渗吸效应对页岩气赋存状态的影响规律

Law of imbibition effect on shale gas occurrence state

目前对于渗吸效应改变页岩气赋存状态的定量化认识尚未形成,对页岩储层中压裂液大量滞留所引起的气水动态置换规律也不明确。为此,开展了气水置换实验以模拟水力压裂后近井区域页岩含水状态的变化情况,借助于含氢流体低场核磁共振谱分析技术(1H-NMR)动态监测页岩储层中甲烷的赋存状态,并计算不同赋存状态下的甲烷气量,进而研究了渗吸效应对页岩气赋存状态的影响规律。研究结果表明:①页岩饱和甲烷的过程分为吸附主导阶段和孔隙填充阶段,吸附作用和压力梯度作用在页岩饱和甲烷的过程中同时发挥作用;②页岩饱和甲烷过程前期阶段优先饱和吸附气,游离态甲烷作为外部甲烷转换为吸附态甲烷的中间状态在页岩孔隙中赋存,吸附气达到饱和状态后,甲烷在压力梯度作用下填充页岩孔隙直至孔隙内外压力平衡;③渗吸效应使页岩发生气水置换作用,吸附态甲烷部分解吸为游离态甲烷,吸附气占比降低,渗吸时间达到80 h时吸附气占比由63.58%降低至45.87%,而游离气量增加使页岩孔隙压力升高,同时水分占据部分孔隙体积,压缩游离气赋存空间,部分游离气被排出页岩孔隙,储层含气性降低,页岩样品含气量由渗吸开始前的7.91 mL/g下降至7.34 mL/g;④水力压裂过程中,压裂液的大量滞留使页岩储层近井区域处于富含水状态,渗吸效应引发的气水置换作用和排替作用使得页岩孔隙及井筒等外部空间中甲烷游离气量升高,孔隙压力升高使得地层压力上升,在一定程度上有利于页岩气的开采。

So far,how imbibition effect changes the occurrence state of shale gas has not been quantitatively understood,and the law of gas-water dynamic displacement caused by the retention of abundant fracturing fluid in a shale reservoir has not been determined clearly.In this paper,a gas-water displacement experiment was performed to simulate the change in the water content of shale near the wellbore after hydraulic fracturing.Then,based on the low field nuclear magnetic resonance spectrum analysis technique of hydrogen bearing fluid(1 H-NMR),the occurrence state of methane in shale reservoirs was dynamically monitored and methane volume in different occurrence states was calculated.Finally,the law of imbibition effect on the occurrence state of shale gas were studied.And the following research results were obtained.First,the shale saturated methane process is divided into adsorption-dominant phase and pore-filling phase.And adsorption and pressure gradient play a role simultaneously in the shale saturated methane process.Second,in the early stage of the shale saturated methane process,priority is given to saturated adsorbed gas,and free methane exists in shale pores as an intermediate state in which the external methane is converted to the adsorbed methane.After the adsorbed gas reaches the saturated state,methane fills shale pores under the pressure gradient until the pressure inside and outside the pores is balanced.Third,imbibition effect leads tothe occurrence of gas-water displacement in shale.The adsorbed methane is partially desorbed into free methane,and the proportion of adsorbed gas is reduced.After 80 hours’imbibition,the proportion of adsorbed gas is reduced from 63.58%to 45.87%.The increase of free gas volume results in the rise of shale pore pressure.And at the same time,water occupies partial pore volume to compress the storage space of free gas and expel some free gas out of shale pores.In this way,the gas bearing property of the reservoir is deteriorated,and the gas content of the shale sample is reduced to 7.34 mL/g from 7.91 mL/g which is the value before the beginning of imbibition.Fourth,the retention of abundant fracturing fluid in the process of hydraulic fracturing makes the shale reservoir near the wellbore in the state of rich water,the gas-water displacement induced by imbibition effect increases the volume of free methane in the external space(such as shale pore and wellbore),and the increase of pore pressure leads to the rise of the formation pressure,which is beneficial to the exploitation of shale gas to a certain extent.