Are maximum magnitudes of induced earthquakes controlled by pressure diffusion?

There is an ongoing discussion about how to forecast the maximum magnitudes of induced earthquakes based on operational parameters, subsurface conditions and physical process understanding. Although the occurrence of damage caused by induced earthquakes is rare, some cases have caused significant economic loss, injuries and even loss of life. We analysed a global compilation of earthquakes induced by hydraulic fracturing, geothermal reservoir stimulation, water disposal, gas storage and reservoir impoundment. Our analysis showed that maximum magnitudes scale with the characteristic length of pressure diffusion in the brittle Earth's crust. We observed an increase in the nucleation potential of larger-magnitude earthquakes with time and explained it by diffusion-controlled growth of the pressure-perturbed part of faults. Numerical and analytical fault size modelling supported our findings. Finally, we derived magnitude scaling laws to manage induced seismic hazard of upcoming energy projects prior to operation. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'.

Magnitude and nucleation time of the 2017 Pohang Earthquake point to its predictable artificial triggering

A damaging Mw5.5 earthquake occurred at Pohang, South Korea, in 2017, after stimulating an enhanced geothermal system by borehole fluid injections. The earthquake was likely triggered by these operations. Current approaches for predicting maximum induced earthquake magnitude ([Formula: see text]) consider the volume of the injected fluid as the main controlling factor. However, these approaches are unsuccessful in predicting earthquakes, such as the Pohang one. Here we analyse the case histories of induced earthquakes, and find that [Formula: see text] scales with the logarithm of the elapsed time from the beginning of the fluid injection to the earthquake occurrence. This is also the case for the Pohang Earthquake. Its significant probability was predictable. These results validate an alternative to predicting [Formula: see text]. It is to monitor the exceedance probability of an assumed [Formula: see text] in real time by monitoring the seismogenic index, a quantity that characterizes the intensity of the fluid-induced seismicity per unit injected volume.

Groundwater extraction-induced seismicity around Delhi region, India

The non-tectonic deformation, either of natural or anthropogenic origin, may influence the earthquake occurrence process and seismicity rate along the plate-boundary or ‘stable’ plate-interiors domains. The low magnitude but moderate seismicity rate of Delhi region on the stable plate-interiors domains of India, exhibits significant variation both in short-term at annual seasonal scale and in long-term at decadal scale. It correlates with the anthropogenic groundwater pumping for the extensive irrigation, urban activities, and seasonally controlled hydrological loading cycle of Indo-Ganga Basin hosted freshwater aquifers. Our coupled hydro-mechanical simulation and poro-mechanical analysis of basement fault stability suggest that the combined aquifer contraction and basement rock expansion act together to modulate the effective stress regime and anthropogenic seismicity on the basement faults in Delhi region.

Hydraulic Fracturing Experiment Investigation for the Application of Geothermal Energy Extraction

As an attractive renewable energy source, deep geothermal energy is increasingly explored. Granite is a typical geothermal reservoir rock type with low permeability, and hydraulic fracturing is a promising reservoir stimulation method which could obviously enhance the reservoir permeability. Previous hydraulic fracturing studies were mostly conducted on artificial samples and small cylindrical granites. The fracturing pressures of artificial samples and small real rock sample were much lower than that of field operation, and it was difficult to observe morphological changes in small rocks. Hence, this paper presents a hydraulic fracturing experimental study on large-scale granite with a sample size of 300 × 300 × 300 mm under high temperatures. Besides, injection flow rate is an important parameter for on-site hydraulic fracturing; previous studies usually only focused on breakdown pressure, and there is a lack of comprehensive analysis about fracturing pressure curves and fracturing characteristics caused by different injection flow rates. This study aims to investigate the influence of injection flow rate on different pressure curve characteristic parameters which are initiation pressure, propagation time, breakdown pressure, postfracturing pressure, fracture geometry, and fracture permeability. The mean injection power was proposed to roughly estimate the fracture total lengths. These results could provide some guidance for field-scale reservoir stimulation and heat extraction efficiency improvement.

Experimental Study of Fracture Propagation: The Application in Energy Mining

Hydraulic fracturing has been widely used in recent years as a key technology to improve energy mining efficiency in petroleum and geothermal industries. Laboratory hydraulic fracturing experiments recently were completed in six large-scale 300 × 300 × 300 mm rock specimens to better understand this complex process of hydraulic fracturing. When injection flow rate increases from 5 to 30 mL/min. The fracture initiation pressures and breakdown pressures increase, the propagation times and post-fracturing pressures decrease. The fracture geometries are observed and analyzed, mean injection power is proposed and results show that it could be used to roughly estimate the fracture total lengths. Moreover, the fracture permeabilities based on the pressure data are calculated and linearly ascend with the increase of injection flow rates. These results can provide some reasonable advice for implementing hydraulic fracturing reservoir simulations and improving energy production efficiency on application to field-scale operation.

