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Fang, Y. (Institute for Geophysics / Jackson School of Geosciences / The University of Texas at Austin) | Elsworth, D. (The Pennsylvania State University, University) | Ishibashi, T. (Fukushima Renewable Energy Institute / National Institute of Advanced Industrial Science) | Zhang, F. (Tongji University)
ABSTRACT: Our experiments investigate the role of roughness in fracture permeability and frictional behavior using artificial fractures with controlled roughness. The results show that (1) Rougher surfaces indicate higher frictional stability and frictional strength due to the presence of cohesive interlocking asperities during shearing, which suggest that rougher fractures are difficult to reactivate. (2) Rough fracture surfaces show velocity strengthening behavior in the initial shearing stage and their strengthening behaviors evolve to velocity neutral and velocity weakening with greater displacement, which means that rough fractures become less stable with shearing (3) The surface roughness exerts a dominant control on permeability evolution over the entire shearing history. Permeability evolves monotonically for smooth fractures but in a fluctuating pattern for highly roughened fractures. A higher roughness is likely to result in more cycling between compaction and dilation during shearing. Significant permeability reduction may occur for rough samples when asperities are highly worn with wear products clogging flow paths. (4) There is no conspicuous correlation between permeability evolution and frictional behavior for rough fracture samples when fractures are subject to sudden sliding velocity change. These lab-scale experimental results reveal the role of rock surface topography in understanding the reactivation and permeability development of fractures.
Subsurface engineering activities, such as the development of enhanced geothermal systems (EGS), stimulation of unconventional shale gas reservoirs, and geological carbon sequestration (GCS), all involve massive fluid injections, which may reduce the effective normal stress on preexisting faults and fractures and induce microearthquakes (Majer et al., 2007; Moeck et al., 2009; Zoback et al., 2012; Zoback and Gorelick, 2012; Ellsworth, 2013; Fang et al., 2016). The induced seismicity occurs as seismic slip, slow slip and aseismic slip (Cornet et al., 1997; Zoback et al., 2012; Guglielmi et al., 2015), which would result in shear compaction or dilation of fractures or faults and lead to permeability reduction or enhancement (Maini, 1972; Barton et al., 1985; Elsworth and Goodman, 1986; Faoro et al., 2009). Hence, understanding the permeability evolution concerning shear deformations is a key step for optimizing the stimulation and production of unconventional reservoirs and for protecting the geological sealing of fluid disposal repository.
There is wide concern that pressurized CO2 fluid has a potential to induce seismicity in the impermeable caprocks that overlie CO2 injection formations. However, the possible impact of induced seismicity on sustainable CO2 containment from geological CO2 sequestration remains unclear because the earthquakes play a significant but mysterious role in influencing the integrity of the caprocks by hypothesized interrelated friction-permeability interaction processes: (1) the earthquakes may occur seismically (i.e., frictionally unstable), enhancing the permeability of faults instantly and leading to potential breaching and loss of inventory; or (2) the earthquakes may occur aseismically (i.e., frictionally stable), closing the aperture of faults and reducing permeability through creep. In this study, we explore these processes through experiments in which we measure the frictional parameters and hydraulic properties using Green River shale sample as an analogue caprock candidate. We observe that fracture permeability declines during shearing while the increased sliding velocity reduces the rate of decline. The physics of these observed behaviors are explored via parametric study and surface measurement of fractures, showing that both permeability and frictional strength are correlated to the fracture asperity evolution that is controlled by the sliding velocity and fracture material. Through the velocity step, the velocity strengthening behavior is observed for Green River shale, suggesting that for Green River shale, only aseismic slip would occur at a low sliding velocity during which the permeability would decrease.
Geological storage of carbon dioxide (CO2) is considered a viable method to significantly reduce CO2 emissions from energy production and to reduce global impacts on climate (Falkowski et al., 2000). Injection of CO2 into deep saline aquifers or depleted oil and gas reservoirs has the potential to sequester significant mass of CO2 in a sustainable manner (Holt et al., 1995; Bachu and Adams, 2003; Orr, 2009; Szulczewski et al., 2012). One key to the success of long-term CO2 storage is the integrity of the resulting seal of impermeable caprocks that contain the charge to deep saline aquifers and prevents leakage into the atmosphere or potable aquifers (Shukla et al., 2010). However, the presence of preexisting faults and fractures distributed throughout the upper crust may influence the longevity of this storage (Anderson and Zoback, 1982; Curtis, 2002). Fluid injection activities (e.g., hydraulic fracturing, deep disposal of wastewater, enhanced geothermal stimulation, etc.) can reactivate pre-existing faults and induce seismicity (Healy et al., 1968; Raleigh et al., 1976; Kanamori and Hauksson, 1992; McGarr et al., 2002; Shapiro et al., 2006; Majer et al., 2007; Suckale, 2009; Ellsworth, 2013; Walsh and Zoback, 2015; Guglielmi et al., 2015; Im et al., 2016). Likewise, large-scale injection of CO2 that generates overpressures and decreases effective normal stresses may reactivate preexisting faults in caprocks (Fig. 1). As a result, CO2-injection induced seismicity may raise the potential that the rupture of caprocks could jeopardize the seal integrity and ultimately result in loss of charge of CO2 (Chiaramonte et al., 2008; Rutqvist, 2012; Zoback and Gorelick, 2012). Hence, it is of particular interest to understand the evolution of permeability of caprocks as a result of seismic and aseismic deformation in caprocks.
