煤层气直井真三轴水力波及压裂实验研究

Hydraulically spread fracturing experiment under true triaxial stress of vertical wells in coalbed methane

采用取自沁水盆地南部寺河矿的原生高阶煤块制作试样,利用真三轴压裂实验装置,研究了煤岩在不同地应力状态下水力波及压裂裂缝起裂及扩展规律。实验结果表明,水力波及压裂能够充分沟通煤岩天然裂隙,形成较为复杂的裂缝网络。水平主应力差越小,在应力干扰作用下越容易使地应力发生转向,使沟通面端割理更容易,形成的裂缝网络更为复杂。面端割理对裂缝起裂和扩展起主导作用,人工裂缝遇割理易发生转向并沿割理延伸。基质裂缝通常沿最大水平主应力方向起裂延伸,但由于先压裂缝及天然裂缝应力干扰作用,起裂方向可能与最大水平主应力呈一定夹角,这对形成缝网十分有利。井筒周围天然裂隙发育会使井筒出现多处起裂,破裂压力不明显,压裂曲线波动,且后起裂的井破裂压力相对较小。

Taking raw high rank coal sample of Sihe coalmine in Qinshui basin as an example, hydraulic fracturing test system with true triaxial stress is adopted to study the initiation and propagation rule of hydraulically spread fracturing under different confining stress. The research shows that the natural fractures can be adequately connected by hydraulically spread fracturing, and complex frac- ture networks are formed. The smaller horizontal stress difference is, the easier stress orientation will be changed under the action of stress interference. In addition, the face cleat and butt cleat that filled in coalbed are more likely opened to form more complex fracture networks. Furthermore, it is concluded that face cleat and butt cleat are major factors to impact fracture initiation and propagation, and artificial fractures are easier to change direction and extend along butt cleat. Matrix fractures usually propagate along the direction of maximum horizontal principal stress. However, due to stress interference of pre-existing hydraulic fractures and natural fractures, it may occur included angle between fracture initiation direction and maximum horizontal principal stress, which is beneficial to form fracture networks. Natural fractures developed along wellbore may cause lots of fracture initiation, indistinct breakdown pressure and fluetuant fracturing curves, moreover, the pressure of subsequent hydraulic fractures is lower than the pre-existing fractures.