煤层气直井地层破裂压力计算模型

为了高效开发煤层气、成功实施水力压裂作业，针对射孔完井条件下，对水力压裂时煤层的破裂压力进行预测。提出将套管、水泥环的影响纳入到井筒周围应力场计算中，运用坐标转换和线性叠加原理，建立了考虑套管、水泥环影响的煤层直井井周应力场解析模型。结合最大拉应力、张性破坏、剪切破坏3种破裂准则，建立了考虑套管、水泥环影响的煤层气直井破裂压力预测模型，分析了射孔径向深度、割理走向倾角、地应力差对破裂压力的影响。以云南地区X井为例，模型计算结果误差在7%以内，证明了该模型的正确性。研究结果表明：套管、水泥环会引起应力集中效应，使得破裂压力降低；在最大水平主应力方向射孔时，煤层更容易本体破裂；射孔周向角在一定区间时，本体破裂和割理张性破裂同时发生；中等割理走向倾角有较低的张性破裂压力，地应力差较小时，不易发生剪切破裂。

A model for calculating formation breakdown pressure in CBM vertical wells

To effectively develop coalbed methane and ensure successful hydraulic fracturing operation, the breakdown pressure of coal formation under hydraulic fracturing is predicted under the condition of perforated completion. The influence of casing and cement sheath is considered in the calculation of stress field around the wellbore. Based on the principle of coordinate transformation and linear superposition, an analytical model of stress field around the vertical well of coal seam is established considering the influence of casing and cement sheath. In combination with three kinds of breakdown criteria, i.e., maximum tension stress, tension fracture and shear fracture, a breakdown pressure prediction model of coal-seam vertical well is established considering the influence of casing and cement sheath. In this study, the influences of perforation radial depth, cleat striking angle and geo-stress difference on breakdown pressure are analyzed. Taking a well in Yunnan province an example, the calculation error is less than 7%, which proves the correctness of this model. The results show that the casing and cement sheath can cause the stress concentration effect and reduce breakdown pressure. In the case of perforation in the direction of maximum horizontal principal stress, the coal seam itself is more likely to be cracked. In the case of perforation circumferential angle in a certain interval, the coal seam fracture and cleat tensile fracture are simultaneous. When the medium cleat strike angle has a lower tensile breakdown pressure with less geo-stress, shear fracture is not prone to occur.