The
adsorption capacity of coal with respect to carbon dioxide
(CO2) is greater than that of methane (CH4).
Studying the CO2 replacement of CH4 in coal
is essential to understanding the mechanism behind CO2-enhanced
coalbed methane. In this paper, scanning electron microscopy was used
to qualitatively study the influence of CO2 injection pressure
on the fracture characteristics of coal. The time effect of the CH4/CO2 adsorption and the effect of the CO2 injection on the CH4 adsorption were quantitatively studied
using low-field nuclear magnetic resonance. The results suggest three
different CH4 states in the coal samples: CH4 adsorbed on the pore surface, free CH4 in the pore center,
and free CH4 between the coal particles. Indeed, the greater
the CO2 injection pressure, the more developed the fracturing
and network, which, in turn, increases connectivity. As the adsorption
time of CH4/CO2 increases, the adsorption rate
of CO2/CH4 gradually decreases. In essence,
CO2 preferentially replaces the CH4 of the minipores.
As the CO2 injection pressure increases, the difference
between the CO2 equilibrium pressure and the initial injection
pressure increases. Moreover, the CH4 production rate increases,
the transverse relaxation time (T
2) spectrum
of the adsorption-state area decreases, the T
2 spectrum of the free-state area increases, and the number
of adsorption holes gradually decreases, whereas the number of seepage
holes increases.
For coal and gas outburst and difficult extraction in soft, low-permeability, and high-gas seam, the integration technology of drilling and mechanical cavitation is put forward to relieve pressure and improve permeability of coal seam. Through the test of mechanical parameters of the coal body, the physical model of drilling and cave-making in coal seam is established, and the mechanical characteristics of drilling and mechanical cavitation are simulated numerically. The results show that the peak strength of coal increases linearly with confining pressure. When the drill bit just touches the coal, the maximum stress occurs at the center of the coal sample. With the increase in the drilling depth of the drill bit, the maximum stress point shifts to the depth. When the coal at the center of the sample is broken, the position of the maximum stress shifts to the surrounding. With the increase of drilling depth, the maximum contact number between the drill bit and coal body increases sharply. When the bit body basically enters the coal body, the maximum contact number between the drill bit and coal body remains unchanged. When the drill pipe rotates, the reaming tool collides with the hole wall at the reaming position on the axis. In general, the contact area increases with the opening of the reaming tool, but the contact between the reaming tool and the hole wall is random. As time increases, the contact force begins to increase and then basically stabilizes; at this time the reaming tool has been fully opened.
Coal and gas outbursts mostly occur during mining at geostructural belts. Pre-drainage coalbed methane using hydraulic fracturing is one of the methods to prevent outbursts. However, the coal in geostructural belts is to be soft and crushed with special mechanical properties and pore structure. To explore the feasibility of hydraulic fracturing in geostructural belts, a field investigation on enhanced coalbed methane using hydraulic fracturing with vertical well was conducted at the Yangquan Coalfield, China. This case puts forward a method for the location selection of vertical well in geostructural belts. In addition, a triple-control technology for hydraulic fracturing, which is characterized by pressure control, flow control and sand ratio control of fracturing fluid, is presented. The results show that the average gas production and maximum gas drainage capacity of the test well were 5.67 and 12.88 times than those of the regular well, respectively, achieving good drainage effects.
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