| 1. |
- Fang, Zhou, et al.
(författare)
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Forecasting the occurrence of injection-induced heterogeneous slip on rock fractures
- 2023
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Ingår i: Engineering Geology. - : Elsevier BV. - 0013-7952 .- 1872-6917. ; 325
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Tidskriftsartikel (refereegranskat)abstract
- Forecasting the slip behavior of a non-uniformly pressurized, heterogeneous creeping rock fracture, either aseismic creep or dynamic slip, is challenging based solely on laboratory and field measurements. Here we reported a simple, robust method to determine whether an aseismic creep is maintained or transitions to a dynamic slip during fluid injection. We reproduced the non-uniformly distributed fluid pressure and the resulting heterogenous aseismic creep on a critically stressed fracture and revealed the ratio of shear stress and frictional resistance corresponding to the fluid pressure front reaching or exceeding unity at the occurrence of dynamic slip. The determination of frictional resistance is based on the Mohr-Coulomb failure criterion with fluid pressure and friction coefficient on discrete segments of the fracture, and the length of fluid pressure front is calculated from hydraulic diffusivity and elapsed time. We used the experimental results of 9 shale fractures and 3 granite fractures to verify this method. We can also observe how the fluid pressure front propagates until the ratio of shear stress and frictional resistance approaches unity or is constrained with the ratio far below unity. This method has the potential for rapidly forecasting the injection-induced slip on a low-permeability rock fracture and simply characterizing the slip behavior of a natural, large-scale fracture during fluid injection.
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| 2. |
- Jia, Yunzhong, et al.
(författare)
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Hydraulic stimulation strategies in enhanced geothermal systems (EGS) : a review
- 2022
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Ingår i: Geomechanics and Geophysics for Geo-Energy and Geo-Resources. - : Springer Nature. - 2363-8419 .- 2363-8427. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Jia, Yunzhong, et al.
(författare)
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Laboratory characterization of cyclic hydraulic fracturing for deep shale application in Southwest China
- 2021
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Ingår i: International Journal of Rock Mechanics And Mining Sciences. - : Elsevier. - 1365-1609 .- 1873-4545. ; 148
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Tidskriftsartikel (refereegranskat)abstract
- As one of the most promising shale gas reservoirs in China, the Lower Silurian Longmaxi formation in the Sichuan Basin has produced a large amount of shale gas during the last few years. However, two significant concerns have been raised during the contemporary shale gas development in this area: dramatic increases in well depths and potential induced seismicity. The extreme depths have resulted in higher in-situ stress and reservoir breakdown pressures. The other concern is whether recent earthquakes are related to hydraulic fracturing operations since they are close in space and time. An advanced fluid injection protocol-cyclic fluid injection has been proposed and used to lower the breakdown pressure of reservoirs and mitigate the potential seismic risks caused by hydraulic stimulations. In this study, we combine a series of laboratory experiments and a numerical simulation model to investigate the breakdown mechanism, seismic risks, permeability enhancement performance of cyclic fluid injection. The results indicate that the cyclic injection method decreases the shale breakdown pressure by similar to 25% compared with the conventional monotonic rate injection, which is the result of fatigue failure near the fluid injection borehole induced by pore pressure fluctuations lagging behind the periodical injection pressure changes. Even though more acoustic emission events are observed during and post-the cyclic injection, the maximum acoustic emission amplitude decreases by 26%, which is equivalent to transforming AE events with large amplitudes to more AE events with small amplitudes. Moreover, fracture morphology and permeability measurement results indicate that cyclic injection creates hydraulic fractures with higher fracture tortuosity but smaller aperture and lower permeability, which contributes to the fatigue mechanism of rock breakdown. Our experimental results and theoretical analysis initially validate the potential of using cyclic injection methods to perform hydraulic fracturing stimulation in shale reservoirs to lower breakdown pressure and mitigate seismic risks.
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| 4. |
- Jia, Yunzhong, et al.
