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不均匀载荷下天然气井井身结构力学性能试验

Mechanical properties of natural gas wellbore structure under non-uniform load

  • 摘要: 在天然气与煤炭资源叠置区中,高强度的煤层开采造成的覆岩运移使保护煤柱内的天然气井井身结构发生径向挤压变形甚至破坏,从破坏区域扩散的天然气极易导致煤矿矿井发生火灾爆炸或人员中毒事故。为探明在天然气与煤炭交叉开采影响下,天然气井井身结构受到不均匀载荷时的力学性能,依据井身结构设计4类套管−水泥环组合体,将天然气井井身结构复杂的径向应力模型简化为单向受力模型,并利用数字图像相关法(DIC)技术和RMT-150岩石力学实验仪开展套管−水泥环组合体的径向压缩试验和DIC测试试验,最后从不同材料的变形速率和力的传递规律角度对井身结构力学性能机制进行分析。结果表明:最外层为套管的井身结构,其力学性能显著优于最外层为水泥环的井身结构,井身结构的力学性能与材料的变形速率以及力的传递规律密切相关;组合体试件在受到外力作用时,力的传递规律呈现出由外至内逐渐减小的特点,试件最外层受到的力最大;当井身结构最外层为水泥环时,结构整体呈脆性,由于水泥环的变形速率大于套管导致水泥环和套管胶结面处出现裂缝,在拉应力作用下水泥环易发生脆性断裂;当井身结构最外层为套管时,结构整体呈弹塑性,在载荷作用下外层套管先发生压缩变形,内部水泥环在套管的挤压作用下产生内部裂隙,但在套管的约束下未出现断裂,试件整体呈径向压缩变形破坏。

     

    Abstract: In areas where natural gas and coal resources overlap, the movement of overburden caused by high-intensity coal mining causes radial compression and deformation or even destruction of the natural gas well body structure within the protective coal pillar. The natural gas diffused from the damaged area can easily lead to fire, explosion or personnel poisoning accidents in coal mines. In order to verify the mechanical properties of the natural gas well bore structure under the influence of cross-mining of natural gas and coal, four types of casing and cement ring combination were designed according to the well structure, and the complex radial stress model of natural gas well structure is simplified into a one-way force model. Digital Image Correlation (DIC) technique and RMT-150 rock mechanics instrument were used to carry out radial compression test and DIC test of casing cement ring combination. Finally, the mechanical performance mechanism of wellbore structure is analyzed from the perspective of deformation rates and force transfer laws of different materials. The results showed: The mechanical properties of the well structure with the outermost casing layer are significantly superior to those of the well structure with the outermost cement sheath. The mechanical performance of the well structure is related to the material’s rate of deformation and the principles governing force transmission. When the composite specimen is acted on by external force, the force transmission law gradually decreases from the outside to the inside, and the outermost layer of the specimen receives the largest force. When the outermost layer of the wellbore structure is a cement sheath, the structure as a whole is brittle. Since the deformation rate of the cement sheath is greater than that of the casing, cracks appear at the cement sheath and casing bonding surface. The cement sheath is prone to brittle fracture under tensile stress. When the casing is the outermost layer of the wellbore structure, the whole structure is elastic-plastic. Under the load, the outer casing first undergoes compression deformation, and the inner cement ring produces internal cracks under the extrusion of the casing, but it does not break under the constraint of the casing, and the whole specimen is damaged by radial compression deformation.

     

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