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煤层重复采动下天然气井套管力学演化规律研究

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

  • 摘要: 我国鄂尔多斯盆地存在煤与天然气资源重叠区域开采相互扰动问题,以新街一井为例研究煤层重复采动下天然气井套管应力演化特征及位移弯曲规律,分析了不同煤柱宽度下套管损伤破坏区域与应力峰值变化情况。在套管、水泥环、围岩间设置接触面以模拟实际生产中三者的接触关系,利用FLAC3D数值软件模拟不同保护煤柱宽度时套管的应力峰值与弯曲程度,为套管的正常生产提供新的思路。研究结果表明:煤层回采过程中套管经历向左侧采空区偏移再向中心回正的过程,在煤层及顶底板处弯曲变形剧烈,应预先加固并重点监测该区域套管损伤与破坏情况。开采2-2上煤层时覆岩移动剧烈导致套管峰值应力增长速率较大,在煤层及顶底板等软弱岩层层位应力集中,上煤层开采对2-2煤层产生卸压效果使得下煤层开采时应力上升缓慢,应力仍集中于煤层处。基于套管应力分布及位移弯曲情况,分析保护煤柱留设宽度,主要以套管应力集中情况作为判断依据:当保护煤柱宽度为300 m时套管应力峰值为287 MPa;当保护煤柱宽度为200 m时应力峰值为312 MPa,超出套管屈服应力导致套管破坏影响安全生产,留设300 m左右宽度保护煤柱能够保护套管的稳定性,提高煤炭的采出率。

     

    Abstract: 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.

     

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