Terrain uplift due to natural hydrologic overpressure in karstic conduits

Water supply from karst sources is a worldwide natural resource and the exploitation is tied to the knowledge of the positions of the hydrologic channels. We show that surface deformation induced by flood events in karst conduits is observable, and consists in uplift and outward movement from the hydraulic channel. Precipitation events produce the natural occurrence of subsurface hydraulic overpressure up to 1 MPa. Numerical modeling shows that the stresses are so strong to uplift and dislocate the surface by several mm and induce tilts in the order of microradians. The naturally induced deformation is compatible with a transient internal pressure loading of a channel. The results can be used to find new channels with dense GNSS networks. Sea water incursion and channels accessed for tourism could be monitored. Seismicity has been shown to have a seasonal variation in some areas, which could be explained by the subsurface stresses induced by the natural subsurface overpressure. The pressure induced deformation is expected to be observed in all karstic systems worldwide.

Pressure build-up and stress variations within the Earth's crust in the light of analogue models

Strength contrasts and spatial variations in rheology are likely to produce significant stress differences in the Εarth’s crust. The buildup and the relaxation of stresses have important consequences for the state of stress of the brittle crust, its deformational behaviour and seismicity. We performed scaled analogue experiments of a classic wedge-type geometry wherein we introduced a weak, fluid-filled body representing a low-stress heterogeneity. The experiments were coupled to direct pressure measurements that revealed significant pressure differences from their surrounding stressed matrix. The magnitude of the pressure variations is similar to the magnitude of the differential stress of the strongest lithology in the system. When rocks with negligible differential stresses are considered, their pressure can be more than twice larger than the surrounding lithostatic stress. The values of the pressure variations are consistent with the stresses that are estimated in analytical studies. This behaviour is not restricted to a particular scale or rheology, but it requires materials that are able to support different levels of stress upon deformation. For non-creeping rheological behaviours, the stress and pressure variations are maintained even after deformation ceases, implying that these stress variations can be preserved in nature over geological timescales.

The spatial footprint of injection wells in a global compilation of induced earthquake sequences

Fluid injection can cause extensive earthquake activity, sometimes at unexpectedly large distances. Appropriately mitigating associated seismic hazards requires a better understanding of the zone of influence of injection. We analyze spatial seismicity decay in a global dataset of 18 induced cases with clear association between isolated wells and earthquakes. We distinguish two populations. The first is characterized by near-well seismicity density plateaus and abrupt decay, dominated by square-root space-time migration and pressure diffusion. Injection at these sites occurs within the crystalline basement. The second population exhibits larger spatial footprints and magnitudes, as well as a power law-like, steady spatial decay over more than 10 kilometers, potentially caused by poroelastic effects. Far-reaching spatial effects during injection may increase event magnitudes and seismic hazard beyond expectations based on purely pressure-driven seismicity.

The surge of earthquakes in Central Oklahoma has features of reservoir-induced seismicity

The recent surge of seismicity in Oklahoma and Kansas is related to fluid disposal. Evidences suggest that critical parameters are the injection volume as well as injection depth but dominant physical processes and a corresponding model to describe the physics are still not clear. We analyse the spatio-temporal distribution of induced earthquakes in the basement and find visible signatures of pore pressure diffusion and poroelastic coupling, features which strongly resemble seismicity induced by the filling of artificial lakes, so-called reservoir-induced seismicity. We developed a first-principle model of underground reservoir-induced seismicity. The physics of the model are based upon the combined mechanisms of fluid mass added to the pore-space of the injection layer and acting as a normal stress on the basement surface, pore-fluid pressure diffusion in the basement as well as poroelastic coupling contributing to the pore-fluid pressure and stress. Furthermore, we demonstrate that underground reservoir-induced seismicity occurs preferably in normal faulting and strike-slip settings, the latter being prevalent in Oklahoma. Our model explains observed injection volume and depth dependence of the seismicity and should be considered as a basis for future hazard prediction and prevention as well as for planning possible disposal sites.

