液氮冷冲击作用对煤岩微纳米尺度孔隙结构及力学性质的影响

Pore-scale analysis of coal structure and mechanical properties evolution through liquid nitrogen thermal shock

液氮压裂是提高煤层气储层改造效率的潜在可行性技术之一,而目前对于液氮冷冲击作用下煤岩孔喉连通性及微观渗流特征可视化的研究则较少,致使该作用对煤岩微纳米尺度渗流特征与力学性质的影响机制认识尚不明确。为了给液氮压裂提高煤层气储层改造效率技术提供理论依据,基于CT扫描和原子力显微镜研究了液氮冷浸前、后煤岩微纳米尺度孔隙结构及力学性质的变化,揭示了液氮冷冲击作用对煤岩渗流能力的影响机制。研究结果表明:①液氮冷浸后,煤岩孔隙数量和孔隙尺度均增大,本次实验条件下孔隙度增大2倍,微裂缝占主导,微裂缝体积占比由液氮冷浸前的7.7%增至90.0%;②基于CT三维孔隙结构重构模型,液氮冷浸后,煤岩喉道数量、总长度、总体积均显著增加,较之于液氮冷浸前,分别增加了1.7倍、1.4倍、1.3倍,煤岩孔隙连通性得以明显改善;③液氮冷浸后,煤岩的绝对渗透率显著提高,在实验条件下为液氮冷浸前的77倍,热应力形成的微裂缝成为液氮冷浸后煤岩的主要渗流通道;④液氮冷浸后,煤岩基质区域和矿物区域均有孔洞及裂缝形成,煤岩表面粗糙度增加,与此同时煤岩基质区域和矿物区域的弹性模量均有所下降,弹性模量平均值分别降低81%、91%。结论认为,液氮冷冲击作用使得煤岩微观缺陷增多、力学性质劣化;液氮压裂有望成为一种高效、绿色的新型煤层气储层改造技术。

Liquid nitrogen fracturing is one of the potential feasible technologies for improving the stimulation efficiency of coalbed methane(CBM)reservoirs.At present,the visualization of pore-throat connectivity and microscopic seepage characteristics in coal rocks under liquid nitrogen thermal shock is still lack of studying.Hence,the influence of liquid nitrogen thermal shock on the micro-nano pore structure and mechanical property of coal rocks are not understood clearly.In order to provide theoretical basis for the stimulation behavior of liquid nitrogen fracturing in coal beds,this paper investigates the change of micro-nano pore structure and mechanical property of coal rocks before and after liquid nitrogen by treatment means of CT scanning and atomic force microscope(AFM).In addition,the influence of liquid nitrogen thermal shock on the seepage routes of coal rock are revealed.The following research results can be obtained.First,the number and scales of pores in the coal increase after liquid nitrogen thermal shock.In this experiment,porosity is increased by 200%,Micro fracture is dominant and its volume ratio is increased to 90.0%from 7.7%before liquid nitrogen treatment.Second,the three-dimensional pore structure reconstruction model obtained by CT shows that after the liquid nitrogen treatment,the number,total length and total volume of throats in the coal rock are increased by 170%,140%and 130%and the pore connectivity is improved greatly.Third,after is cold soaked in liquid nitrogen,its absolute permeability is improved significantly.In this experiment,the absolute permeability of coal after liquid nitrogen treatment is 77 times higher than that before liquid nitrogen cooling.The micro fracture induced by thermal stress is the main percolation routes in coal after liquid nitrogen cooling.Fourth,pores and fractures are newly formed on both matrix and mineral domains,and the surface roughness is increased.In the meantime,the elastic modulus in matrix and mineral domains of coal drops,and the average elastic modulus drops by 81%and 91%,respectively.In conclusion,liquid nitrogen thermal shock leads to the increase of microscopic defects in coal and the deterioration of mechanical property.Liquid nitrogen fracturing is expected to be a new kind of efficient and green CBM reservoir stimulation technology.