我国煤层渗透率低、瓦斯压力高、含量大，原始煤层瓦斯抽采困难。为提高煤层瓦斯抽采率、缩短预抽时间，必须实施人工增透。而众多压裂增透技术中，液态CO2压裂兼具压裂增透和驱替置换的双重瓦斯强化抽采作用，是当前低渗透煤层压裂改造方法的一个研究热点。为探索液态CO2（L-CO2）压裂增透技术在高瓦斯低透气性煤层中的应用及效果，类比水力压裂施工工艺，提出了井下高压（30 MPa）液态CO2压裂增透煤岩成套装备系统架构，确定了决定装备性能的5个关键参数。基于弹性力学理论、相似模拟实验和装备自身结构特点，综合考虑L-CO2低粘强渗透性和其相变特征，建立了L-CO2压裂煤岩起裂压力计算模型，推导了水和L-CO2压裂注液量的量化表征关系，得到了关键参数的科学确定方法。采用该方法，针对淮南潘三矿C13-1煤层地质赋存条件，研发了国内首套井下高压（30 MPa）L-CO2压裂增透煤岩成套装备，进行了井下上向穿层钻孔的L-CO2压裂增透煤层现场试验。结果表明：L-CO2压裂增透影响半径约25 m，此范围内单孔瓦斯抽采体积分数 提高约1.47倍，抽采纯量提高1.75~3.3倍；抽采钻孔出现明显的压裂增流效应，且随抽采孔与压注孔间距离的增加，增流效应逐渐减弱。

Coal seams in China have many features,such as low permeability,high gas pressure and large gas content,and it is extremely difficult to extract gas from the original coal seam.The permeability has to be increased in order to improve the efficiency of extracting gas,shorten the time of extracting gas in advance.Among many technologies of increasing the permeability by cracking coal rock,liquid CO2 can not only contribute to the increase of permeability,but also substitute for gas,which can improve the efficiency of extracting gas significantly.In order to explore the application and effect of the L-CO2 fracturing technology in high gassy and low permeability coal seams,referring to the hydraulic fracturing technique,the technical framework of a complete set equipment of permeability enhancements induced by high pressure (30 MPa) L-CO2 fracturing is proposed.And,then 5 key parameters that determine set equipment performance were identified.At the same time,based on the elastic mechanics theory,similar simulation experiments,and the structural characteristics of the equipment,considering the low viscosity and strong permeability of L-CO2 and its phase transition characteristics,an initiation pressure calculation model for L-CO2 fracturing was established and the quantitative characterization relationship between the water injection volume of fresh water and L-CO2 fracturing was derived.As a result,the scientific determination method of 5 key parameters was obtained.With this method,in terms of the geological conditions of the C13-1 coal seam in Pansan Coal Mine,Huainan,the first domestic complete set equipment of permeability enhancements induced by high pressure (30 MPa) L-CO2 fracturing was developed.And,its field test was carried out underground.The results show that the influence radius of L-CO2 fracturing can be reach to 25 m approximately.Within this range,the single-hole gas drainage concentration is increased by about 1.47 times,and the drainage volume is increased by 1.75-3.30 times.There is a significant increase of the flow rate in drainage holes,induced by the L-CO2 fracturing.Moreover,the effect of flowrate increasing is gradually weakened with the increasing of the distance between the drainage holes and the injection hole.

1 压裂增透煤岩成套装备系统架构

2 关键参数的确定方法

   2.1 增压泵出口压力Pfout确定

   2.2 增压泵出口流量Qcout确定

   2.3 L-CO2增压泵进液压力Pin与回气压力Pback确定

   2.4 L-CO2储罐补气压力Psup确定

3 试验应用

   3.1 试验地点概况

   3.2 试验方案设计

   3.3 高压L-CO2压裂增透煤岩成套装备组装

   3.4 试验过程温度和压力变化曲线

4 煤层压裂增透效果

   4.1 K3孔瓦斯抽采效果分析

   4.2 压裂影响半径分析

   4.3 L-CO2压裂前后单孔瓦斯抽采效果分析

   4.4 K5，K6和K7钻孔瓦斯抽采效果分析

5 结论