纳米孔缝下压裂液对页岩中甲烷运移的影响

Effect of Fracturing Fluid on the Migration of Methane in Shale with Nano-sized Fractures

压裂体积改造以及页岩自身性质都会产生大量纳米级微孔缝,通过模拟计算、实验数据并结合现场数据研究水滞留在页岩中对甲烷解吸运移的影响,通过经验公式计算得出,纳米环境下甲烷以间隙填充和水合溶解方式与水形成水溶气。分子动力学模拟显示水在纳米环境中分子有序度增加,链间氢键力减少,流动阻力小,扩散系数比宏观环境增大2个数量级;纳米通道内水中溶入甲烷后,扩散系数比单纯纳米环境中水大2个数量级,链间氢键力更小,流动几乎无阻力。页岩的毛细渗透可以使水进入纳米孔缝,吸附在页岩表面无法返排,造成返排率低。水进入页岩中竞争吸附置换甲烷,减少甲烷吸附量,有利于产能,但在低含水饱和度环境下,甲烷在水中溶解会造成一定永久残留,预吸水活性炭吸附解吸实验数据显示,当水量与孔容体积相当时,与干活性炭比较,甲烷吸附量减少60%,而溶解等综合因素无法解吸的甲烷残留最高为13.5%,几乎无自由水返出,综合结果水的存在正向贡献大。

Spatial fracture-network generated fracturing and the characteristics of shale itself produce large amount of nano-sized fractures in the shale. The effect of water retained in shale on desorption and migration of methane was studied through simulation, experiment and data from fled operations. Calculation with empirical equations showed that in nano-sized environment, methane flls the nano-sized fractures and is dissolved into water to form a water dissolved gas. Simulation with molecular dynamics showed that in nano-sized environment the degree of order of water molecules is increased, the hydrogen bond force between molecular chains is decreased. Low fow resistance in the nano-sized environment led to a coeffcient of diffusion that was two orders of magnitude greater than that of the macro environment. When methane was dissolved in the water existed in nano-sized channels, the coeffcient of diffusion of water molecules was two orders of magnitude greater than that of the water with no dissolved methane in nano-sized environment, and the hydrogen bond force was much lower. In this condition there is almost no fow resistance. Capillary penetration take place in shale formation drives water into nano-sized fractures, and is therefore adsorbed onto the surfaces of shale. The adsorption of water makes it diffcult for the water to fow back, resulting in very low fowback effciency. Water entering into shale competes with methane for adsorption onto the surfaces of shale and displaces methane, reducing the adsorption quantity of methane and is hence benefcial to increasing production rate. On the other hand, at low water saturation, some small amount of methane dissolved permanently in water. Data from adsorption-desorption experiment with active carbon with pre-absorbed water showed that, when the volume of water is equivalent to the pore volume of the active carbon, the adsorption quantity of methane is 60% less than the adsorption quantity of methane on the surface of dry active carbon, and the amount of residue methane that is unable to desorb because of dissolution in water is 13.5% at most. In this condition there is almost no water fowback. All factors considered, the existence of water is benefcial to hydrocarbon production.