12 research outputs found

    Injection-induced slip heterogeneity on faults in shale reservoirs

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    Managing fluid stimulation protocols is an effective means to mitigate the risk of injection-induced earthquakes during shale gas development. The success of these protocols is dependent on our understanding of fluid pressure heterogeneity and the associated inhomogeneous slip on critically stressed faults. Here we show the evolution of velocity-weakening zone on a simulated fault, derived from fluid injection and velocity stepped experiments, and the corresponding non-uniform fluid pressure distribution, recovered from coupled hydro-mechanical simulations. Our results indicate that the sharp extension of velocity-weakening zone occurs before the nucleation of fault rupture, which could be an indicator to avoid the reactivation of other fault patches beyond the stimulated zone. The dynamic rupture is estimated to extend much faster than the maximum speed of the velocity-weakening zone front. We infer that the velocity-weakening zone may further expand and fully control the fault behavior after multiple slip events.Nanyang Technological UniversityWei Wu gratefully acknowledges the support of Start-up Grant from Nanyang Technological University, Singapore

    Experimental evidence for multiple controls on fault stability and rupture dynamics

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    The stability of frictional sliding affects the spectrum of fault slip, from slow-slip events to earthquakes. In laboratory experiments, the transition from stable sliding to stick-slip is often explained by the ratio of the stiffness of the loading system to a critical value that depends on effective normal stress and other physical properties. However, theoretical considerations indicate other controls on fault stability that have not been validated experimentally. Here, we exploit the dependence of frictional properties on load-point velocity to explore the dynamics of frictional sliding with gradual variations of frictional properties. We use the period-multiplying and chaotic cycles that appear at the transition between stick-slip and stable sliding as a sensitive indicator of fault stability. In addition to the stiffness ratio, we find that the ratio of the parameters that describe the dependence on velocity and state constitutes another control on the stability of faulting and rupture dynamics. Variations of these two non-dimensional parameters among faults may help explain the wide range of rupture styles and recurrence patterns observed in nature.Ministry of Education (MOE)This study was supported by Ministry of Education, Singapore, under award number RG169/16. SB acknowledges funding from the National Science Foundation, under award number EAR-1848192

    Slip velocity dependence of friction-permeability response of shale fractures

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    Fluid injection-induced fracture slip during hydraulic stimulation of shales may be seismic or aseismic with the slip mode potentially influencing the evolution of permeability and subsequent shale gas production. We report a series of friction-permeability tests with constant and stepped velocities on planar saw-cut fractures of Longmaxi shale, Green River shale and Marcellus shale. In particular we explore the additive effect of stepped velocity on fracture permeability evolution relative to the background permeability driven at constant velocity. Fracture permeability decreases at larger slip displacement at constant velocity presumably due to asperity degradation and clay swelling. Sudden up-steps in slip velocity temporarily enhance fracture permeability as a result of shear dilation on hard minerals, but permeability net decreases with increasing slip displacement as wear products fill the pore space. Fracture surface roughness is the link between the fracture permeability and friction coefficient, which are both influenced by mineralogical composition. The fractures and sheared-off particles in the tectosilicate-rich and carbonate-rich shales dilate to increase fracture permeability, whereas asperity comminution readily occurs in the phyllosilicate-rich shale to reduce fracture permeability. The results potentially improve our ability to facilitate shale gas extraction and to mitigate the associated seismic risks

    Permeable rock matrix sealed with microbially-induced calcium carbonate precipitation : Evolutions of mechanical behaviors and associated microstructure

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    Microbially-induced calcium carbonate precipitation (MICP) is a promising grouting material for subsurface remediation due to its water-like viscosity and excellent penetration. Current studies of MICP-grouting for subsurface remediation of both rock fractures and highly-permeable rock matrix focus on the spatio-temporal distribution of precipitated bio-CaCO3 and the resulting reduction in permeability. Conversely, we focus on the improvement of mechanical response following MICP-grouting. We contrast the improved mechanical response of MICP-treated Berea sandstones with distinctly contrasting initial mechanical properties - contrasting associated pre- and post-treatment microstructures with various durations of MICP-grouting. Results indicate that although the precipitated CaCO3 mass with time within these two rock types is similar, significant differences exist in the evolution of mechanical properties (UCS, Young's modulus and brittleness). The evolution of mechanical properties for the low-strength sandstone (initial UCS 25.7 MPa) exhibits three contrasting phases: an initial slow increase, followed by a rapid-increase and then saturation and asympotic response. After ten cycles of MICP-grouting, UCS, elastic modulus and brittleness index for low-strength sandstone increase by 229%, 179% and 177% compared with before grouting. In contrast, the mechanical properties for the high-strength sandstone (initial UCS 65.1 MPa) are not significantly enhanced, increasing UCS by only 22%, 14% and 12%. Imaging by scanning electron microscopy (SEM) indicates that the cementing minerals fill the quartz framework for the high-strength sandstone but are sparse for the low-strength sandstone. Sandstone is a elastic sedimentary rock consisting of a framework of quartz grains bonded by cementing minerals. For the high-strength sandstone infused with a large mass of cementing minerals, the calcium carbonate crystals only precipitate in the gaps between the cementing minerals or adhere to the cementing minerals. This is only capable of relatively limited enhancement in the bio-bonding strength and volume of the quartz framework. For the low-strength sandstone with fewer cementing minerals, the precipitated calcium carbonate is evenly distributed on the surfaces of the quartz gains. The bulk strength is progressively increased with the ongoing bio-cementation between quartz gains. Cementing mineral contents not only exert a considerable control on the integral mechanical properties and penetration for the sandstone, but also have a direct influence on the microscopic distribution of bio-accumulated CaCO3, controlling the effectiveness of bio-cementation by incrementing the mechanical properties

