Relative permeabilities of gas and water for different rank coals

Abstract

Characteristics of gas–water two-phase flow through coal seams play crucial roles in water depletion and gas production associated with coalbed methane (CBM) recovery. One of the most important characteristic is the relative permeability of gas and water which is largely dependent on gas/water saturations in coal, varying with coal ranks. For better understanding of the seepage mechanism of the gas–water flow in coal, the relative permeabilities of gas and water in different rank coals selected from south Qinshui Basin have been investigated under various gas/water saturations through water replacement with methane using an unsteady-state method. The results have shown that the ratio of effective methane permeability and absolute permeability is obviously increasing with rank, implying that the gas slippage of high rank coal has more significant effect than the low rank coal. A series of relative permeability curves for selected coals have been obtained. All of these curves show that the selected coals are featured by smaller methane permeabilities and narrow spans of two-phase flow regions and lower relative permeability, and have low methane permeabilities under irreducible water condition as well. The experiments also revealed that the selected coals exhibit high residual water saturation with low relative permeabilities of gas and water. With increasing of the maximal vitrinite reflectance, the irreducible water saturation exhibits a U-shaped tendency whereas the methane permeability under the irreducible water condition generally increases. The irreducible water saturation slightly increases with increasing of vitrinite and weakly decreases as inertinite increases, while the methane permeability under irreducible water condition is negatively related with vitrinite and positively related to inertinite to some extent. The experimental data were further parameterized to correlate the relative permeabilities of methane and water to gas saturation, showing that a correlation of power function can fit the experiments well. As a result, a permeability model incorporated with coal rank and maceral compositions with gas saturation was developed to predict the relative permeabilities of gas (methane) and water in coals.