A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones

Despite its enormous cost, large-scale carbon capture and storage (CCS) is considered a viable strategy for significantly reducing CO2 emissions associated with coal-based electrical power generation and other industrial sources of CO2 [Intergovernmental Panel on Climate Change (2005) IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change, eds Metz B, et al. (Cambridge Univ Press, Cambridge, UK); Szulczewski ML, et al. (2012) Proc Natl Acad Sci USA 109:5185–5189]. We argue here that there is a high probability that earthquakes will be triggered by injection of large volumes of CO2 into the brittle rocks commonly found in continental interiors. Because even small- to moderate-sized earthquakes threaten the seal integrity of CO2 repositories, in this context, large-scale CCS is a risky, and likely unsuccessful, strategy for significantly reducing greenhouse gas emissions.

/pdf/earthquake-triggering-and-large-scale-geologic-storage-of-135zd764p3.pdf

Earthquake triggering and large-scale geologic storage of carbon dioxide

CHANGES IN PERMEABILITY CAUSED BY TRANSIENT STRESSES: FIELD OBSERVATIONS, EXPERIMENTS, AND MECHANISMS Michael Manga, 1 Igor Beresnev, 2 Emily E. Brodsky, 3 Jean E. Elkhoury, 4 Derek Elsworth, 5 S. E. Ingebritsen, 6 David C. Mays, 7 and Chi-Yuen Wang 1 Received 7 November 2011; revised 15 February 2012; accepted 10 March 2012; published 12 May 2012. [ 1 ] Oscillations in stress, such as those created by earth- quakes, can increase permeability and fluid mobility in geo- logic media. In natural systems, strain amplitudes as small as 10 A6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to prestimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the prestimulated value is governed by the time to reblock pores, or for geochemical processes to seal pores. Monitor- ing permeability in geothermal systems where there is abun- dant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution. Citation: Manga, M., I. Beresnev, E. E. Brodsky, J. E. Elkhoury, D. Elsworth, S. E. Ingebritsen, D. C. Mays, and C.-Y. Wang (2012), Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms, Rev. Geophys., 50, RG2004, doi:10.1029/2011RG000382. INTRODUCTION [ 2 ] The permeability of Earth’s crust is of great interest because it largely governs key geologic processes such as advective transport of heat and solutes and the generation of elevated fluid pressures by processes such as physical com- paction, heating, and mineral dehydration. For an isotropic Department of Earth and Planetary Science, University of California, Berkeley, California, USA. Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Department of Civil and Environmental Engineering, University of California, Irvine, California, USA. Department of Energy and Mineral Engineering, Center for Geomechanics, Geofluids, and Geohazards, EMS Energy Institute, Pennsylvania State University, University Park, Pennsylvania, USA. U.S. Geological Survey, Menlo Park, California, USA. Department of Civil Engineering, University of Colorado Denver, Denver, Colorado, USA. Corresponding author: M. Manga, Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720, USA. (manga@seismo.berkeley.edu) material, permeability k is defined by Darcy’s law that relates the fluid discharge per unit area q to the gradient of hydraulic head h, q ¼A kgr rh; m where r is the fluid density, m the fluid viscosity and g is gravity. The permeability of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 A23 m 2 in intact crystalline rock, intact shales, and fault cores, to values as high as 10 A7 m 2 in well-sorted gravels. Nevertheless, despite being highly heterogeneous, perme- ability can be characterized at the crustal scale in a manner that provides useful insight [e.g., Gleeson et al., 2011]. [ 3 ] The responses of hydrologic systems to deformation provide some insight into controls on permeability, in par- ticular its evolution in time. For example, the water level in wells and discharge in rivers have both been observed to change after earthquakes. Because earthquakes produce stresses that can change hydrogeologic properties of the crust, hydrologic responses to earthquakes are expected, especially in the near field (within a fault length of the Copyright 2012 by the American Geophysical Union. Reviews of Geophysics, 50, RG2004 / 2012 1 of 24 Paper number 2011RG000382 8755-1209/12/2011RG000382 RG2004

/pdf/changes-in-permeability-caused-by-transient-stresses-field-2mxwg2xepm.pdf

Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

The variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability (k) of tectonically active continental crust decreases with depth (z) according to log k ! )14‐3.2 log z, where k is in m 2 and z in km. Other independently derived, crustal-scale k‐z relations are generally similar to this power-law curve. Yet there is also substantial evidence for local-to-regional-scale, transient, permeability-generation events that entail permeabilities much higher than these mean k‐z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log k ! )11.5‐3.2 log z, suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k‐z curves. Several lines of evidence suggest geologically rapid (years to 10 3 years) decay of high-permeability transients toward background values. Crustal-scale k‐z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.

