煤岩定向射孔对水力压裂裂纹扩展影响的研究

Study on effect of directional perforation in coal rock on crack propagation by hydraulic fracturing

瓦斯抽采是制约煤矿高效生产的关键。为提高低渗透、难抽采煤层瓦斯抽采效率,针对煤岩定向射孔对压裂裂纹扩展的影响,采用扩展有限元方法对水力致裂煤层流固耦合模型进行求解,研究并揭示了应力差、射孔方位角及射孔深度对裂纹扩展规律、起裂压力及裂纹宽度的影响。研究结果表明:射孔转压裂时,裂纹先在射孔的孔底处起裂,并最终转向了与最大水平主应力一致的方向上继续扩展;平面应力差增大时,更容易形成长而宽的裂纹,平面应力差由2 MPa增加到7 MPa的过程中,起裂压力增加了94.4%,但最终形成的裂缝宽度增加了375%;射孔方位角越大,所形成的裂纹越短,射孔方位角从0°增加到60°的过程中,裂纹的起裂压力增加了187%,而最终形成的裂纹宽度降低了84%;较深的射孔能形成较长和较宽的裂纹,当射孔深度与钻孔半径的比值从1.0增加到3.5的过程中,裂纹的起裂压力降低了51.4%,而裂纹的宽度增加了145%。因此建议在实际的施工中,要尽可能准确地取得煤层地应力分布情况,选择与最大主应力一致的方向上射孔,并选择合适的射孔参数,以达到较为理想的射孔深度。

Gas drainage is the key to restrict the efficient production of coal mines.In order to improve the gas extraction efficiency in lowpermeability and difficult-to-drain coal seams,the extended finite element method was used to solve the fluid-solid coupling model of hydraulically fractured coal seams according to the influence of directional perforation in coal on the propagation of fracturing cracks.The research reveals that the effect of stress difference,perforation azimuth and perforation depth on crack propagation law,crack initiation pressure and crack width.The study results show that when the perforation is switched to fracturing,the cracks begin at the bottom of the perforation hole first and eventually turns to the direction consistent with the maximum horizontal principal stress and continue to expand.When the plane stress difference increases,it is easier to form long and wide cracks.When the plane stress difference increases from 2 MPa to 7 MPa,the initiation pressure increases by 94.4%,but the final crack width increases by 375%.The larger the azimuth angle of the hole,the shorter the crack formed.When the azimuth angle of the perforation is increased from 0°to 60°,the crack initiation pressure increases by 187%,and the width of the crack eventually formed is reduced by 84%.Deeper perforations can form longer and wider cracks.When the ratio of perforation depth to drilling radius increases from 1.0 to 3.5,the crack initiation pressure of the cracks decreases by 51.4%,and the width of the cracks increases by 145%.Therefore,it is recommended that in actual construction,the distribution of in-situ stress of coal seam should be obtained as accurately as possible,perforating in the direction consistent with the maximum principal stress,and appropriate perforation parameters should be selected to achieve a more ideal perforation depth.