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Abstract
The hydrated shales under cyclic loading and unloading conditions are common for the shale reservoir development; corresponding mechanical properties and permeability evolution are very significant and should be deeply researched. Firstly, the experiments of the hydrated shales under the above conditions are discussed, showing that the peak strength is lower and corresponding permeability is higher for more days of hydrating treatment. Secondly, the damage theory is proposed to analyze the shale permeability evolution due to hydromechanical damage and get permeability variation under initial loading and unloading conditions, observing that the permeability in the loading process decreases with increasing confining pressure and increases in the unloading process with decreasing confining pressure; however, the former changes much greater than the latter considering the same confining pressure, indicating that the irreversible damage for the hydrated shales in this cyclic condition has resulted in obvious difference of the permeability. Furthermore, the curves between the permeability and confining pressure based on the experimental data are fitted as negative exponential functions under initial loading conditions and power functions under more cyclic loading conditions, showing that more loading process will change the permeability evolution model. However, the permeability while unloading changes smoothly and can be fitted as a power function with the confining pressure. And in addition, the loss ratio and recovery ratio of the permeability have been deeply researched under five cyclic loading and unloading conditions, thoroughly explaining the permeability decreasing variation with more cyclic processes. Finally, the sensitive coefficients of the permeability have been investigated to observe the largest coefficients under initial cyclic conditions and less and less with more cyclic processes, especially the coefficients while loading which are more sensitive to lower confining pressure and smaller while unloading, which is in accordance with the shale permeability loss and recovery variation, revealing the permeability evolution of the hydrated shale under complex extracted environment.
Highlights
China Mineral Resources (2018) has reported that accumulated proven geological reserves of shale gas from 2015 to 2017 are, respectively, 5441, 7643, and 9168 billion cubic meters, and corresponding productions are, respectively, 45, 78.82, and 100 billion cubic meters
The curves k ~ D in Figures 5(a)–5(c) show that the shale permeability in Figure 5(c) with 10-day hydrated days is the greatest under the same loading conditions compared with other shales with less hydrated days; the main reason is concluded that initial damage is the greatest for 10-day hydrated shales resulting in the greatest initial permeability and it was kept the greatest even with the same damage variable, explaining the effect of hydration on the shale permeability
(2) In the primary loading stage, the shale permeability increases with more hydrated days and decreases with increasing confining pressure, indicating that larger confining pressure makes the shale pores and fractures more compacted to form less seepage channels
Summary
China Mineral Resources (2018) has reported that accumulated proven geological reserves of shale gas from 2015 to 2017 are, respectively, 5441, 7643, and 9168 billion cubic meters, and corresponding productions are, respectively, 45, 78.82, and 100 billion cubic meters. The above studies indicate that the hydration effect on the mechanical and permeable properties in laboratory conditions and in situ compression tests should not be ignored, and deep investigations about complex load combination on the hydromechanical behavior of the reservoir shales should be deeply conducted. Sun et al [26] and Yan et al [27] carried out the seepage tests to reveal the strong relationship of shale permeability and real-time damage, providing useful information for research on the permeability characteristics of hydrated shale under cyclic loading and unloading conditions. Taking a reservoir shale as a case study, considering the shale structures and hydration effect, corresponding permeability experiments under cyclic loading and unloading conditions are conducted to investigate the relationship of crack propagation and permeability of reservoir shales. Some models describing the permeability loses, permeability recoveries, and corresponding stress-sensitive coefficients are proposed to indicate the permeability evolution under cyclic loading and unloading conditions, revealing the permeability evolution mechanism of the hydrated shale under complex conditions
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