Effects of Cyclic Loading on the Pore Structure of Anthracite Coal

In the process of improving coalbed permeability through pulse hydraulic fracturing, the cyclic loading effect influences the characteristics of micropores in coal matrix, thus affecting the process of gas migration. Therefore, it is essential to investigate the effect of cyclic loading on the pore structure of coal. Seven groups of loading tests at different frequencies and amplitudes were conducted on anthracite coal obtained from Shanxi Province, China, using a fatigue-testing machine. Subsequently, using a PoreMaster GT-60 Mercury-intrusion apparatus, the influence of the frequency and amplitude on the structural characteristics (including mercury-injection and mercury-ejection curves, pore size distribution, porosity, and specific surface area) of pores in coal samples was analyzed. Finally, the law and mechanism of action of the loading frequency and amplitude on pores in coal samples were comprehensively analyzed. The test results showed that, in the case of maintaining the sine-wave amplitude unchanged during loading while altering the loading frequency, the overall porosity and pore volume rise at different degrees. The growth of the loading frequency presents a more significant promotive effect on the initiation and development of pores and fractures. Moreover, it drives the transformation of micropores and transition pores into mesopores and macropores, thus increasing the proportion of seepage pores. Under the condition of large sine-wave amplitude during loading, macropores and mesopores are subjected to the repeated action of the external force, thereby reducing the overall porosity. In addition, the volume of the seepage pores declines, and the number of the coalesced pores decreases. Finally, in light of these results, the implications of frequency and amplitude selection in the process of pulse hydraulic fracturing are discussed. Therefore, the results of this research will provide an important theoretical basis for the field application of pulse hydraulic fracturing technology in coal mines.

Interaction between Hydraulic Fracture and Pre-Existing Fracture under Pulse Hydraulic Fracturing

Summary Pulse hydraulic fracturing technology can greatly improve the effect of fracture propagation in rock and form complex fracture networks in reservoirs. The interaction mechanism between hydraulic fractures and pre-existing fractures under pulse hydraulic pressure is unclear. The induced laws of pre-existing fractures on the propagation direction of hydraulic fractures under different pulse frequencies and pulse hydraulic pressures are revealed in this work. We have carried out traditional hydraulic fracturing (THF) tests and pulse hydraulic fracturing tests with rock-like specimens. We compared the interaction between hydraulic fractures and pre-existing fractures in the two hydraulic fracturing tests. Acoustic emission (AE) characteristics of the interaction between hydraulic fractures and pre-existing fractures during pulse hydraulic fracturing are analyzed. The results show that pre-existing fractures in the rock-like specimen can induce the direction of propagation of hydraulic fractures. The influence of pre-existing fracture tips on hydraulic fracture propagation is greater with low pulse frequencies than with traditional hydraulic pressures and high pulse frequencies. When the pulse frequency is 1 Hz, hydraulic fractures are easily induced by pre-existing fracture tips. With increasing pulse frequency, the hydraulic fracture propagation direction gradually moves away from the pre-existing fracture tips and extends perpendicularly to the direction of the minimum principal stress. Under pulse hydraulic loading, more hydraulic fractures are generated around the wellbore than under THF and extend to the pre-existing fracture, and more hydraulic fractures around the wellbore are created with low-frequency pulse loading than with high-frequency pulse loading. Compared with traditional hydraulic pressures, hydraulic fracture propagation with low pulse frequencies (1 and 3 Hz) is more complex than hydraulic fracture propagation with traditional hydraulic pressures and high pulse frequencies (5 Hz). Under high pulse hydraulic pressure and pulse frequency, hydraulic fractures easily extend along the direction perpendicular to the direction of the minimum principal stress like propagation under traditional hydraulic pressure. The study of the interaction mechanism between hydraulic fractures and natural fractures under pulsating hydraulic pressure can provide a method for the formation of fracture network systems in large-scale fracturing and may improve the fracturing efficiency.

Study and Application of Pulse Hydraulic Fracturing for Tight Oil Reservoir

Pulse hydraulic fracturing is a promising stimulation technology to enhance the effectively permeability of coal seams. The fundamental of pulse hydraulic fracturing is that fracturing fluids with a certain frequency are injected into coal, resulting in the rupture of coal and forming a well-distributed fracture network due to the pulse loading. Better effects of gas extraction using pulse hydraulic fracturing had been gotten compared with that of hydraulic fracturing. Accordingly, how to apply pulse hydraulic fracturing technology to improve the fracturing effect of tight and shale reservoirs is a question worth thinking about, although this is very challenging due to the totally different downhole operating conditions. In this paper, experimental apparatus for fatigue damage of quasi-triaxial rock under alternating loads was established. The maximum injection pressure is 50MPa, while the pulse pressure amplitude is greater than 5MPa, and the pulse frequency is adjustable from 0 to 50Hz. Rock failure experiments under pulsating load were carried out and the effects of different hydraulic pulse parameters and rock properties on rock damage were studied. Experimental results show that hydraulic pulse has different effects on rock compressive strength and fracture pressure of different properties. With the increase of hydraulic pulse frequency, the influence on rock compressive strength increases firstly and then decreases. With the increase of pulse pressure amplitude, the influence on rock strength increases. With the increase of hydraulic pulse processing time, the influence on rock fracture pressure increases firstly and then tends to stabilize. Hydraulic pulse has the greatest influence on the compressive strength and fracture pressure of He 8 reservoir, followed by Chang 8 and Chang 6 reservoir of Changqing Oilfield in China. Based on the experimental results, hydraulic pulse frequency is preferred to be about 18-20Hz, accordingly, a downhole hydraulic pulse generator is designed and manufactured. The indoor test results show that the generator performance meets the design requirements. Field tests of pulse hydraulic fracturing were carried out in 3 wells in Changqing tight oil reservoir. Encouraging results were obtained, the average construction pressure was reduced obviously and average daily production per well increased significantly compared to adjacent wells.