Study On Creep Mechanics Characteristics And Its Influence On Shaft Wall Stability In

Coal bed gas is a kind of clean energy, my country is rich in coal bed gas reserves, exploitation and utilization of coal bed gas resources can not only restraint mine gas disasters effectively and reduce the greenhouse effect by gas, but also optimize the energy structure alleviate energy shortage in our country. However, due to the special mechanical properties of coal and the complex geological environment characteristics of the coal bed gas reservoir, the stability of the wall of the coal bed gas well has seriously restricted the exploration and development of coal bed gas resources in China. The hole shrinkage, sticking of tool, which caused by the creep of coal seam gas well, and even the collapse of borehole is particularly prominent. The laboratory rock physical mechanics experiment, the creep model theory and numerical simulation methods are used in this article, the physical and mechanical properties and the creep mechanical properties of the coal seam gas reservoir are studied systematically and deeply. The model of coal rock elastic-viscous-plastic creep and the creep model based on energy angle are established. At the same time, it is compared with the existing rock creep model and find the most suitable one to describe the creep characteristics of coal rock. Finally, using the COMSOL numerical simulation software to analyze the influence of elastic modulus, Poisson’s ratio, in-situ stress distribution and well diameter of coal bed methane gas well. The main conclusions are as follows:(1) The physical properties such as dry density, natural density, water content, specific surface area, total pore volume and average pore size of coal and rock are obtained by laboratory tests. The analysis shows that the total pore volume of coal and rock is related to specific surface area, and both of them have a linear positive correlation.(2) The experiments of coal rock uniaxial creep show that when the axial stress increases from 0.98 mpa to 5.98 MPa, coal sample test creep process of performance attenuation creep; when the axial stress is 6.49 MPa,coal test creep process performance for non-attenuation creep, coal and rock of the long-term strength between the 5.98~ 6.49 MPa.(3) According to the experimental results of the creep of coal rock, not only improve the original model, but also obtain the creep mechanical model which based on the elastic viscous plastic behavior of coal and derive the constitutive equation and creep equation. The improved model can reflect the two exponential decay characteristics of the creep curves of coal and rock.(4) Based on the energy angle, the coal rock creep mechanics model is proposed. The model is based on the original model, which is connected in series with a delta element. The element is characterized by strain and time in the case of t3. And the constitutive equation and creep equation of the coal rock creep model are derived based on the energy angle.(5)The direction of maximum creep deformation is the maximum horizontal stress direction in the coal bed gas reservoir, borehole deformation shows the maximum principal stress direction, which is caused by the compression of the hole shrinkage. With the decrease of the elastic modulus, the creep deformation of the coal bed gas well wall increases gradually; When the Poisson’s ratio is less than 0.25, the creep deformation is not obvious, and the creep deformation of the wall rock is significantly increased with the increase of Poisson’s ratio; With the increase of the well diameter, the creep of the borehole wall is gradually increasing; With the increase of the maximum horizontal stress, the creep deformation of the coal bed gas well wall is gradually increasing.According to the coal rock creep mechanics model, a numerical simulation method is established to simulate the creep law of coal bed gas borehole, which helps to realize coal bed gas borehole instability mechanism through the coal rock creep and have important practical significance to the safe drilling of coal bed gas and the effective development of well completion and cementing.