基于断裂力学的煤层气井破裂压力计算方法

Calculation method for fracture pressure of coalbed methane well based on fracture mechanics

为了提高煤层气井破裂压力预测精度,运用断裂力学理论推导裂缝性地层破裂压力计算公式,结合分形岩石力学理论建立裂缝性地层破裂压力解析模型,提出一种实用的破裂压力计算方法.结果表明:在非均匀地应力下,煤岩井壁更易沿最大水平主应力方向起裂,解析模型与传统模型在表达形式上具有统一性和更广泛的适用性.煤岩裂缝具有分形特征,分形效应提高岩石阻碍裂缝扩展的能力,当分形维数越大、相似比越小时破裂压力越大.在不同裂缝状态下,缝长对破裂压力的影响规律不同,当裂缝长度较小时破裂压力随裂缝长度增加而迅速减小;当裂缝长度增加到一定数值后,闭合裂缝破裂压力随缝长增加而增大,开启裂缝破裂压力随缝长增加而减小并趋于定值.郑庄地区5口煤层气井实例计算表明:该计算方法预测的破裂压力误差在10%以内,破裂压力对缝长变化特别敏感,合理确定裂缝长度对准确预测破裂压力尤为重要.

In order to improve the prediction accuracy of the coalbed methane well(CBM)fracture pressure,the fracture pressure calculation formula of fractured formation is derived by fracture mechanics,the analytical model of fracture pressure in fractured formation is established based on fractal rock mechanics,and a practical calculation method for fracture pressure is obtained.The result shows that the wellbore would be more likely to be fractured along the direction of maximum horizontal stress under non-uniform in-situ stress.The analytical model has expression uniformity with the traditional model and a wider applicability.Coal fracture has fractal characteristics,the fractal effect enhances the ability of rock to hinder crack propagation,the greater the fractal dimension is,the smaller the similarity ratio and the greater the fracture pressure is.The influences of crack length on fracture pressure are different when the crack states are different,when the crack length is small,fracture pressure decreases rapidly with the increase of crack length.After the crack length increases to a certain value,fracture pressure of closed fracture increases with the increase of crack length,fracture pressure of open fracture decreases and tends to constant value with the increase of crack length.Example of 5CBM wells in Zhengzhuang block shows that the error of this method is less than 10%,fracture pressure is very sensitive to crack length,a reasonable choice of crack length is particularly important to predict fracture pressure accurately.