蠕变-渗流耦合作用下有效抽采半径研究

It is of great significance to accurately determinate effective drainage radius for optimizing boreholes arrangement and enhancing gas drainage effect. Gas drainage practice indicates that gas drainage effect in soft coal seam is far worse than hard coal, analysis of its causes, borehole shrinkage phenomenon occurs over time in both hard and soft coal seam because of rheological properties, but the ability of soft coal to resist the destruction is weak, and its creep deformation is more intense, the unstable failure of boreholes can occur in a short time, blocking gas drainage channel, even extending the extraction time, also it is difficult to improve the drainage effect; although borehole diameter in hard coal seam reduces but still in a stable condition，and high concentration gas can be extracted for a long time. Therefore, it is necessary to study borehole aperture variation for determining gas drainage radius and the effective drainage time. The project intends to establish visco-elastic-plastic model of coal around boreholes considering plastic softening and dilatancy, study borehole aperture variation of different intensity coal, and determine the effective drainage time; The project intends to establish the fluid-structure coupling model of gas drainage process considering the adsorption characteristics on the basis of comprehensively analyzing the influence of creep deformation and matrix shrinkage and effective stress changes on permeability, and determinate effective drainage radius of different coal seams. The project can provide a theoretical basis for the gas migration law studies , but also provide a new way to determine effective drainage radius for different coal seams.

准确确定有效抽采半径对于优化钻孔的布置方式，提高瓦斯灾害防治效果具有重要意义。瓦斯抽采实践表明，软煤层抽采效果远差于硬煤层，一个重要原因就是由于含瓦斯煤的流变特性，软硬煤层钻孔均会产生缩孔现象，但软煤抵抗破坏的能力较弱，蠕变变形剧烈，钻孔短时间就可能发生失稳破坏，阻塞抽采通道，即使延长抽采时间，也很难提高抽采效果；硬煤层钻孔直径虽有缩小但仍处于稳定状态，可长时间抽取高浓度瓦斯。因此，在确定有效抽采半径时，应首先研究抽采钻孔的孔径变化规律，确定有效抽采时间。项目拟建立考虑煤的塑性软化和扩容特性的钻孔周围煤体黏弹塑性模型，研究不同强度煤层钻孔孔径变化规律，确定有效抽采时间；综合分析蠕变变形、基质收缩和有效应力变化对渗透率的影响，建立考虑吸附作用的瓦斯抽采流固耦合模型，确定不同煤层条件的有效抽采半径。本项目可为瓦斯运移规律研究提供理论基础，也可为不同煤层条件有效抽采半径的确定提供新方法。

准确确定有效抽采半径对于优化钻孔的布置方式，提高瓦斯灾害防治效果具有重要意义。瓦斯抽采实践表明，软煤层抽采效果远差于硬煤层，一个重要原因就是软煤层钻孔的蠕变变形更为剧烈，容易产生缩孔甚至堵孔现象，钻孔一旦被堵塞，即使延长抽采时间，也很难提高抽采效果，因此，在确定有效抽采半径时，应首先研究抽采钻孔的孔径变化规律，确定有效抽采时间。项目在煤的蠕变压缩实验的基础上，建立了考虑煤的塑性软化和扩容特性钻孔周围煤体的黏弹塑性模型，对比分析了软硬煤层钻孔的卸压效果，查明了软硬煤层钻孔的孔径变化规律，确定了不同煤层条件的有效抽采时间；建立了考虑蠕变变形和基质收缩效应的瓦斯抽采流固耦合模型，确定了不同煤层条件的有效抽采半径，并对其进行了验证。研究结果表明：（1）软煤层钻孔具有更好的卸压效果，水力冲孔钻孔的卸压范围Rb和冲煤量m具有线性关系，Rb = 1.6064 m + 1.9336，煤体强度、埋藏深度和钻孔孔径是控制钻孔卸压效果的主要因素。（2）软硬煤层钻孔均会发生缩孔现象，但是软煤层钻孔蠕变变形更为剧烈，在短时间内就可能发生失稳破坏，导致瓦斯抽采浓度快速衰减；硬煤层钻孔孔径虽有缩小但仍处于稳定状态，并不发生堵孔现象，可以长时间的抽取高浓度的瓦斯，试验地点软煤钻孔的有效抽采时间为12d，而硬煤钻孔所有抽采时间均为有效抽采时间；地应力和煤体强度是控制钻孔缩孔幅度的主要因素，硬煤层钻孔到井田深部由于所受应力增大，蠕变变形加剧，在埋深800m处钻孔仅32d即被堵塞，其有效抽采时间即缩小为32d。（3）确定了不同煤层条件的有效抽采半径，其与孔径有：R抽=0.0067r0 + 0.2748，与初始瓦斯压力有：R抽= -0.835P0 + 2.228，与冲煤量有：R抽 = 1.8m + 2.2667，埋深400m、600m和800m煤层的有效抽采半径分别为2.88m、1.62m和0.82m；现场实测的有效抽采半径表明计算结果合理可靠；煤体强度、埋藏深度、初始瓦斯压力、抽采时间、初始渗透率和钻孔孔径是影响有效抽采半径的主要因素。本项目可为瓦斯运移规律研究提供理论基础，也可为不同煤层条件有效抽采半径的确定提供新方法。