Mechanism of coal fine-bubble coupling in the unsaturated flow stage of coalbed methane drainage
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摘要:
揭示煤层气排采储层非饱和流阶段煤粉与气体相互作用机理，对制定排采制度和提高产气量具有重要意义。通过气泡–煤粉微观作用实验装置，系统开展了不同直径大小的气泡对不同粒度和密度煤粉的作用实验，分析了气泡对煤粉运移轨迹和速度的影响及捕获煤粉特征。结果表明，气泡产出能够影响煤粉的运移轨迹，甚至能够捕获煤粉；煤粉通过气泡时会产生3种运动类型：沿着气泡表面运移到气泡底部最后被捕获、沿着气泡表面运移到气泡底部最后脱落及接近气泡时被排斥而轨迹发生偏转。煤粉若被气泡捕捉，则运动速度呈现出减小–增大–减小的变化特征；若未被气泡捕获，速度呈现出减小–增大–减小–增大的变化特征。不同条件下气泡对煤粉的捕获效率高达64.38%~86.64%；在气泡表面最高点附近发生碰撞煤粉被捕获的概率最大，并且随着偏离角度的增大，气泡捕获效率均呈现出逐渐减小的趋势；在相同的碰撞位置下，气泡对煤粉的捕获效率随着煤粉密度、煤粉粒径的增大而减小，随着气泡直径的增大而增大。煤层气产气初期应根据储层的实际导流能力合理控制降压速率，若储层导流能力较强，应加大排采速率，增大气体解吸对煤粉的扰动和捕获作用，促使大量煤粉随地下水或气泡产出；若储层导流能力较弱，应该适当降低排采速率，以防气体快速解吸而引起大量煤粉启动运移；同时，可应用大气泡携煤粉能力强的特性，促使近井地带的煤粉产出以增大储层导流能力。
Abstract:To reveal the mechanism of interaction between coal fines and gas in the unsaturated flow stage of coalbed methane (CBM) drainage, it is essential to formulate the drainage system and improve the gas production. The effect of air bubbles with different diameters on coal fines with different particle sizes and densities was systematically experimented through the experiment device for microscopic interaction between the air bubble and the coal fines. Meanwhile, the influence of air bubbles on the migration trajectory and velocity of coal fines and the characteristics of coal fine capturing by air bubbles were also analyzed. The results show that the production of air bubbles could affect the migration trajectory of coal fines, and even capture and carry coal fines. Typically, three types of motion are generated in the coal fines through the air bubbles: the coal fines may migrate along the surface of the air bubble to the bottom to be captured or fall off at last, or be repelled upon its approaching to the air bubble with the migration trajectory deflected. If the coal fines are captured by air bubbles, the velocity will decrease at first, then increase, and finally decrease. If the coal fines are not captured by air bubbles, the velocity will decrease at first, followed by increase and decrease successively, and increase finally. The capture efficiency of air bubbles to coal fines under different conditions ranges from 64.38% to 86.64%. The maximum capture probability occurs near the highest point of the bubble surface, and with the increase of the deviation angle, the capture efficiency of bubble shows a decreasing trend. At the same collision position, the capture efficiency of air bubbles on coal fines decreases with the increase of coal fines in density and particle size, but increases with the increase of bubble diameter. In the initial stage of CBM production, the depressurization rate should be reasonably controlled according to the actual conductivity of the reservoir. In case of large reservoir conductivity, the drainage rate should be increased to increase the disturbance and capture of gas desorption to coal fines, so that a large amount of coal fines will be produced with groundwater or air bubbles. In case of small reservoir conductivity, the drainage rate should be appropriately reduced to prevent the migration of a large amount of coal fines due to the rapid desorption of gas. At the same time, the production of coal fines in the near-wellbore area may be stimulated with the great carrying capability of air bubbles for coal fines, so as to increase the conductivity of the reservoir.
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Keywords:
- coalbed methane /
- unsaturated flow /
- coal fine /
- bubble /
- migration
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图 1 气泡与煤粉作用原理与实验装置
Fig. 1 Mechanism of interaction between air bubbles and coal fines and its experiment device
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图 7 不同煤粉粒度条件下煤粉捕获效率
Fig. 7 Capture efficiency of coal fines at different particle sizes
表 1 实验方案
Table 1 Experimental program
编号 煤粉粒径/μm 煤粉密度/(g·cm−3) 气泡直径/mm 1 75~96 −1.3，1.3~1.4，1.4~1.5，1.5~1.6，1.6~1.7，+1.7 1 2 120~180，96~120，48~75 1.4~1.5 1 3 75~96 1.4~1.5 0.75，1.25，1.50 
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