    Hydraulic stimulation strategies in enhanced geothermal systems (EGS) : a review

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    In enhanced geothermal systems (EGS), the natural permeability of deep rocks is normally not high enough and needs to be increased. Permeability increase can be achieved through various stimulation methods, such as hydraulic, chemical, and thermal stimulation. Among these, hydraulic stimulation is the most commonly used technique to increase both reservoir permeability and the specific area for heat exchange. A comprehensive understanding of the underlying processes towards an optimization of hydraulic stimulation performance while minimizing the potential of unwanted induced seismicity is a critical prerequisite for a successful development of any EGS site. In this paper, we review the hydraulic stimulation strategies that have been developed and implemented for EGS. We begin with a description of the underlying mechanisms through which the permeability and heat exchange area increases are achieved. We then discuss the mechanisms of fluid injection-induced seismicity during and after a hydraulic stimulation operation. After that, alternative hydraulic stimulation strategies, namely conventional hydraulic stimulation, multi-stage fracturing, and cyclic soft stimulation, are reviewed based on current research in theoretical studies as well as, laboratory, and in-situ field experiments. Finally, some representative EGS projects are reviewed, focusing on fluid injection strategies, seismic responses, and reservoir permeability enhancement performance. The review shows the importance and need of (a) a comprehensive geological characterization of the natural fracture system including the nearby fault zones as well as the in-situ stress conditions, prior to the development of the site, (b) a proper design of the well arrangement, such as the positioning of the injection and production wells, and (c) the selection of an appropriate fluid injection strategy for the system at hand

    Mechanical-Chemical-Mineralogical Controls on Permeability Evolution of Shale Fractures

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    We report experimental observations of permeation of CO2-rich aqueous fluids of varied acidic potential (pH) on three different shales to investigate mechanical, chemical, and mineralogical effects on fracture permeability evolution. Surface profilometry and SEM-EDS (scanning electron microscopy with energy-dispersive X-ray spectroscopy) methods are employed to quantify the evolution in both roughness on and chemical constituents within the fracture surface. Results indicate that, after 12 hours of fluid flow, fracture effective hydraulic apertures evolve distinctly under different combinations of shale mineralogy, effective stress, and fluid acidity. The evolution of roughness and transformation of chemical elements on the fracture surface are in accordance with the evolution of permeability. The experimental observations imply that (1) CO2-rich aqueous fluids have significant impact on the evolution of fracture permeability and may influence (and increase) shale gas production; (2) shale mineralogy, especially calcite mineral, decides the chemical reaction and permeability increasing when CO2-rich aqueous fluids flow through fractures by free-face dissolution effect; (3) clay mineral swelling reduces fracture aperture and additively calcite pressure solution removes the bridging asperities, which are the main reasons for fracture permeability decrease; (4) competition roles among clay mineral swelling, mineral pressure solution, and free-face dissolution determine how fracture permeability changes. Furthermore, a multiple parameter model is built to analyze effective hydraulic aperture evolution in considering above three mechanisms, which provide a reference to forecast fracture permeability evolution in shale formations

    Mechanical Responses of a Deeply Buried Granite Exposed to Multilevel Uniaxial and Triaxial Cyclic Stresses : Insights into Deformation Behavior, Energy Dissipation, and Hysteresis

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    This article presents the results for cyclic uni/triaxial tests on the deeply seated granite samples drilled from a -915m deep tunnel in Sanshandao (SSD) gold mine. The monotonic and cyclic tests were carried out to observe the mechanical responses of the granite samples under different loading regimes. The disparities concerning the strain evolution and compressive strength of granite samples considering monotonic and cyclic uniaxial and triaxial loading are presented. Deformation behaviour, dissipated energy, and hysteresis are documented and evaluated. Quantitative correlations between strain evolution and cyclic stress levels are revealed. The amount of energy transformation during uniaxial and triaxial cyclic loading is determined. The impacts of confining pressure level on ultimate strain, energy dissipation, and stress-strain phase shift are presented. The mechanical responses of the granite samples subjected to different stress paths and loading strategies are summarised, and corresponding interpretations are given to clarify the differences of mechanical behaviour encountered in distinct loading methods

    Fracture Propagation and Morphology Due to Non-Aqueous Fracturing: Competing Roles between Fluid Characteristics and In Situ Stress State

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    Non-aqueous or gaseous stimulants are alternative working fluids to water for hydraulic fracturing in shale reservoirs, which offer advantages including conserving water, avoiding clay swelling and decreasing formation damage. Hence, it is crucial to understand fluid-driven fracture propagation and morphology in shale formations. In this research, we conduct fracturing experiments on shale samples with water, liquid carbon dioxide, and supercritical carbon dioxide to explore the effect of fluid characteristics and in situ stress on fracture propagation and morphology. Moreover, a numerical model that couples rock property heterogeneity, micro-scale damage and fluid flow was built to compare with experimental observations. Our results indicate that the competing roles between fluid viscosity and in situ stress determine fluid-driven fracture propagation and morphology during the fracturing process. From the macroscopic aspect, fluid-driven fractures propagate to the direction of maximum horizontal stress direction. From the microscopic aspect, low viscosity fluid easily penetrates into pore throats and creates branches and secondary fractures, which may deflect the main fracture and eventually form the fracture networks. Our results provide a new understanding of fluid-driven fracture propagation, which is beneficial to fracturing fluid selection and fracturing strategy optimization for shale gas hydraulic fracturing operations

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