Control of effective stress on permeability in high-rank coal reservoirs

In order to discuss the anisotropy of stress sensitivity of permeability in the high rank coal reservoir, five high-rank coal samples were measured under overburden pressure to reveal the control mechanism of effective stress. The coal mass has the highest permeability in the direction parallel to the face cleat and bedding plane by using 3.5 MPa to simulate the original formation pressure, while it has the lowest permeability in the direction perpendicular to the bedding planes. The coal permeability declines(or increase) in power function with the increase(or decline) of effective stress. The permeability damage/loss anisotropy indicates that in the direction parallel to the face cleat, coal mass has the highest stress sensitivity, and the stress sensitivity in different directions is controlled by the width of the crack and the direction of its distribution. High rank coal is of high density, poor development of pore and fracture, and it is very difficult to compress, the anisotropy of cleat compressibility is not obvious. With the increase of effective stress the cleat compressibility showed a decreasing trend. The essence of effective stress on permeability is that the reduction of the coal reservoir pore fracture volume results in the reduction of permeability and leads to larger irreversible damage of permeability in all directions.