基于离散元方法的煤层压裂裂缝模拟研究

The research of coalbed fracturing fracture simulation based on DEM

我国煤层气资源丰富,但由于煤层气赋存与产出机理独特,要实现煤层气的工业开发必须采取有效的增产措施,水力压裂是众多增产措施中使用比较多的煤层气开发技术。裂缝延伸状态是水力压裂的关键,国内外学者采用多种数值方法开展裂缝性储层裂缝延伸的模拟研究,但由于煤岩中割理比较发育、非连续特征比较明显,采用常规的有限元、有限差分、边界元等方法模拟时与实际情况存在偏差。为此,考虑离散元方法模拟的优势,根据离散元原理开展了煤岩水力压裂裂缝扩展的数值模拟。模拟结果表明,煤岩压裂后产生了多裂缝,主要集中在近井筒附近。随着压裂液排量的增加,裂缝宽度也不断增加,压力的波及范围和裂缝扩展范围也增大。当水力裂缝遭遇割理时,水力裂缝容易被割理抑制造成裂缝发生转向。而转向压力高于裂缝延伸压力下,致使水力裂缝在另一侧延伸,形成非对称裂缝;随着压裂液黏度的增加,注入压力、裂缝宽度明显增加;随着压裂液黏度的增加,裂缝延伸的长度增加,分支裂缝张开的长度和张开的数量明显减小;随着水平最大最小应力比的增加,水力裂缝在最大水平主应力方向延伸长度增加,在最小水平主应力方向出现减小的趋势。说明在高应力比下,不利于割理的张开与多裂缝的产生,而更有利于形成对称双翼对称裂缝。

China has an enormous amount of coalbed methane.But due to its special oc- curence and producing mechanism, its industrial exploitation acquires effective approaches to increasing production,among which hydraulic fracture is rather a widely used coalbed methane development technology.The condition of cracks extension is critical to hydraulic fracture.Academics home and abroad apply various numerical methods to model that of frac- tured reservoir.However,ordinary methods such as FEM （Finit Element Method）,FDM（Finit Difference Method） and BEM （Boundary Element Method） cannot produce results that are perfectly consistent with the actual practices, since cleats in coalbed are fairly mature and its uncontinuity are relatively obvious.Therefore,DEM （Discrete Element Method） is a better choice for the numerical simulation of the cracks extension in coalbed hydraulic fracture giv- en its advantage.The outcome indicates that fracturing causes lots of cracks that are mainly gathtering near the wellbore.With more fracture fluid delivered,the cracks outspread, extend- ing to a larger area,and result in an expansive influencing area.When encountering cleats,hy- draulic fracturing tend to be suppressed easily and thus lead to fractures deflected. While the deflection pressure becomes higher than that for streching cracks,cracks caused by hy- drafrac will extend to the other side,turning asymmetrical.When the viscosity of fracture fluid increase,injection pressure and the width of cracks rise significantly, and the length of cracks grow,while the length and the number of splayed branch cracks decline evidently, with the growth of the ratio of horizontal maxmum and minimum stress,cracks in the direction of max- mum horizontal principal stress expand while that of the minimum one seem to reduce.All suggest that a high stress ratio prevents cleat from stretching and slow down the formation of quantities of cracks,but is benefit to establishing symmetic two-wing cracks.