studies have suggested that injection into deeper geo-reservoirs, at the brittle-ductile transition, could reduce (or suppress) induced earthquakes. In this context, this research has aimed at bettering our understanding of reservoir deformation during different fluid injection strategies. Laboratory experiments were performed on porous reservoir rock under different reservoir conditions and fluid injection strategies. In the first section, it is demonstrated that water-saturated conditions, compared to dry conditions, cause a reduction in uniaxial compression strength in five sandstones. Additional experiments suggest that this water weakening of sandstones is due to the reduction of the fracture toughness and of the static friction of the materials. In the second section, it is shown that under drained conditions, pore fluid pressure oscillations affect the long-term mechanical behaviour of intact porous rocks. More than the amplitude of the oscillations, the period controls the time-to-failure, failure strength and dilatancy rate of the sample. Additionally, the pore fluid pressure oscillations control the AE events (a proxy for the seismicity): the AE events rate oscillates in-phase with the pore fluid pressure variations. Increasing the differential stress, the amplitude and the period of the oscillations accentuates this behaviour. In the third section, it is demonstrated that under drained conditions, pore fluid pressure oscillations strongly affect the mechanical behaviour of faults. The pore fluid pressure signal directly controls the instabilitiesâ (i.e., stick-slip and AE events) distribution, with an increase of events at elevated pore fluid pressure. For initially stable sliding faults, the pore fluid pressure oscillation signal controls the onset of unstable slip: the higher the pore fluid pressure oscillationâs amplitude, the lower the stress and displacement at the onset of a seismic event. In the fourth section, it is shown that under quasi-drained conditions, a pore fluid pressure increase within a rock undergoing ductile deformation causes instantaneous dilation of the system. Further increasing the pore fluid pressure leads to the development of localized deformations and the shear fracturing of the rock. The macroscopic shear fracturing is not instantaneous when the ductile to brittle transition is passed. Prior to this, a creeping phase is necessary. The time and strain length of this creeping phase are controlled by the rate of pore fluid pressure increase: the faster the injection, the longer the creeping phase. This work provides new insight into the mechanical behaviour of rock masses and faults under various crustal conditions during pore fluid pressure variations. It helps to constrain the mechanisms involved in deep geo-reservoirs submitted to different injection strategies.

EPFL2021