基于扩展有限元分析的页岩水力压裂裂缝扩展规律探究

Investigation of shale hydraulic fracturing propagation laws based on extended finite element analysis

水力压裂是开采地下页岩气资源的有效技术手段,探究页岩水力压裂裂缝的扩展规律,可为页岩气的高效开采提供科学的指导依据。通过运用大型有限元软件ABAQUS中的扩展有限元模块,针对不同地应力差工况条件下均质页岩中初始裂缝的位置、方位角、数量和含层理页岩中层理的构造方向、内部倾角、岩性对水力裂缝扩展的影响进行探究。结果表明:对于垂向扩展的水力裂缝,水平主应力增大使裂缝更不易扩展,裂缝扩展长度减小、起裂压力增大;在注液体积流量相同时,向初始裂缝两端同时起裂所形成的水力裂缝长度大于仅向一侧起裂;当初始裂缝处于页岩中部且呈45°方向时,裂缝会向最大水平主应力方向偏转,且偏转程度随最大水平主应力的增大而增大;分时多簇压裂时,裂缝间的扩展会相互干扰,且会较大地影响裂缝扩展的形态和起裂压力,但对裂缝注液点裂缝宽度的影响较小;对于含水平和竖直构造层理的页岩,改变层理内部倾角,水力裂缝会出现不同程度偏转,且其偏转程度随着层理内部倾角的增大而减小;对于含45°方向构造层理的页岩,水力裂缝在层理分别为砂岩、煤岩和泥岩中的偏转程度依次增大,且裂缝偏移比随着最大水平主应力的增大而增大。

Hydraulic fracturing is an effective technical method to exploit subsurface shale gas resources.To investigate the law of fracture propagation in shale hydraulic fracturing can provide proper guidance for high‑efficient exploitation of shale gas.By using the extended finite element module of ABAQUS which is a large finite element software to investigate the influence of the position,azimuth and number of the initial fractures in homogeneous shale,and the structural direction,internal dip angle and lithology of the bedding shale on hydraulic fracture propagation at different ground stress deviations.The results show that for vertical hydraulic fractures,with the increase of the horizontal principal stress,the fracture is more difficult to propagate,the fracture propagation length decreases,and the initiation pressure increases.At the same injection volume,the length of hydraulic fractures formed by simultaneous initiation at both ends of the initial fracture is larger than that formed only on one side.When the initial fracture is located in the middle of shale with a direction of 45°,the fracture will deflect to the direction of the maximum horizontal principal stress,and the degree of deflection increases with the maximum horizontal principal stress.In the process of time‑sharing multi‑cluster fracturing,the propagation of fractures will interfere with each other,and it will greatly affect the attitude of fracture propagation and initiation pressure,but it has little influence on the fracture width at the injection point.For shale with horizontal and vertical bedding structures,hydraulic fracture deflection will be deflected to different degrees when the internal dip angle of bedding is changed,and the deflection degree of hydraulic fractures decreases with the increase of the internal dip angle of bedding.For shale with structural bedding in the direction of 45°,the deflection degree of hydraulic fractures increases successively in sandstone,coal rock and mudstone,and the fracture migration ratio increases with the maximum horizontal principal stress.