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The structure of caprocks is often greatly altered by different scales of faults or fissures in long-term geological tectonic evolution, and the sealing performance may be deteriorated. In this paper, a simplified geological model characterized as multilayered sandstone and fault-bearing caprocks extracted from the Shuang 6 underground gas storage located in the Liaohe oilfield was established. Different fault geometry (e.g., fault length, fault dip angle, and fault type) and seepage attributes (porosity and permeability) were considered to illustrate their impacts on natural gas migration during the cyclic high rate of injection and production of natural gas. The results showed that the seepage anisotropy and the natural gas front are strongly affected by the formation properties and, especially, are hindered by the low permeability sandstone layers. The difference in the lateral migration distance of natural gas in different layers can reach 110 m at the end of the injection period, with an annual injection volume of 108 m3. The migration of natural gas along the fault zone is mainly controlled by the permeability of faults, followed by fault scale, fault dip angle, and fault type. The sealing failure of caprocks in the fault zone does not occur based on the simulated gas migration distribution, showing that a very limited amount of natural gas migrates into the caprocks. [ABSTRACT FROM AUTHOR]

Gas breakthrough pressure is a key parameter to evaluate the sealing capacity of caprock, and it also plays important roles in safety and capacity of CO2 geological storage. Based on the published experimental results, we present numerical simulations on CO2 breakthrough pressure in unsaturated low‐permeability rock under 9 multiple P‐T conditions (which can keep CO2 in gaseous, liquid and supercritical states) and thus, a numerical method which can be used to accurately predict CO2 breakthrough pressure on rock‐core scale is proposed. The simulation results show that CO2 breakthrough pressure and breakthrough time are exponential correlated with P‐T conditions. Meanwhile, pressure has stronger effects on experimental results than that of temperature. Moreover, we performed sensitivity studies on the pore distribution index λ (0.6, 0.7, 0.8, and 0.9) in van Genuchten‐Muale model. Results show that with the increase of λ, CO2 breakthrough pressure and breakthrough time both show decreasing trends. In other words, the larger the value of λ is, the better the permeability of the caprock is, and the worse the CO2 sealing capacity is. The numerical method established in this study can provide an important reference for the prediction of gas breakthrough pressure on rock‐core scale and for related numerical studies. [ABSTRACT FROM AUTHOR]

In the current international situation, energy storage is an important means for countries to stabilize their energy supply, of which underground storage of natural gas is an important part. Depleted gas reservoir type underground gas storage (UGS) has become the key type of gas storage to be built by virtue of safety and environmental protection and low cost. The multi-cycle high injection and production rate of natural gas in the depleted gas reservoir type UGS will cause the in-situ stress disturbance. The slip risk of fault in the geological system increases greatly compared with that before the construction of the storage engineering, which becomes a great threat to the sealing of the gas storage. Reasonable injection and production strategy depend on the reliable assessment of the shear behavior of the fault belt, which can guarantee the sealing characteristics of the UGS geological system and the efficient operation of the UGS. Therefore, the shear behavior of the fault is studied by carrying out experiments, which can provide important parameters for the evaluation of fault stability. However, there is a large gap between the rock samples used in the previous experimental study and the natural faults, and it is difficult to reflect the shear failure characteristics of natural faults. In this paper, similar fault models based on high-precision three-dimensional scanners and engraving machines, filled with three types of fault gouge, are prepared for a batch of representative direct shear tests. The results show that the peak shear strength of the fault rocks with a shear surface is higher than that of the fault rocks with a tensile surface. Compared with the clay mineral content, the roughness of the fault surface is much more significant for the shear strength of the fault rock. For the fault rocks with similar fault surface morphology, the higher the clay content in the fault gouge, the greater the shear strength of the fault rocks. For the fault rocks with different fault surface morphology and the same fault gouge, the cohesion and internal friction angle of the tensile type is generally smaller than that of the shear type. [ABSTRACT FROM AUTHOR]

Frequent micro-seismic events or even the severely destructive earthquakes can be induced by the subsurface fluid injection or production. In this paper, the induced seismicity related with four types of classical subsurface fluid injection or production engineering projects (i.e., wastewater injection into deep wells, CO2 geological storage, oil and gas exploitation, deep geothermal energy production) was reviewed to understand the position, occurrence time of induced seismicity, range of influence and the trigger mechanism. Furthermore, targeting at the deep geothermal energy production, the determination methods of the maximum earthquake magnitude, and the challenges in reducing the induced earthquake risks were reviewed. China is rich in deep geothermal energy, especially the reserves of hot dry rocks is considerable. Multiple geophysical observation methods, e.g., seismic, electrical and magnetic methods, can be applied to obtain the multi-scale geophysical and geological information. Laboratory experiments and numerical simulations should be combined to obtain a clear understanding on the triggering mechanism of seismicity. Identification technologies of faults or concealed faults can be widely applied to reduce the destructive earthquake risks associated with the production of deep geothermal energy. For the development of high-efficiency connection control technology between injection and production wells, it is possible to develop an integrated reservoir stimulation method, i.e., alternative thermal stimulation of hot and cold water-chemical corrosion-hydraulic fracturing technology. The microseismic monitoring technology should be used to adjust the reservoir stimulation scheme in time in the field. Through the development of a series of enhanced geothermal system (EGS) pilot projects, key exploitation technologies can be obtained to ensure the safety and efficiency of the EGS project in China. [ABSTRACT FROM AUTHOR]