水平井多段压裂应力场计算新模型

为了准确认识水平井多段压裂过程中井筒周围应力场变化，指导裂缝起裂和延伸机制研究，综合考虑井筒内压、地应力、压裂液渗流、热应力、射孔和先压开裂缝等因素的影响，根据应力叠加原理，建立了水平井多段压裂应力场计算模型。该模型利用位移不连续法，重点考虑了先压开裂缝在水平井筒周围水平面上产生的诱导应力大小，克服了裂缝诱导应力解析模型只能计算裂缝高度平面诱导应力大小的局限。结果表明，通过应力场模型进行的破裂压力预测与实际破裂压力值吻合度较高，验证了建立模型的准确性；人工裂缝形成后会对水平井筒周围应力场造成很大影响，在裂缝尖端出现应力集中，造成应力分量降低，在裂缝周围一定区域造成应力分量有较大的升高；多段压裂压开人工裂缝越多，应力之间干扰越严重，水平井筒周围应力分布越复杂。该研究对认识多段压裂存在先压裂缝干扰下后续裂缝的起裂和延伸机制，指导压裂施工具有一定意义。

A new calculation model for stress field due to horizontal well staged fracturing

A calculation model for stress field due to horizontal well staged fracturing is proposed according to stress superposition principle. The model considers the influence of the wellbore pressure, in-situ stress, fracturing fluid seepage, thermal stress, perforation and artificial cracks. It can accurately adapt to the change of stress field around the wellbore in the process of staged fracturing in horizontal wells, and guide the research of the fracture initiation and extension mechanism. The model focuses on the induced stress caused by artificial cracks in the plane around the horizontal wellbore with the method of the displacement discontinuity. The model can also overcome the limitation of ordinary induced stress analytical models which can only calculate the induced stress in the fracture height plane. Results show that the fracture pressure calculated with the proposed model is consistent with the actual fracture pressure, which demonstrates the accuracy of the model. The artificial fractures can cause great impact on stress field around the wellbore. The stress concentration at the crack tips can result in the reduction of stress components and the obvious increasing of the stress components around a given area of fractures. Meanwhile, the more artificial fractures are fractured, the more serious the stress interference is and the more complex the stress distribution around wellbore is. The research has significance in analyzing the crack initiation and propagation mechanism under the influence of crack interference and provides guidance to fracturing design as well.