/pdf/permeability-of-the-continental-crust-dynamic-variations-4ndk3gtsw7.pdf

Permeability of the Continental Crust: Dynamic Variations Inferred from Seismicity and Metamorphism

Analysis of induced seismicity in geothermal reservoirs – An overview

[1] The temporal and spatial pattern of seismicity associated with reservoir water level fluctuations, injection of high-pressure fluids in deep boreholes, and seasonal groundwater recharge provide a unique setting to study the hydrological properties of the seismogenic fractures. Pore pressure diffusion is primarily responsible for the build up of fluid pressures and the onset of seismicity. The hydrologic property controlling pore pressure diffusion is hydraulic diffusivity c, which is directly related to intrinsic permeability k. By analyzing more than 90 case histories of induced seismicity, we determined the hydraulic diffusivity value of fractures associated with seismicity to lie between 0.1 and 10 m2/s. This range of values of c corresponds to a range of intrinsic permeability values between 5 × 10−16 and 5 × 10−14 m2. We call this range the seismogenic permeability ks. Fractures with ks were found to be associated with Darcian flow. Fractures with permeability less than ks were aseismic, as the pore pressure increase was negligible. In fractures with permeability larger than ks, aseismic non-Darcian flow was observed. Seismicity was uniquely associated with fractures with seismogenic permeability. Thus seismogenic permeability is an intrinsic property of fractures where pore pressure diffusion is associated with seismicity.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JB004665

Seismogenic permeability, ks

Induced seismicity has been observed near Monticello Reservoir, South Carolina, since December 1977. Deployment of a seismic network before impound- ment allowed for detection and accurate location of pursuant seismicity since its inception. Corroborative fault-plane solutions, together with geological and borehole data on fracture orientations, made it possible to determine the structures associated with the initial seismicity. Earlier descriptions attributed this seismicity to the un- drained elastic response to impoundment of the reservoir or to a coupled poroelastic response, where diffusion of pore pressure and subsequent weakening was the pre- dominant cause. Quantitative evaluation of strength changes at hypocentral locations of a subset of 53 well-located earthquakes that followed the initial impoundment led to the following results: (1) The rocks in the vicinity of Monticello Reservoir are critically stressed, and strength changes less than or equal to 0.1 MPa are adequate to trigger seismicity; (2) except at locations on the periphery of the reservoir, and at shallow depths within it (1 km), impoundment of the reservoir led to strengthening at hypocentral locations due to the undrained elastic effect; (3) diffusion of pore pressure is the dominant mechanism for the observed seismicity; and (4) the inferred permeability of the fractures associated with seismicity, 5 10 14 m 2 (50 mD), lies within the range of seismogenic permeability associated with induced seismicity.

/pdf/mechanism-of-initial-seismicity-following-impoundment-of-the-33cdyrlqwl.pdf

Mechanism of Initial Seismicity Following Impoundment of the Monticello Reservoir, South Carolina

A review of case histories of reservoir-induced seismicity (RIS) in China shows that it mainly occurs in granitic and karst terranes. Seismicity in granitic terranes is mainly associated with pore pressure diffusion whereas in karst terranes the chemical effect of water appears to play a major role in triggering RIS. In view of the characteristic features of RIS in China, we can expect moderate earthquakes to be induced by the construction of the Three Gorges Project on the Yangtze River.

/pdf/reservoir-induced-seismicity-in-china-3i03mcxbng.pdf

Reservoir-induced Seismicity in China

A surprising increase in seismicity started in and around Monticello Reservoir, South Carolina in December 1996, and by the end of 1999, over 700 earthquakes with –0.4 ≤ M L ≤ 2.5 had been located. This seismicity occurred in a new hypocentral region and filled the gaps in earlier seismicity at depths shallower than 2 km. The seismicity occurred in four episodes each with at least one earthquake of magnitude M L ∼ 2.0. The new seismicity started at depths of 0.8–2 km within a previously a seismic granofel body and in its surrounding volume (episodes I and II). Episode III began more than a year later and also occurred in granofels. It was located to the east of the first two episodes and at shallower depths (from the surface to ∼1.4 km deep). The seismicity then migrated less than 1 km to the north and south and occurred in granodiorites (episode IV). We speculate that the rocks in the new hypocentral regions were isolated from the regions of earlier seismicity by fractures filled with zeolites. Twenty years of reaction with water led to the reopening and weakening of the zeolite-filled fractures, allowing fluids to enter the previously aseismic regions and triggering these episodes of intense seismicity. We suggest that pore pressure migration was associated with these episodes of seismicity. Each of the four episodes was associated with two stages with different temporal and spatial patterns. In the first stage, there was a rapid increase in seismicity in a small volume. We interpret this to be associated with a rapid build-up of pore pressure. In the second stage the seismicity spread, and its activity rate decayed. We interpret this stage to be associated with the equilibration of pore pressure.

/pdf/renewed-seismicity-near-monticello-reservoir-south-carolina-1pxh3c6pbo.pdf

Renewed Seismicity near Monticello Reservoir, South Carolina, 1996–1999

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Seismogenic permeability, ks

Mechanism of Initial Seismicity Following Impoundment of the Monticello Reservoir, South Carolina

Reservoir-induced Seismicity in China

Renewed Seismicity near Monticello Reservoir, South Carolina, 1996–1999