单相水流诱发裂缝内煤粉启动机理与防控对策

Incipient motion mechanisms and control measures of coal fines during single-phase water flow in coalbed fractures

煤层气井排水阶段是产煤粉的关键时期,煤粉运移不但会降低煤层渗透率,而且还会堵塞井筒和损坏设备。基于水动力学原理、扩展DLVO理论和JKR接触理论,开展了单相水流阶段裂缝面黏附煤粉的受力分析,建立了以"压力梯度"为判决条件的煤粉启动力学模型,分析了颗粒尺寸、颗粒类型、裂缝缝宽和离子强度对煤粉启动的影响,并围绕"适度出煤粉"和"阻止煤粉运移"两大思路提出了相应的防控对策。结果表明:煤粉启动压力梯度随着粒径的增加,先减小、后增大;裂缝缝宽越大,煤粉启动压力梯度越小;在不利化学黏附条件下有机质颗粒的启动压力梯度小于黏土矿物微粒,但在有利条件下却大于黏土矿物微粒;有利条件下的煤粉启动压力梯度远大于不利条件下的启动压力梯度。基于"适度出煤粉理论"设计煤层合理压力梯度区间时,应以优先改善最大缝宽裂缝的渗透性为原则,向煤层中注入煤粉稳定剂可提高煤粉-裂缝面间的黏附力,从而有效阻止煤粉运移。

The dewatering phase of coalbed methane（CBM）well is the critical period for coal fines production.Coal fines migration can not only reduce the coalbed permeability,but also block the shaft and break down devices.Based on hydrodynamics,extended-DLVO theory and JKR contact theory,force analysis was carried out on the coal fines attached to fracture surface at the single-phase water flow stage.A mechanical model for incipient motion of coal fines were established with＂pressure gradient＂as the judgment criterion,so as to analyze the influences of fines size,fines type,fracture aperture and ionic strength on incipient motion of coal fines.Additionally,corresponding control measures were proposed aiming at＂proper coal fines discharge＂and＂the inhibition of coal fines migration＂.The results show that the critical pressure gradient（CPG）for incipient motion of coal fines first decreases and then increases with fines size;the larger the fracture aperture is,the less the CPG for incipient motion of coal fines will be;the CPG for incipient motion of organic fines is less than that for mineral fines under unfavorable conditions,whereas it is greater than that for mineral fines under favorable conditions;the CPG for incipient motion of coal fines under favorable conditions is rather greater than that under unfavorable conditions.When setting a reasonable range of coalbed pressure gradient using the theory of＂proper coal fines discharge＂,to preferentially improve the permeability of the fracture with maximum aperture is taken as a principle.On the other hand,the injection of coal fines stabilizer to coalbed can increase the adhesive force between coal fines and fracture surface to prevent coal fines migration effectively.