Natural Gas Industry B (Dec 2020)
Law of imbibition effect on shale gas occurrence state
Abstract
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 (1H-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 was studied. And the following research results were obtained. First, the process of shale saturation with methane is divided into adsorption-dominant phase and pore-filling phase. And adsorption and pressure gradient play a role simultaneously in the process of shale saturation with methane. Second, in the early stage of the process of shale saturation with methane, 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 to the 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 h' 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.
