瓦斯抽采钻井的采动裂隙场阻塞特性及其二次增透机理

During the process of extracting depressurized gas with gob gas ventholes (GGVs), the particles such as coal fines or rock dust carried in the gas flow drop in the mining fracture, making the permeability of the fracture field reduce, the gas production drop and even the borehole close. In regard to this problem, the applicant proposed permeability-enhancement technology of gas-liquid combination: by enhancing the permeability of fracture field for a second time, the gas production could be increased or restored and favorable engineering results have been achieved. However, so far the blocking characteristics of the mining-induced fracture field and the mechanism of enhancing permeability for a 2nd time are unclear. Consequently, the engineering design of the permeability-enhancement technology of gas-liquid combination is completely dependent on the experience, which severely restricts the widespread use of this technology. In this program, the motion characteristics of the particles in the mining-induced fracture were taken as the research object to study the sedimentation of the particles and to reveal the blocking characteristics of the fracture field such as the blocking mode and the type of the zone liable to be blocked. In addition, the influence rules of the pressure fluctuations on the shear stress variation of water flow wall were investigated to deduce the mechanism of the additional value of shear stress fluctuations on the resuspension of sediment particles. Besides, the inlets distribution rules of particles transport in the intersecting area of fractures were analyzed. Furthermore, the key factors affecting the effect of permeability-enhancement of fracture field were discussed to reveal the mechanism of second-enhancement of permeability based on the gas-liquid combination technology. The research results could help extend service life and increase utilization of GGVs, thus providing scientific guidance for the wide use of the technology of extracting depressurized gas with GGVs.

地面钻井抽采卸压瓦斯期间，抽采气流所携带的煤（岩）粉屑等颗粒极易沉降于采动裂隙中，造成裂隙场渗透率降低、钻井产气量下降甚至闭井。对此，申请人提出了气-液相组合增透技术，对裂隙场进行二次增透，实现钻井增产或复产，取得了较好的工程实效。但是，目前对瓦斯抽采采动裂隙场的阻塞特性、二次增透机理还未能阐释清楚，造成气-液相组合增透技术的工程设计完全依赖经验，严重制约技术推广应用。本项目以采动裂隙中颗粒的运动特性为研究对象，研究裂隙场内抽采气流颗粒的沉降行为，揭示裂隙场的阻塞模式、易阻塞区类型等阻塞特性；研究压力脉动对水流壁面切应力变异的影响规律，揭示切应力脉动附加量对沉积颗粒起悬的作用机制；研究裂隙相交区颗粒运移的汊道分配规律；研究裂隙场增透效果的主控影响要素，揭示基于气-液相组合增透的裂隙场二次增透机理。研究成果可显著延长钻井寿命、提高钻井有效利用率，为钻井抽采卸压瓦斯技术的推广应用提供技术支撑。

瓦斯抽采气流携带的煤岩颗粒在采动裂隙内沉积是导致地面钻井瓦斯抽采流量骤降的重要因素之一。本项目以利用水流冲刷作用减少采动裂隙内沉积颗粒数量提高地面钻井瓦斯抽采效果为背景，开展了采动裂隙微观形态及节理面粗糙度分布特性、抽采气流影响下煤岩颗粒在采动裂隙中的沉积特性、水流作用下采动裂隙内沉积颗粒的固液耦合作用机制与起动行为等方面的系列研究。本项目成果能够为优化基于气-液相组合增透的裂隙场二次增透工艺参数提供科学依据，为延长钻井寿命、提高钻井利用率奠定理论基础。本项目的具体研究成果如下：.（1）采动裂隙微观形态和节理面粗糙度分布特性.建立了采动裂隙缩径的结构模型，并得到了裂隙缩径的关键特征参数，揭示了采动裂隙以缓变缩径为主的结构特征；提出了基于最小二乘法的裂隙节理面粗糙度修正算法，分析了采动裂隙节理面粗糙度的分布特性。 .（2）抽采气流中煤岩颗粒在采动裂隙缩径区域的沉积特性.研究了煤岩颗粒在采动裂隙典型形态下的运移行为及沉积模式，分析了输送气速、固气比、缩径特征参数等因素对裂隙内颗粒沉积规律的影响，揭示了裂隙内颗粒呈“三段式”沉积的时空演化特性。.（3）水流作用下采动裂隙沉积颗粒的起动行为特性.建立了湍流状态下裂隙内沉积颗粒临界起动流速数学模型，揭示了湍流边界层对颗粒起动的“隐蔽”效应；研究了裂隙内不同雷诺数下沉积颗粒的起悬机制，探明了沉积颗粒在分叉裂隙内的运移与汊道分配规律，提出了水流条件下颗粒临界起动判定指标及临界值，为优化基于气-液相组合增透的裂隙场二次增透工艺提供了理论基础；研究了裂隙内压力脉动的传递特性，分析了压力脉动特征参数对颗粒临界起动流速的影响，探明了影响裂隙场压力脉动增透效果的主控因素。.（4）水流作用下采动裂隙沉积颗粒的固液耦合作用机制.研究了颗粒起动过程中流体与颗粒耦合力矩、曳力、颗粒速度等因素与粒径、流体粘度和流体密度间的关系，提出了提高流体介质密度以提升堵塞裂隙水流冲刷效果的有效方法。