On the effective stress law for rock-on-rock frictional sliding, and fault slip triggered by means of fluid injection

Fluid injection into rocks is increasingly used for energy extraction and for fluid wastes disposal, and can trigger/induce small- to medium-scale seismicity. Fluctuations in pore fluid pressure may also be associated with natural seismicity. The energy release in anthropogenically induced seismicity is sensitive to amount and pressure of fluid injected, through the way that seismic moment release is related to slipped area, and is strongly affected by the hydraulic conductance of the faulted rock mass. Bearing in mind the scaling issues that apply, fluid injection-driven fault motion can be studied on laboratory-sized samples. Here, we investigate both stable and unstable induced fault slip on pre-cut planar surfaces in Darley Dale and Pennant sandstones, with or without granular gouge. They display contrasting permeabilities, differing by a factor of 10⁵, but mineralogies are broadly comparable. In permeable Darley Dale sandstone, fluid can access the fault plane through the rock matrix and the effective stress law is followed closely. Pore pressure change shifts the whole Mohr circle laterally. In tight Pennant sandstone, fluid only injects into the fault plane itself; stress state in the rock matrix is unaffected. Sudden access by overpressured fluid to the fault plane via hydrofracture causes seismogenic fault slips.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.

How will induced seismicity in Oklahoma respond to decreased saltwater injection rates?

In response to the marked number of injection-induced earthquakes in north-central Oklahoma, regulators recently called for a 40% reduction in the volume of saltwater being injected in the seismically active areas. We present a calibrated statistical model that predicts that widely felt M ≥ 3 earthquakes in the affected areas, as well as the probability of potentially damaging larger events, should significantly decrease by the end of 2016 and approach historic levels within a few years. Aftershock sequences associated with relatively large magnitude earthquakes that occurred in the Fairview, Cherokee, and Pawnee areas in north-central Oklahoma in late 2015 and 2016 will delay the rate of seismicity decrease in those areas.

Gas injection may have triggered earthquakes in the Cogdell oil field, Texas

Between 1957 and 1982, water flooding was conducted to improve petroleum production in the Cogdell oil field north of Snyder, TX, and a contemporary analysis concluded this induced earthquakes that occurred between 1975 and 1982. The National Earthquake Information Center detected no further activity between 1983 and 2005, but between 2006 and 2011 reported 18 earthquakes having magnitudes 3 and greater. To investigate these earthquakes, we analyzed data recorded by six temporary seismograph stations deployed by the USArray program, and identified 93 well-recorded earthquakes occurring between March 2009 and December 2010. Relocation with a double-difference method shows that most earthquakes occurred within several northeast-southwest-trending linear clusters, with trends corresponding to nodal planes of regional focal mechanisms, possibly indicating the presence of previously unidentified faults. We have evaluated data concerning injection and extraction of oil, water, and gas in the Cogdell field. Water injection cannot explain the 2006-2011 earthquakes, especially as net volumes (injection minus extraction) are significantly less than in the 1957-1982 period. However, since 2004 significant volumes of gases including supercritical CO2 have been injected into the Cogdell field. The timing of gas injection suggests it may have contributed to triggering the recent seismic activity. If so, this represents an instance where gas injection has triggered earthquakes having magnitudes 3 and larger. Further modeling studies may help evaluate recent assertions suggesting significant risks accompany large-scale carbon capture and storage as a strategy for managing climate change.

Hydrogeologic controls on induced seismicity in crystalline basement rocks due to fluid injection into basal reservoirs

A series of Mb 3.8-5.5 induced seismic events in the midcontinent region, United States, resulted from injection of fluid either into a basal sedimentary reservoir with no underlying confining unit or directly into the underlying crystalline basement complex. The earthquakes probably occurred along faults that were likely critically stressed within the crystalline basement. These faults were located at a considerable distance (up to 10 km) from the injection wells and head increases at the hypocenters were likely relatively small (∼70-150 m). We present a suite of simulations that use a simple hydrogeologic-geomechanical model to assess what hydrogeologic conditions promote or deter induced seismic events within the crystalline basement across the midcontinent. The presence of a confining unit beneath the injection reservoir horizon had the single largest effect in preventing induced seismicity within the underlying crystalline basement. For a crystalline basement having a permeability of 2 × 10(-17)  m(2) and specific storage coefficient of 10(-7) /m, injection at a rate of 5455 m(3) /d into the basal aquifer with no underlying basal seal over 10 years resulted in probable brittle failure to depths of about 0.6 km below the injection reservoir. Including a permeable (kz  = 10(-13)  m(2) ) Precambrian normal fault, located 20 m from the injection well, increased the depth of the failure region below the reservoir to 3 km. For a large permeability contrast between a Precambrian thrust fault (10(-12)  m(2) ) and the surrounding crystalline basement (10(-18)  m(2) ), the failure region can extend laterally 10 km away from the injection well.