液氮冷却煤变形-破坏-渗透率演化模型及数值分析
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国家自然科学基金(51274079,51274110,51474121);河北省自然科学基金(E2013208148)


Permeability evolution model and numerical analysis of coupled coal deformation, failure and liquid nitrogen cooling
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    摘要:

    如何定量评估液氮冷却后煤储层的渗透率演化是液氮冷却增透煤储层技术的关键。为分析液氮注入煤后的变形、破坏和渗透率演化过程,将煤视作弹脆塑材料,其变形过程包括弹性变形、脆性跌落和残余塑性流动3个阶段,结合单元强度退化指数、扩容指数和Mohr-Column准则,建立了考虑围压对煤单元峰后力学行为影响的本构模型。根据煤岩单元变形过程,将煤岩单元渗透率演化分成2个阶段,即弹性压缩煤岩单元渗透率减小阶段及煤岩单元破坏后的渗透率增加阶段。分析了单元弹性变形、剪切破坏和拉破坏与渗透率之间的关系。煤岩单元弹性压缩和拉伸引起单元内孔隙空间的变化,进而影响单元渗透率;煤岩单元剪切破坏在单元内形成共轭剪切带,在剪切带内的流体流动服从平行板定律,给出了基于单元体应变的剪切带宽度和渗透率计算公式;煤岩单元拉破坏在单元体内形成“十”字型裂隙,在裂隙内的流动也服从平行板定律,给出了基于单元体应变的裂隙宽度和渗透率计算公式。结合热传导理论建立了液氮冷却煤层的温度-变形-破坏-渗透率演化模型,并在FLAC下利用Fish函数方法予以实现。数值算例研究了液氯注入辽宁王营子矿某煤层气抽放井后煤层的变形、破坏和渗透率演化过程。结果表明:1)煤受液氮冷却作用后发生体积收缩,越靠近钻孔温度梯度越大,收缩变形越大,温度拉应力越大,越容易破坏,形成拉破坏区。液氮注入冷却10 d后的拉破坏区约0.65 m宽。2)在拉破坏区,单元内形成了贯通的裂隙,单元体渗透率显著增长,液氮冷却10 d的单元渗透率最大增长幅度可达1.97×105倍。3)远离钻孔区域,拉应力也使得煤的渗透率有所增加,增加幅度为1%~14%,远小于破坏区。4)随着冷却时间增加,破坏区域扩大,但增长速率逐渐减缓,这表明在工程实践中冷却时间过长,不一定能取得更好的冷裂效果。5)液氮冷裂的主要影响区域在1.0 m左右,但实际工程中钻孔内压力、煤岩体内水的相变等对煤岩的实际变形和破坏也有很大影响,从而使得液氮冷裂的影响区域更大。6)模型能较好地反映液氮冷却煤体变形-破坏-渗透率演化过程,从而为评估液氮冷却煤岩增透效果提供一种简便、可行的方法。

    Abstract:

    How to quantitatively evaluate the permeability change of coalbed subjected to liquid nitrogen cooling is a key issue of enhanced-permeability technology of coalbed. To analyze the evolution process of permeability of coupled coal deformation, failure and liquid introgen cooling, the coal is supposed as elastic, brittle and plastic material. Its deformation process includes elastic deformation stage, brittle strength degradation stage and residual plastic flow stage. Combined with strength degradation index, dilatancy index of the element and Mohr-Column strength criterion, the element scale constitutive model with the effects of confining pressure on peak-post mechanical behaviors is built. Based on the deformation process of coal rock, there exist two stages of permeability evolution of the element including decrease of permeability due to elastic contraction and increase due to coal rock element's failure. The relationships between the permeability and elastic deformation, shear failure and tension failure for coal are studied. The permeability will be influenced by the change of pore space due to elastic contraction or tension of element. Conjugate shear zones appear during the shear failure of the element, in which the flow follows so-called cubic law between smooth parallel plates. The calculation formulas of the permeability and the aperture of the fractures are given out based on the volumetric strain. When tension failure criterion is satisfied with the rock element fails and two orthogonal fractures appear. The calculation formulas of the permeability and the width of the fractures are given out based on the volumetric strain. Further, combined with the thermal conduction theory the permeability evolution model of coupled coal deformation, failure and liquid nitrogen cooling is presented. Then Fish function method in FLAC is employed to perform the model. The permeability's evolution process for coal bed cryogenically stimulated by flowing liquid nitrogen through gas production well in Wangyingzi mine, Liaoning province, is simulated and the results include: 1) When liquid nitrogen(LN2) is injected into a rock at warm reservoir temperature, heat from the rock will quickly transfer to the liquid nitrogen resulting in rapid cooling and contraction of coal bed. The nearer the position is to bore hole, the bigger the shrinkage deformation and thermal stress and coal fail when tension stress sufficiently built up. In this paper the tension failure band after 10 days' LN2 cooling is 0.65 m. 2) In tension failure area the cracks from cooling stimulation lead to the significant growth of permeability. The maximum permeability for element is 1.97×105 times more than that before cooling. 3) Apart from the bore hole, the thermal tensile stress leads to the growth of permeability at a rate of 1%~14%, far less than that in the tension failure area. 4) With increasing time the failure area gradually slowly grows up. It indicates that the longer cooling time does not mean better effects. 5) The cooling fracturing area is found to have a 1.0 m band. In practical engineering the pressure in hole bore and phase transition of water also influences the deformation and failure of coal, which leads to much more failure zone of cooling. 6) The evolution process of permeability of coupled coal deformation, failure and liquid introgen cooling can be better reflected by the model in this paper. This study is hoped to provide a simple but reasonable description of the permeability evolution of rocks subject to liquid nitrogen cooling.

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张春会,王来贵,赵全胜,李伟龙.液氮冷却煤变形-破坏-渗透率演化模型及数值分析[J].河北科技大学学报,2015,36(1):90-99

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  • 收稿日期:2014-10-23
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  • 在线发布日期: 2015-01-22
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