The occurrence state of moisture in coal and its influence model on pore seepage

Moisture is one of the most important factors that influences coal seepage and coal-bed methane (CBM) extraction. To obtain the water occurrence state and dynamic processes of water change in coal, a series of microscopic observation experiments of Wei Jiagou coal by using field-emission environmental scanning-electron microscopy (ESEM) was conducted under the condition of a fixed point. Afterwards, a mathematical model to explain the influence of water on porosity and permeability was proposed based on the ESEM observations. It was found that there were three main types of water occurrence state: a crescent shape, a full filled shape and an annulus shape, which can provide powerful evidence to explain the influence of water on porosity and starting pressure gradient. As well as this, the box counting reached a minimum at a chamber pressure of 520 Pa and the box counting reduced after water wetting. Based on the mathematical model analysis, the water-occupied area of crescent shapes would reach a peak value with an increase of the contact angle, which has a critical impact on the effective porosity. The influence model that we built matched well with experimental data, which in turn demonstrated the validity of the mathematical model. The prominent combined effect of strain and water saturation appeared on the ridge of the permeability contour, while strains have little influence on permeability at a large initial porosity. Furthermore, a model for contact angle and wetting height was proposed and discussed, and contact angles with different improving fluids were tested. It also can be shown that using better wettability improving-fluid can save the cost of volume and have a good performance on the results of hydraulic technology based on model and experimental tests.