考虑裂缝特性指标的煤层气“垂直井”起裂压力计算方法

Calculation of initiation pressure of vertical well for coalbed methane considering crack characteristic index

为探究碎裂煤储层中裂缝特性指标对垂直井起裂压力的影响规律,解决垂直井起裂压力的理论计算问题,首先基于断裂力学和弹性力学原理,考虑原生裂缝特性和压裂液渗透特性,建立了垂直井起裂压力模型。其次定量化分析原生裂缝特性对起裂压力的影响,提出了"裂缝特性指标T"的概念,并推导出T的数学表达式。最后对起裂压力做了原生裂缝特性参数敏感度分析,得出影响起裂压力的最敏感性因素为裂缝宽度b,且当b在100~700μm区间内时地层的裂缝特性指标对起裂压力有实际意义,解决了压裂目标层的选择问题。研究结果表明,垂直井起裂压力的影响因素不仅与储层岩石力学性质、应力场、压裂液性质、井孔尺寸有关,还与原生裂缝的宽度、长度、密度以及裂缝面的粗糙度有关,尤其是裂缝宽度的小范围波动都会引起垂直井起裂压力较大的变化。根据焦作矿区恩村5口试验井田实际地质勘查资料和压裂施工资料,对垂直井起裂压力进行了计算和验证,发现实际起裂压力与大部分计算结果较为吻合,且与井筒埋深的变化趋势一致,表明该理论模型对于垂直井起裂压力的实际控制具有一定的指导意义。

In order to explore the influence of fracture characteristics index on crack initiation pressure in vertical well and solve the problem of theoretical calculation about vertical well initiation pressure. First of all, the vertical well initiation pressure model is established based on the principles of fracture mechanics and elastic mechanics, considering the characteristics of primary fractures and fracturing fluids. Secondly, the influence of primary crack characteristics on initiation pressure is analyzed quantitatively; meanwhile, proposing the concept of "crack characteristic index control coefficient T" and deriving the mathematical expression of T. At last, sensitivity analysis of the primary crack characteristic parameters is made for the initiation pressure. It is concluded that the most sensitive factor affecting the initiation pressure is the crack width b. When b is in the range of 100-700 μm, the fracture characteristics of the formation have the practical significance for the initiation pressure, which solves the problem of the choice of fracturing target layer. The results show that: The influence factors of vertical well initiation pressure are related not only to the mechanical properties of the source rock, the stress field, the fracturing fluid properties and the well size, but also the width, length, density and roughness of the crack. Especially the small fluctuations in the width of the crack will cause vertical wells crack pressure changes. According to the actual geological prospecting data and fracturing construction data of the 5 test wells in Jiaozuo mining area, the vertical well initiation pressure is calculated and verified. It is found that the actual initiation pressure is in good agreement with the calculated results. It is shown that the theoretical model has some guiding significance for the actual control of the vertical well initiation pressure.