超临界CO_2温变对低渗透煤层孔渗变化的实验研究

Experiment study on the porosity and permeability of low permeability coal by supercritical CO_2 temperature variation

为解决我国高瓦斯煤层渗透性差导致瓦斯抽采率低的难题,利用超临界二氧化碳强扩散和溶解增透等独特优点,采用自制三轴渗透实验装置,开展不同温度下超临界二氧化碳作用后煤的宏观增透实验,在宏观增透实验基础上进行煤微观扫描成像实验。结果表明:恒定体积应力和孔隙压力条件下,不同温度超临界二氧化碳作用后,煤的渗透率较增透前提高一个数量级,但在二氧化碳的超临界温度范围内,煤的渗透率随温度增加呈负指数变化规律。超临界二氧化碳作用后,煤微观孔隙率较增透前提高两个数量级,随着温度增加,煤微观孔裂隙的演化速率减慢,孔隙率随温度增加呈负指数变化规律。宏微观实验数据同时表明,煤宏观渗透率随微观孔隙率增加而增大。超临界二氧化碳增透过程中,孔隙压力对低渗透煤层的增透效果起主控作用。

In order to solve the universal problem of low permeability and low gas recovery in China＇s high gassy seams, combining the strong diffusion, dissolution-enhancement and other unique advantages of super-critical carbon dioxide. The three axis penetration experimental device was self-made, the macro increasing permeability experiment of coal was carried out after the action of super-critical carbon dioxide with different temperatures, the microscopic scanning imaging experiment of coal was conducted based on macro increasing permeability experiment. The results show： after the action of super-critical carbon dioxide with constant volume stress, constant pore pressure and different temperatures, the permeability is increased by one order of magnitude than that before the increasing permeability experiment, but in the super-critical temperature range of carbon dioxide, the permeability of coal shows a negative exponential change with the increase of temperature. After the action of super-critical carbon dioxide, the micro porosity of coal increases by two orders of magnitude than that before the increasing permeability experiment, the evolution rate of micro pore-fissure slows down with the increase of temperature, the porosity presents a negative exponential changing law with the increasing temperature. The macro-micro experimental data also show that the macro-permeability of coal increases with the increase of micro-porosity. During the process of increasing permeability by the super-critical carbon dioxide, pore pressure plays the main control function in increasing the permeability of low permeability coal seam.