Fracture evolution in coalbed methane reservoirs subjected to liquid nitrogen thermal shocking

液氮冷冲击作用下煤层气储层的裂隙演化

Thermal shocking effect occurs when the coalbed methane(CBM)reservoirs meet liquid nitrogen(LN2)of extremely low temperature.In this study,3D via X-ray microcomputer tomography(μCT)and scanning electron microscope(SEM)are employed to visualize and quantify morphological evolution characteristics of fractures in coal after LN2 thermal shocking treatments.LN2 thermal shocking leads to a denser fracture network than its original state with coal porosity growth rate increasing up to 183.3%.The surface porosity of theμCT scanned layers inside the coal specimen is influenced by LN2 thermal shocking which rises from 18.76%to 215.11%,illustrating the deformation heterogeneity of coal after LN2 thermal shocking.The cracking effect of LN2 thermal shocking on the surface of low porosity is generally more effective than that of high surface porosity,indicating the applicability of LN2 thermal shocking on low-permeability CBM reservoir stimulation.The characteristics of SEM scanned coal matrix in the coal powder and the coal block after the LN2 thermal shocking presented a large amount of deep and shallow progressive scratch layers,fracture variation diversity(i.e.extension,propagation,connectivity,irregularity)on the surface of the coal block and these were the main reasons leading to the decrease of the uniaxial compressive strength of the coal specimen.

当煤层气储层遇到温度极低的液氮时,会发生冷冲击效应。本文利用三维X射线显微成像系统和扫描电镜对液氮冷冲击引起的煤样裂隙形态演化特征开展可视化和量化分析。在液氮冷冲击作用后煤样裂隙网络比初始状态致密,孔隙率增加了183.3%;煤样CT扫描层表面孔隙率在液氮冷冲击影响下增加18.76%~215.11%,说明了液氮冷冲击后煤样变形的非均质性。液氮冷冲击作用,总体上对低表面孔隙率比高表面孔隙率扫描层致裂的效果更显著,这表明液氮冷冲击对低渗煤层气储层增产的适用性。液氮冷冲击前、后煤粉和煤块的电镜扫描实验发现,煤基质呈现大量深浅递进分层擦痕特征以及煤块表面裂隙变化的多样性,如扩展、产生、连通和不规则,这是导致液氮冷冲击后煤样单轴抗压强度降低的主要原因。