ABSTRACT: Massive fluid injection can reactivate pre-existing faults or fractures and induce deformation as either seismic slip, slow slip or aseismic slip. These shear deformations, controlled by frictional strength and stability, may lead to different shear permeability evolutions. Previous studies have explored frictional stability-permeability relationships of carbonate-rich and phyllosilicate-rich samples during shear deformation, suggesting that phyllosilicate-rich shale has a lower frictional strength, but higher frictional stability and larger permeability reduction than that of carbonate-rich shale. This qualitative result is sufficient to identify the role of individual mineral group (i.e., tectosilicate, carbonate, and phyllosilicate) in prompting this response. Indeed, it is still uncertain whether or not a quantitative relationship of frictional stability-permeability relationships of fractures exists. In this study, we perform a series of hydroshearing experiments on saw-cut fractures of natural rocks (Green River shale, Opalinus shale, Longmaxi shale, Tournemire shale, Marcellus shale, and Newberry tuff) with distinct mineralogical compositions to understand the frictional stability-permeability relationships with respect to individual mineral groups. Our experimental results indicate that permeability change increases non-linearly with frictional strength while decreases non-linearly with frictional stability. These relationships imply that clay-rich fractures may be easily reactivated with aseismic deformation due to low frictional strength and high frictional stability, meanwhile, the permeability may decline due to clay swelling and wear product compaction. On the contrary, tectosilicate-rich fractures show the opposite trend. These results are significant for providing valuable references for understanding how permeability evolves in engineering activities like shale reservoir stimulation and CO2 caprock integrity evaluation.
The stimulation of shale gas reservoir, of enhanced geothermal system (EGS), and the long term geological sequestration of CO2 involve massive fluid injection, which may reactivate pre-existing faults and fractures by hydroshearing and induce seismicity as seismic slip, slow slip and aseismic slip (Cornet et al., 1997; Zoback et al., 2012; Guglielmi et al., 2015b; Fang et al., 2016). A number of in-situ observations show that these deformations may affect the transport characteristics of the reservoir formation. For example, rapid flow at seismogenic depths is observed in IODP drill holes on the Cascadia margin (Davis et al., 1995). In-situ experiments show permeability enhancement that are associated with small dilatant slip event in reactivation of both Tournemire shale and carbonate faults (Guglielmi et al., 2015a, 2015b). At laboratory scale, experimental observations indicate that when a fracture slips, permeability may increase due to significant dilation or decrease as a result of progressive formation of gouge (Barton et al., 1985; Faoro et al., 2009). These relationships between fluid flow and fault slip pose a ubiquitous question in understanding how fault permeability evolves during fault movement, which would further provide significant insight of how fluids may be trapped by seal layers, how hydrocarbons may migrate within fractures, and how integrity of seal system may lose or enhance.
Enhanced geothermal system (EGS) has the potential to offer a large amount of clean energy by extracting stored thermal energy from the subsurface. The effectiveness of heat extraction is dependent not only on the permeability of fractured rocks but also on the stability of preexisting and induced fractures. A better understanding of fracture slip in granite during fluid injection is critical to optimize the strategies of hydraulic stimulation. We experimentally investigated the shear behaviors of a sawcut fracture, a gouge-filled fracture, and a natural fracture in Bukit Timah granite in response to fluid injection under a constant normal stress and a constant critical shear stress, respectively. In the most cases, the pore pressure at the injection-induced failure exceeds that predicted by the Mohr-Coulomb failure criterion. This is attributed to the nonuniform distribution of fluid over the fracture plane, which is associated with lower permeability of fracture and host rocks and higher injection rate. The shear behaviors of sawcut and natural fractures show a complex combination of creep and stick before injection failure, which is presumably dependent on the state of asperity contacts. The gouge-filled fracture always creeps preceding the injection-induced failure, because the stiffness of testing system overweighs the critical rheologic stiffness of fracture. For these three fractures, the slip rate at injection failure increases with higher injection rate, releasing more strain energy. The slip rate induced by fluid injection in this study falls within the slip rate range of slow slip events observed in natural faults.
