Visualizing experimental investigation on gas-liquid replacements in a microcleat model using the reconstruction method

Cleats are the main channels for fluid transport in coal reservoirs. However, the microscale flow characteristics of both gas and water phases in primary cleats have not been fully studied as yet. Accordingly, the local morphological features of the cleat were determined using image processing technology and a transparent cleat structure model was constructed by microfluidic lithography using the multiphase fluid visualization test system. Besides, the effect of microchannel tortuosity characteristics on two-phase flow was analyzed in this study. The results are as follows: (1) The local width of the original cleat structure of coal was strongly nonhomogeneous. The cleats showed contraction and expansion in the horizontal direction and undulating characteristics in the vertical direction. (2) The transient flow velocity fluctuated due to the structural characteristics of the primary cleat. The water-driven gas interface showed concave and convex instability during flow, whereas the gas-driven water interface presented a relatively stable concave surface. (3) The meniscus advanced in a symmetrical pattern in the flat channel, and the flow stagnated due to the influence of undulation points in a partially curved channel. The flow would continue only when the meniscus surface bypassed the stagnation point and reached a new equilibrium position. (4) Enhanced shearing at the gas-liquid interface increased the gas-injection pressure, which in turn increased residual liquids in wall grooves and liquid films on the wall surface. The visualization of microcleat gas-liquid-drive characteristics can provide flow information that cannot be obtained by conventional laboratory measurements. The tortuous channel leads to fluctuations in the instantaneous flow velocity, the sudden reduction in the local opening of the cleat exacerbates the flow hysteresis, and the continuous change in the opening also leads to unequal and dynamic changes in the contact angle on both sides of the meniscus. The above phenomena actually increase the energy loss in the repulsion process, and the conventional experimental measurements in the laboratory can misrepresent the physics of unsaturated flow. image Geometric characterization and reconstruction of a three-dimensional transparent cleat model were performed. Gas-liquid repulsion characteristics were determined in a visualized microcleat model. Transient flow velocity hysteresis is evident in tortuous channels. Dynamic variation of the contact angle shows diversity. Pressure changes the groove liquid content and liquid film thickness.