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本文描述了采用瞬时流动机理测定煤-甲烷系统的扩散系数(D)的一项新技术,并且检验了这项技术对那些随着流量-压力和气体浓度连续变化而变化的系数的依赖性。尽管该技术的开发主要是针对煤层气储层和采空区中的煤,由于利用了逆扩散原理,它也可应用于在煤中注入了第二种气体的情况。结果表明,D值随着煤中甲烷平均浓度的降低而连续减小。D值的对数与压力大小成正比。D值的减小可能是由两个效应引起的。第一是流动机理有可能随着煤中甲烷浓度不断下降而变,甲烷浓度下降是由于煤中孔径变化造成的。另一个效应是由气体解吸引起的固体煤基质的体积应变,即所谓“收缩效应”。这种基质收缩可能导致孔径缩小,因而,D值减小。
This paper describes a new technique for determining the diffusion coefficient (D) of a coal-methane system using an instantaneous flow mechanism and examines the dependence of this technique on those coefficients that vary with the continuous change in flow-pressure and gas concentration. Although this technology has been developed mainly for coal in coalbed methane reservoirs and goafs, it can also be applied to the case where a second gas is injected into coal due to the use of the principle of inverse diffusion. The results show that the value of D decreases continuously with the decrease of average methane concentration in coal. The logarithm of the D value is proportional to the pressure magnitude. The decrease in D value may be caused by two effects. The first is that the flow mechanism is likely to change as the concentration of methane in the coal decreases. The decrease in methane concentration is due to the change in pore size in the coal. Another effect is the volumetric strain of the solid coal matrix caused by gas desorption, the so-called “shrinkage effect”. This contraction of the matrix may cause the pore size to decrease, and thus, the value of D decreases.