Coal fines are commonly produced in the form of aggregates
during
the development of coalbed methane (CBM), which significantly restricts
the well productivity. Specifically, the reunion state of coal fines
plays a key role in impacting the conductivity of propping fractures
after fracturing. In this work, the anionic surfactant sodium dodecyl
benzene sulfonate (SDBS) was selected to control the reunion state
of coal fines. Visualized simulations of coal fines’ transport
behavior in proppants with different particle sizes were carried out
under low constant pressure based on sand pack experiments. Moreover,
the characteristics of coal fine filling and production in propping
fractures were obtained. The results indicate that SDBS has a dispersion
effect on coal fine aggregates in the KCl fracturing fluid, with an
average reduction of 36% in particle size. There is a positive correlation
between the proppant size and the permeability coefficient of propping
fractures. Coal fines reduce the permeability coefficients obviously,
which first rapidly decrease and then slowly reach a stable stage.
In the first stage, the changes of permeability coefficients are affected
by the relative particle sizes of the proppant and the coal fine,
which is consistent with the classic 1/3 sealing mechanism. In the
second stage, SDBS increases the amount of coal fines between the
proppant pore throats, and the thickness of the filter cake formed
from coal fines at the entrance of the propping fracture increases,
resulting in the inhibition of fracture conductivity. Additionally,
the output of coal fines is increased dramatically induced by SDBS.
Finally, the inhibitory effect of SDBS on fluid production was verified
by comparing the water production and coal fine content of two field
wells CW # 1 and CW # 2. This study has guiding significance for the
output and control of coal fines in low-pressure production of CBM
wells.
The fracture behavior of coal-seam roof rock in coal mining is a key and controlling factor for the mode optimization of the artificial roof caving. However, the fracture mechanism of roof rock under loading is not clear. In the work, the split Hopkinson pressure bar (SHPB) experiment was carried out using semicircular bending samples from the sandstone of coal-seam roof rock in the Junger mining area of Inner Mongolia at the loading rate of 0.35–3.78 GPa·m0.5·s−1, and the dynamic fracture behavior and energy dissipation mechanism of samples under different loading rates were investigated. The result shows that the dynamic stress–strain process of the hard roof rock includes four stages: linear instantaneous compaction, linear elastic compression, failure, and fracture extension, in which the failure forms changes from brittle fracture to ductile fracture with the increase of loading rate. The mode I fracture toughness increases linearly under confining pressure. In addition, the propagation orientation of induced fractures is parallel to the loading direction, and the gravel in the samples can inhibit fracture extension, resulting in changing the fracture extension path. Further, the energy absorption efficiency of the samples during the fracture process decreases with the increase in the loading rate.
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