Harvesting heat trapped in igneous rocks offers us an affordable and sustainable solution to reduce our dependence on fossil fuels. Because of the extremely low permeability of igneous rocks, fluid injection has been used to create and/or activate fractures, enhancing the permeability of the host rocks. Besides the permeability evolution of rock fractures, the effectiveness of heat extraction is also dependent on the frictional stability of preexisting and induced fractures, because frictional instabilities of fractures result in seismic events. For example, fluid injection-induced seismicity in an enhanced geothermal system (EGS) in Basel, Switzerland led to the closure of the project (Majer et al. 2007), so did the California project in the United States also aiming at extracting underground geothermal energy (Elsworth et al. 2016). Therefore, a better understanding of the frictional stability evolution of fractures in igneous rocks is of critical importance to optimize the hydraulic stimulation strategy for EGS.
The fracture is induced to slip when the shear stress acting on the fracture exceeds the shear strength of it according to the Mohr-Coulomb failure criterion (Zoback 2010) and the effective stress law (Terzaghi 1923). When a fracture starts to slip, the slip can either be seismic or aseismic, depending on the friction rate parameter and relative magnitude of critical rheologic stiffness of the fracture and the stiffness of elastic surroundings (Marone 1998). Specifically, when the friction rate parameter is positive, the fracture is intrinsically rate strengthening and can always slip stably (aseismic), while a fracture with a negative friction rate parameter is conditionally rate weakening, tending to slip unstably (seismic) if the critical rheologic stiffness of the fracture is larger than the stiffness of elastic surroundings.
Ye, Zhi (The University of Oklahoma) | Ghassemi, Ahmad (The University of Oklahoma) | Ji, Lujun (Occidental Petroleum Corporation) | Sen, Vikram (Occidental Petroleum Corporation) | Mailloux, Jason (Occidental Petroleum Corporation)
Shear reactivation of pre-existing fractures can play a crucial role in hydraulic stimulation to enable production from unconventional shale reservoirs. However, the mechanisms of permeability enhancement contributed by fracture shear slip are still poorly understood, and the laboratory experiments on shale fractures are insufficient. In this work, injection-induced shear tests were conducted on two reservoir shale samples (from the depth >10,000 ft.) each having a single rough fracture to characterize permeability evolution during fracture shear slip. In the tests, remarkable enhancement of flow rate/permeability have been achieved on both samples through fracture shear slip induced by elevating injection pressure. It is shown that ∼100 times increase in flow rate was induced by a ∼0.1 mm scale of shear slip on the two cylindrical shale samples (with ∼38 mm diameter). The permeability enhancement was retained on the sheared samples even with the decline of fluid pressure. This means that the permeability increase by fracture shear slip may be permanent. In addition, significant stress drops were induced by fracture shear slip in both tests, resulting in further fracture aperture/permeability enhancement. In one sample, a new fracture plane was formed during the shear slip of the original pre-existing fracture, which may help generate an interconnected fracture network. Therefore, dilatant shear slip, stress drop, and the new fractures formation are important and often integral mechanisms of permeability enhancement contributed by pre-existing fractures during shale reservoir stimulation. The results improve the understanding of shear slip in enhancing permeability of shale fractures, and would help engineer solutions for maintaining these fractures open, reducing costs (proppant/water and additive cost savings).
Shear slip of pre-existing fractures has been recognized as a major permeability creation mechanism of reservoir stimulation for a long time (e.g. Mayerhofer et al., 1997; Pine & Batchelor, 1984; Rutledge et al., 2004; Zoback et al., 2012). Most reservoir rocks contain abundant pre-existing fractures, some of which may be sealed with calcite or other infill minerals. Usually, these fractures are inactive and without sufficient permeability before stimulation. A number of modeling and field studies have shown that when the pre-existing natural fractures are favorably oriented with respect to the in-situ stresses, an injection or the leak-off from a hydraulic fracture (even with low fluid pressure) can cause the fractures sliding and propping due to asperities. Also, the interpretations of induced microseismic events during hydraulic injection have indicated that shear slip of pre-existing fractures around hydraulic fractures can help generate a large stimulated reservoir volume and benefit the production performance (e.g. Fisher et al., 2004; Mayerhofer et al., 2010). However, the fundamental mechanisms of permeability creation by fracture shear slip underlying in shale reservoir stimulation are still poorly understood.