液氮冷浸对煤体致裂增透作用机制实验研究

For the further study on the mechanism of fracturing and permeability improvement of coal, which has been immersed in cryogenic liquid nitrogen, the raw coal samples are taken as the research objects, experiments of different metamorphic degree coal samples immersed in cryogenic liquid nitrogen will be developed under drying condition and different water contents, experiments of coal permeability before and after immersing in cryogenic liquid nitrogen will be conducted, and experiments of the physical and mechanical parameters, such as, the shear strength and compression modulus under compression of single axis and three axis will also be carried out. The tools of atomic force microscope and field emission scanning electron microscope are used to analyse the change law of primary void-fracture structure of coal samples before and after immersing in cryogenic liquid nitrogen. The results are expected to reveal the extension mechanism of primary structure of coal samples with different metamorphic degree and the competition mechanism between the effect of moisture and its cooling-fracturing.According to the damage mechanics, fracture mechanics and thermodynamics theory, the paper expounds the hot-cold shock damage principle caused by immersing in cryogenic liquid nitrogen in consideration of the volume expansion deformation because of the frozen of water inside the coal body when it is immersed in the cryogenic liquid nitrogen and the difference in the conductivity coefficient of the coal mineral particles and the cementing After the stress analysis of coal matrix element immersed in liquid nitrogen, a permeability evolution model unit of coal matrix element was established in consideration of the effect of moisture influence, basing on the plate theory. This paper puts forward a finite element simulation method on the basis of this model and provides theoretical support for the study of effect of fracturing and permeability improvement of liquid nitrogen injection.

为深入研究液氮冷浸对煤体致裂增透作用机制，本课题以原煤试样为研究对象，开展干燥、不同含水率条件下不同变质程度煤样液氮冷浸实验、液氮冷浸前后煤体渗透率实验、单轴和三轴压缩条件下煤体抗剪强度和压缩模量等力学物理参数测试实验研究，利用原子力显微镜和场发射环境扫描电镜等手段对比分析液氮冷浸前后煤体试样原生孔隙-裂隙结构的变化规律，揭示液氮冷浸对不同变质程度煤体原生结构的扩展机制及煤体水分影响与自身冷裂作用的竞争机制；根据损伤力学、断裂力学和热力学等理论，在考虑液氮冷浸时煤体内部水冰相变产生体积膨胀变形及煤体矿物颗粒和其胶结物热的传导系数存在差异的情况下，阐述液氮冷浸致裂作用对煤体的热冷冲击破坏原理；在对液氮冷浸后煤基质单元体受应力分析的基础上，以平板理论为桥梁，建立考虑水分影响的液氮冷浸作用下煤基质单元体渗透率演化模型；并提出基于该模型的有限元模拟方法，为研究液氮注入煤层致裂增透效果提供理论支撑。

本课题对液氮冷浸致裂作用下煤体内原生孔隙-裂隙扩展规律及对煤基质热冷破坏作用开展系统深入的实验和基础理论研究。主要研究内容包括，利用自行研制的自增压式可视化液氮冷浸煤体实验装置，开展了液氮冷浸煤体实验，采用核磁共振、CT扫描、扫描电子显微镜等手段对比分析了液氮冷浸前后煤体试样孔裂隙结构的变化规律，研究了水冰相变产生的冻胀力对冷浸过程中煤体内部微观结构及损伤变化情况的影响，对比分析了液氮冷浸对干燥状态和不同含水率状态下煤体试样的破坏形式，并利用利用三轴渗流实验系统，系统研究了液氮冷浸前后煤体的渗透率演化特征，建立考虑水分影响的液氮冷浸作用下煤基质单元体渗透率演化模型，揭示了液氮冷浸致裂作用下煤体水分影响与自身冷裂作用的竞争机制。依据项目申请书内容，经过系统的研究，总体上达到了预期目标。研究取得了三个方面的代表性成果：（1）搭建了自增压式可视化液氮冷浸煤体实验装置，以原煤试样为研究对象，开展了干燥、不同含水率条件下不同变质程度煤样液氮冷浸实验研究，揭示了液氮冷浸致裂作用对不同变质程度煤体内原生孔隙-裂隙扩展规律，得到了煤体水分影响与自身冷裂作用的竞争机制。（2）利用三轴渗流实验系统，采用控制变量法，研究了循环次数、含水饱和度、冷浸时间和煤种对増透效果的影响，得到了液氮冷浸对干燥状态和不同含水率状态下煤体试样破坏形式。（3）在推导温度应力的基础上，根据施加温度应力后煤单元破坏的体积变化，建立了合适液氮冷浸后煤基质单元体的考虑围压条件下的损伤本构模型，以平板理论为桥梁，建立了考虑水分影响的液氮冷浸作用下煤基质单元体渗透率演化模型，得到了液氮冷浸作用后煤体渗透率的演化规律，并利用实验数据对模型的准确性进行验证，模型计算结果能够与实验数据相吻合。项目研究成果为为液氮冷浸致裂增透方法提供理论基础。