Study On Gas And Water Flow Behavior Of Coal Under In-situ Conditions

Coalbed methane(CBM)as a clean energy,its development can not only make up for the shortage of natural gas supply,but also reduce the hazard of explosion in the process of coal mining.The micropores of coal are the main space of coalbed methane adsorption.The fractures are the main seepage channel of gas and water in coal seam.The seepage of gas and water in coal rock reservoir is an important factor affecting gas well production,and the coal rock reservoir has the characteristics of poor porosity and poor permeability,fragile,strong adsorption and heterogeneity.And the permeability of coal seam is strongly affected by stress sensitivity.Therefore,research on gas and water flow behavior under in-situ conditions of coal seam is of great significance.This paper focuses on the No.15 coal seam in Qinshui Basin,analyzed of coal mineral composition,and the pore-fracture structure are characterized by scanning electron microscopy and nitrogen adsorption.The results show that the coal contains a great number of micro-fractures but poor in connectivity,the coal samples are fragile and the permeability is low.Microporous development and the pore size distribute mainly in the range of 1～10nm,concentrated on 2 nm.The effect of coal adsorption characteristics on coal physical property is analyzed.The adsorption characteristics of balanced coal-water sample on single-component gas was measured using HKY-Ⅱ automatic adsorption gas content test system.The results show that the adsorption capacity on coal is CO2>CH4>N2.Compared with the untreated coal,the wetting contact angle between coal which has the adsorption of CH4 and the simulated formation water is bigger,the swelling amount increases with the adsorption capacity.The adsorption increases the hydrophobicity of the coal.The mechanism of effective stress and pore pressure on the seepage capacity of coal and rock fluid is clarified respectively.With the increase of effective stress,the permeability of gas and liquid phase of coal both decreases gradually.When the effective stress increases to a certain value,the permeability will not change again.When the effective stress is constant,with the increase of pore pressure,the gas permeability of coal is gradually reduced,the stronger the gas adsorption,the more obvious the decrease of permeability.The CH4 and CO2 permeability have a critical value with the pore pressure change.Before the critical value,the expansion of the matrix dominates the change of permeability.After the critical value,the effective stress dominates the change of permeability.The permeability of helium is manifested as increasing with increasing pore pressure during the whole process of pore pressure change.After the adsorption of methane into the coal,the wettability is weakened,the adhesion of the surface of the coal to the liquid is weakened,the fluidity of the liquid is increased and the permeability of the core liquid is increased.The relative permeability curves of Helium-water and CH4-water relative permeability curves were measured by self-developed coal gas-water two-phase seepage test device respectively.The results show that compared with the relative permeability of gas and water in helium test,the relative permeability of gas phase with CH4 test is decreased,the relative permeability of liquid phase is increased and the saturability of irreducible water is reduced.In the study of gas-water flow behavior in coal,the sample should consider using full-diameter cores.The core pretreatment should first saturate the adsorbed gas and then saturate the formation water.During the test,control the pore pressure by reducing the back pressure to simulate the dynamic production condition.