Hydraulic Fracturing Fracture Initiation Model for a Vertical CBM Well
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摘要: 煤岩内部存在大量割理、裂隙等结构弱面,水力压裂时裂缝起裂可能发生于煤岩本体或割理裂隙处,造成其裂缝起裂机理与常规砂岩储层有较大差别,需要建立新的适用于煤层气井的裂缝起裂压力计算模型。考虑煤岩割理系统交割网状分布特性以及割理的空间位置变化关系,基于岩石力学和断裂力学理论,推导了煤层裸眼井壁和射孔孔壁周围及割理壁面处的应力分布方程;基于张性和剪切起裂条件,建立了煤岩不同完井方式下的裂缝起裂压力计算模型。应用该模型对2口压裂井进行了计算,裸眼和射孔完井裂缝起裂压力计算值与实测值之间的误差分别为3.96%和4.72%,表明该模型的计算结果与实测结果吻合较好。研究结果表明,煤层压裂能够从割理处起裂,起裂压力与煤层割理倾角、割理壁面内摩擦系数以及煤层水平主应力差值等因素密切相关。Abstract: There are many cleats, fractures and other structural weak planes in coal seams. Fractures may origin from coal body or cleat cracks during hydraulic fracturing. Consequently, mechanisms related to fracture initiation may be significantly different from those observed in conventional reservoir formations. In this regard, a new calculation model of fracture initiation pressure suitable for gas wells in coal-bed methane formations should be established. Considering the delivery network distribution characteristics of the coal seam cleat system and change of cleats in their spatial positions, the stress distribution around the perforated holes and cleats walls was determined based on the rock mechanics and fracture mechanics theory. According to conditions related to tensile and shear failures, the calculation model for fracture initiation pressure of coal was established under different well completion methods. The calculation model was used for two fractured wells and the fracture initiation pressure difference between the calculated value and the measured value in the conditions of open hole completion and perforated completion were 3.96% and 4.72%, respectively. It could be seen that the calculated results coincided well with measured values. The results showed that a seam fracture could be generated from cleats, and the fracture initiation pressures were closely related to coal bed cleat angle, coefficient of internal friction of cleat walls, coal bed horizontal principle stress differences and other factors.
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