高压脉冲放电激励超临界CO_(2)压力动态响应分析

Pressure Dynamic Response of Supercritical CO_(2) under High Voltage Pulse Discharge Excitation

我国煤层渗透率低、瓦斯压力高、含量大,原始煤层瓦斯抽采困难。为提高煤层瓦斯抽采率、缩短预抽时间,必须实施人工增透;而众多压裂增透技术中,超临界CO_(2)压裂兼具压裂增透和驱替置换的双重瓦斯强化抽采作用,是当前低渗透煤层压裂改造方法的一个研究热点。为探索高压脉冲放电激励CO_(2)(L-CO_(2))压裂增透技术压裂机理,采用自主开发的高压脉冲放电激励超临界CO_(2)试验装置,对基于高压脉冲放电激励液态CO_(2)相变过程的电、热、力多场耦合的复杂过程进行定量研究,确定高压脉冲放电能量对液态CO_(2)转化超临界态压力动态响应规律。通过Span&Wagner CO_(2)状态方程,对液态CO_(2)→超临界CO_(2)气化需要的能量进行了分析与计算,得出在反应釜内放电能量达到20 kJ、40 kJ、50 kJ工况下超临界CO_(2)压力将分别达到11 MPa、18 MPa、26 MPa。通过改变起爆电压等级分别为1500 V,2000 V,2500 V实现三个等级放电能量实验,放电能量分别为20 kJ、40 kJ、50 kJ三种工况下对反应釜内超临界CO_(2)压力动态响应压力进行监测,获得压力与时间关系曲线,分析了压力曲线变化规律。研究表明:不同放电能量下,反应釜内液态CO_(2)气化程度是不同的,随着放电能量的增加反应釜内超临界CO_(2)压力随之增大。研究成果对实现高压放电激励超临界CO_(2)持续压裂煤层增透技术应用提供了一定的借鉴意义。

Coal seams in China have many features,such as low permeability,high gas pressure and large gas content,so it is extremely difficult to extract gas from the original coal seam.The permeability has to be increased in order to improve the efficiency of extracting gas and shorten the time of extracting gas in advance.Among many technologies of increasing the permeability by cracking coal rocks,liquid CO_(2) can not only contribute to the increase of permeability,but also substitute for gas,which can improve the efficiency of extracting gas significantly.In order to explore the application and effect of the L-CO_(2) fracturing technology in high gassy and low permeability coal seams,a self-developed high voltage pulse discharge excitation supercritical CO_(2) test device is used.The complex process of electric,thermal and force coupling in liquid CO_(2) phase transformation induced by high voltage pulse discharge is quantitatively studied,and the dynamic response law in the supercritical state during the process is determined.According to the Span&Wagner equation state of CO_(2),the energy needed for liquid CO_(2) to supercritical CO_(2) gasification is analyzed and calculated,and the results show that the pressure of the supercritical CO_(2) will reach 11 MPa,18 MPa and 26 MPa respectively under the condition that the discharge energy in the reactor reaches 20 kJ,40 kJ and 60 kJ.The pressure of supercritical CO_(2) in the reactor is monitored by adjusting the initiation voltage level to 1500 V,2000 V and 2500 V respectively,and discharge energy of 20 kJ,40 kJ and 50 kJ respectively.The relation curve between pressure and time is then obtained,and the changing rule of the pressure curve is analyzed.The results show that the degree of liquid CO_(2) gasification is different under varying amounts of discharge energy,and the supercritical CO_(2) pressure in the reactor increases with the increase of discharge energy.The research results provide a reference for the application of anti-reflection technology of continuous and repeated cracking of coal seam under high pressure discharge excitation of supercritical CO_(2).