(författare)
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Laboratory geomechanical and petrophysical characterization of Longmaxi shale properties in Lower Silurian Formation, China
- 2021
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Ingår i: Marine and Petroleum Geology. - : Elsevier. - 0264-8172 .- 1873-4073. ; 124
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Tidskriftsartikel (refereegranskat)abstract
- The Lower Silurian Longmaxi formation is one of the most promising shale gas reservoirs in China. A comprehensive understanding of the shale geomechanical and petrophysical properties is crucial for the successful exploration and extraction of shale gas. We select four representative locations to acquire Longmaxi formation shale samples for the laboratory experiments, to investigate the geomechanical and petrophysical properties through a series of X-ray diffraction (XRD), scanning electron microscope (SEM), uniaxial compression, triaxial compression, tensile strength, and fracture toughness measurements. Laboratory results indicate that: (1) The quartz is the dominant mineral, and phyllosilicate mineral contents vary largelly from 7.30% to 47.80% in Longmaxi shale, which enables a higher brittleness index and fracbility. SEM results show that the high gas storage potential and well micro-fractures development of Longmaxi shale rocks. (2) The phyllosilicate content is vital in determining the uniaxial compressive strength, triaxial strength and elastic properties due to its weaker mechanical properties than tectosilicate minerals; (3) Fracture toughness of Longmaxi shale are relatively higher than shale formations in the USA, which indicate a higher potential to form fracture networks during hydraulic fracturing operations. (4) The anisotropy affects Longmaxi shale mechanical properities extensively due to the high-density bedding planes, which may further influence the fracture network formation during hydraulic fracturing operations. Our results revealed significant non-linear mechanical response as a consequence of shale fabric and mineralogy, which provides necessary information for the in-situ hydraulic fracturing and wellbore stability application during shale gas development in Longmaxi shale formation.
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| 5. |
- Jia, Yunzhong, et al.
(författare)
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The effect of fluid pressure on frictional stability transition from velocity strengthening to velocity weakening and critical slip distance evolution in shale reservoirs
- 2021
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Ingår i: Geomechanics and Geophysics for Geo-Energy and Geo-Resources. - : Springer Nature. - 2363-8419 .- 2363-8427. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- Seismic activities have been reported by the large-scale fluid injection in shale reservoirs both during hydraulic fracturing operations and wastewater disposal processes. Fluid overpressure has been regarded as the primary cause for the injection-induced seismicity since the fluid lubricates the fault and decreases the effective normal stress applied to the pre-existing faults. However, how fractures/faults slip after the activation remains unclear. The rate-and-state friction law has been widely used to describe the fracture stability during slip. Hence, we performed a series of velocity-stepping slip experiments under various combinations of fluid pressure and normal stress states with shale samples, which aims to investigate the role of fluid pressure on the rate-dependent parameter (a-b) and critical slip distance (D-c) evolution. We observed the frictional stability transits from velocity strengthening to velocity weakening with the increase of fluid pressure in shale samples. Moreover, the critical slip distance increases dramatically due to the fluid pressure increases, which is the result of the fluid oscillation phenomenon. Through the calculation of critical fracture rheologic stiffness of shale samples under fluid pressure, the results indicated that a higher possibility for fluid injection-induced seismicity with the increase of fluid pressure. Our experimental observations suggest that the fluid pressure can change the frictional stability characteristics of shale fractures and favor the potential seismic slip, which could be a possible mechanism for the fluid injection-induced seismicity, especially in unconventional shale reservoirs.
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| 6. |
- Jia, Yunzhong, et al.
(författare)
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The Frictional Restrengthening and Permeability Evolution of Slipping Shale Fractures During Seismic Cycles
- 2022
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Ingår i: Rock Mechanics and Rock Engineering. - : Springer Nature. - 0723-2632 .- 1434-453X. ; 55:4, s. 1791-1805
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Tidskriftsartikel (refereegranskat)abstract
- The fluid injection-induced seismicity has drawn widespread concern due to the dramatic rise in seismicity rate worldwide, especially recent events associated with hydraulic fracturing operations during shale gas development. The frictional restrengthening is a prerequisite for the seismic cycles and the rate-and-state friction law is commonly used to describe the frictional behaviour of fractures and faults. However, the permeability evolution of faults/fractures during the seismic cycles remain finitely understood. In this study, we perform a series of slide-hold-slide experiments with concurrently permeability measurement to explore the frictional restrengthening, and permeability response to seismic cycles with Longmaxi shale fractures. The results indicate that even though the Longmaxi shale fractures exhibit a lower frictional healing rate than granite fractures, they still have the potential for seismic activities with a relatively lower seismic moment or low-rate creep. Similar to the in-situ observations, the Longmaxi shale fracture permeability gradually decays during the whole seismic cycle. The permeability response due to the reactivation and repose is complicated, which is largely controlled by the fracture slip history and matching conditions. Fracture permeability enhancement due to reactivation results from the shear dilation, mineral particle mobilisation, and the destruction and breaching of the fracture sealing. In contrast, permeability decay mainly results from asperity degradation. These observations highlight that the small-scale fracture surface properties may largely affect the permeability recovery and decay during seismic cycles, which provides a deeper understanding of fracture frictional behaviours and mitigating seismic risks in shale reservoirs.
