The Characteristics Of Gas Migration In High Rank Coal Under Reservoir Conditions

Based on the physical properties of coal reservoirs of different ranks and the gas migration mechanism of coal-bed research results, combine the test results of the pore fracture morphology observation and pore structure parameters of different ranks coal, this paper through research of reservoir pores and cranny property, adsorption and desorption characteristics of high-rank coal, the results show that: the micropore content increases gradually with the deepening of coalification degree, but the macropore content displayed contrary trend, the transitional and meso pore content changed slightly; High rank coal pore specific surface area is also greater than low rank coal, but the porosity and average pore radius are letter; The micropore distribute densely and directivity in high rank coal, however, the pore connectivity is worse and the semi-closed pore is in the majority. The change law of pore morphology and structure parameters show the increase process of coalification degree, that is the process of micropore content in coal increased, the macropore content decreased and the pore directionality increased. Further study found that the development is not balanced in the pore fracture system of high rank coal, which the exogenous pore fracture is more developed, endogenous and microstructure part is scarce, and the secondary pore fracture is lack. Adsorbability of high rank coal is greater than the middle and low rank coal as the whole, but increased the difficulty of gas desorption accordingly.This paper use Qinshui basin as an example to explore the evolution law of gas migration, diffusion and seepage in the discharge and mining process of coal bed gas with the conditions of high rank coal reservoirs, and form the gas diffusion and seepage model fit the research area. Study holds that: the gas-bearing property of gas reservoir is good in high rank coal, but relative desorption rate and desorption efficiency is low that difficult to develop; the gas diffusion model of inner pore is a dynamic changing process which changes diffusion model in the same pore size with the change of pressure. Due to the specific pore properties of high rank coal, Knudsen diffusion model is less, Transitional diffusion model throughout the entire discharge and mining process , Fick diffusion model mainly occurs in the beginning period of depressurization, and the diffusion rate in the small pore size is higher than the low rank coal, but the contrary in the large pore size. The development of seepage channels in high rank coal is out of line which seepage rate there is a rapidly growing process, but is still lower than low rank coal in general. In the mining stage it is controlled by the effective stress and self-regulating effect of coal matrix, the permeability of reservoir showed regular changes.