如何定量評估液氮冷卻后煤儲層的滲透率演化是液氮冷卻增透煤儲層技術的關鍵。為分析液氮注入煤后的變形、破壞和滲透率演化過程，將煤視作彈脆塑材料，其變形過程包括彈性變形、脆性跌落和殘余塑性流動3個階段，結合單元強度退化指數、擴容指數和Mohr-Column準則，建立了考慮圍壓對煤單元峰后力學行為影響的本構模型。根據煤巖單元變形過程，將煤巖單元滲透率演化分成2個階段，即彈性壓縮煤巖單元滲透率減小階段及煤巖單元破壞后的滲透率增加階段。分析了單元彈性變形、剪切破壞和拉破壞與滲透率之間的關系。煤巖單元彈性壓縮和拉伸引起單元內孔隙空間的變化，進而影響單元滲透率；煤巖單元剪切破壞在單元內形成共軛剪切帶，在剪切帶內的流體流動服從平行板定律，給出了基于單元體應變的剪切帶寬度和滲透率計算公式；煤巖單元拉破壞在單元體內形成“十”字型裂隙，在裂隙內的流動也服從平行板定律，給出了基于單元體應變的裂隙寬度和滲透率計算公式。結合熱傳導理論建立了液氮冷卻煤層的溫度-變形-破壞-滲透率演化模型，并在FLAC下利用Fish函數方法予以實現。數值算例研究了液氯注入遼寧王營子礦某煤層氣抽放井后煤層的變形、破壞和滲透率演化過程。結果表明：1）煤受液氮冷卻作用后發生體積收縮，越靠近鉆孔溫度梯度越大，收縮變形越大，溫度拉應力越大，越容易破壞，形成拉破壞區。液氮注入冷卻10 d后的拉破壞區約0.65 m寬。2）在拉破壞區，單元內形成了貫通的裂隙，單元體滲透率顯著增長，液氮冷卻10 d的單元滲透率最大增長幅度可達1.97×105倍。3）遠離鉆孔區域，拉應力也使得煤的滲透率有所增加，增加幅度為1%～14%，遠小于破壞區。4）隨著冷卻時間增加，破壞區域擴大，但增長速率逐漸減緩，這表明在工程實踐中冷卻時間過長，不一定能取得更好的冷裂效果。5）液氮冷裂的主要影響區域在1.0 m左右，但實際工程中鉆孔內壓力、煤巖體內水的相變等對煤巖的實際變形和破壞也有很大影響，從而使得液氮冷裂的影響區域更大。6）模型能較好地反映液氮冷卻煤體變形-破壞-滲透率演化過程，從而為評估液氮冷卻煤巖增透效果提供一種簡便、可行的方法。
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 1.97×105 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.