Study on mechanical evolution law of natural gas well casing under repeated mining of coal seam

In the Ordos Basin of China, there exists the problem of mutual disturbance between coal and natural gas resources in overlapping areas. Taking Xinjie No. 1 well as an example,we studied the casing stress evolution characteristics and displacement and bending law of natural gas wells under the repeated mining of coal seams,and the damage and destruction area of the casing and the change of the stress peak value under different coal pillar widths were analysed. A contact surface was set among casing,cement ring and surrounding rock to simulate the contact relationship among the three in actual production,and FLAC3D numerical software was used to simulate the peak stress and bending degree of casing with different widths of protective coal pillar,which provided a new idea for the normal production of casing. The research results show that the casing undergoes the process of offsetting to the left side of the mining area and then back to the centre during the process of coal seam winning, and the bending and deformation at the coal seam and the top and bottom plate is intense,so it should be reinforced in advance and focus on monitoring the damage and destruction of the casing in this area. When mining 2-2 upper coal seam,the overlying strata moves drastically,which leads to the casing peak stress growth rate is larger,and the stress is concentrated in weak strata such as coal seam and the top and bottom plate. The upper coal seam mining has the effect of unloading the 2-2 coal seam,so that the stress rises slowly in the mining of the lower coal seam,and the stress is still concentrated in the coal seam. On the basis of stress distribution and displacement and bending situation of the casing,we analyze the width of protective coal pillar,mainly based on the stress concentration situation of the casing. When the width of protective coal pillar is 300 m,the peak stress of the casing is 287 MPa. When the width of protective coal pillar is 200 m,the peak stress is 312 MPa,which is beyond the yield stress of casing,leading to the casing destruction and affecting the safety production. Setting up the protective coal pillar of about 300 m width can protect the stability of the casing and increase the coal extraction rate.