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| 7. |
- Lu, Yiyu, et al.
(författare)
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Investigating the Mineralogical and Chemical Effects of CO2 Injection on Shale Wettability at Different Reservoir Temperatures and Pressures
- 2021
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 35:18, s. 14838-14851
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Tidskriftsartikel (refereegranskat)abstract
- The wettability of shale, which determines the success of carbon dioxide-enhanced shale gas recovery (CO2-ESGR) and the safety of CO2 geo-storage, is influenced due to shale-CO2 chemical interactions. To date, the wettability alteration mechanism of shale by its surficial chemical groups is not well understood. In this study, the variations in the wettability, mineralogy, and infrared spectrum of shale after interaction with CO2 at different temperatures and pressures were studied using the sessile drop method, X-ray diffraction, and Fourier transform infrared spectroscopy analysis. The effect of the contents of minerals and chemical groups on shale wettability was investigated by correlation analysis. It is found that the pressure of CO2 has a larger influence on shale wettability than the temperature, whereas the temperature has a greater effect on the infrared spectra of shale. Quartz has the greatest effect on wettability followed by carbonate and clay minerals. Quartz is originally water-wet, and the negative correlation between the relative quartz content and the wettability of shale implies the deterioration of the quartz hydrophilicity after CO2 treatment, and the same applies to the oxygen-containing groups. These findings indicate the interactions among functional groups and their redistribution on the surface during shale-CO2 interactions. Si-O groups are derived more from the hydrophobic Si-O-Si groups than hydrophilic Si-OH groups after CO, treatment, reducing the contribution of quartz to shale hydrophilicity. The weakened absorption ability of CO32- ions to water may be caused by the rotation and redistribution of the CO32- groups on the surface after the CO2 treatment. A longer aliphatic hydrocarbon length reduces the shale wettability. Among the hydrophilic -OH groups, free -OH has the largest influence on shale wettability followed by -OH-O, -OH-OH, and -OH-Pi. This study has important theoretical significance for understanding the wettability alteration mechanism of shale.
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| 8. |
- Lu, Zhaohui, et al.
(författare)
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Microseismic Monitoring of Hydraulic Fracture Propagation and Seismic Risks in Shale Reservoir with a Steep Dip Angle
- 2022
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Ingår i: Natural Resources Research. - : Springer Nature. - 1520-7439 .- 1573-8981. ; 31:5, s. 2973-2993
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Tidskriftsartikel (refereegranskat)abstract
- Hydraulic fracturing is an essential technique to increase reservoir permeability and enhance the production of shale gas. When the dip angle is steep and geological condition is complex, hydraulic fractures may behave complexly, and research on this topic is critical for the shale gas industry. This paper reports a case study of hydraulic fracturing in a shale reservoir with a steep dip angle. We monitored pump data, including the injection rate and fluid pressure. Microseismic monitoring was also used to record the seismic events and monitor the hydraulic fracture propagation. Our results validated that microseismic monitoring is a feasible technique to monitor the hydraulic fracture propagation in shale reservoirs with steep dip angles. Moreover, the variation in depth of shale reservoir induces significant alternation of local in situ stress states, in which cases the fracture propagation pathway is more complex, and where microseismic monitoring is necessary to acquire the hydraulic fracture distribution. Besides, all sound sources, including quarries and rivers, should be eliminated during microseismic station arrangement to improve accuracy of microseismic signals. Moreover, the relationship between the maximum magnitude of seismic event and fluid injection volume was validated further in this study. Finally, unexpected faults and aquifers may affect hydraulic fracture propagation due to the steep dip angle of the target shale reservoir. Thus, a comprehensive geological survey is essential for better hydraulic fracturing design. Our results provide first-hand in situ hydraulic fracturing data and provide important implications for shale gas development, especially for those shale reservoirs with steep dip angles.
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| 9. |
- Mei, Cheng, et al.
(författare)
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Experimental evidence for multiple controls on fault stability and rupture dynamics
- 2022
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Ingår i: Earth and Planetary Science Letters. - : Elsevier. - 0012-821X .- 1385-013X. ; 577
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Song, Chenpeng, et al.
(författare)
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Permeable rock matrix sealed with microbially-induced calcium carbonate precipitation : Evolutions of mechanical behaviors and associated microstructure
- 2022
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Ingår i: Engineering Geology. - : Elsevier. - 0013-7952 .- 1872-6917. ; 304
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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