深水浅部非均匀受力地层破裂压力预测模型

Prediction Models for Fracture Pressure of Deep Sea Shallow Formation under Non-uniform Stress

深水浅部地层尚未固结成岩,上覆岩层压力较低,破裂压力也较低,发生井壁破裂与坍塌的风险较高,目前尚无完善的力学理论来预测深水浅层的破裂压力。为此,引入黏土力学超孔隙压力及其破碎性系数来对井眼周围有效应力进行修正,建立非均匀受力的圆形井眼模型、椭圆形井眼模型及其圆形井眼弹塑性力学模型,结合弹性力学理论、塑性力学理论、屈服条件及黏土力学理论,推导出深水浅层非均匀受力情况下的破裂压力计算公式,通过实际测井数据及文献数据完成模型验证,进一步对破裂压力影响因素进行分析。结果表明,深水浅部非均匀受力地层破裂压力受孔隙压力系数、岩石破碎性系数、椭圆长短轴比等因素的影响,孔隙压力系数与岩石破碎性系数越大,破裂压力越小;椭圆长短轴比越大,破裂压力越大,当长短轴比为1.0时,破裂压力最小。

Deep water shallow formation is unconsolidated yet under low overburden pressure and fracture pres-sure,so it has a higher risk of fracture and collapse of wellbore.There is no perfect mechanical theory for predicting the fracture pressure of the deep water shallow formation at present.Excess pore pressure and its fragmentation coefficient of clay mechanics was introduced to correct effective stress around the borehole,and a circular borehole model,an ellipse borehole model and an elastic-plastic mechanics model with circular borehole under non-uniform stress were set up.Combined with the theory of elastic mechanics,plastic mechanics,yield conditions and clay mechanics,the fracture pressure calculation formula for the deep water shallow formation under the non-uniform stress has been derived.The model was verified through the actual logging data and literature data,and influencing factors of the fracture pressure were analyzed further.The results show that the fracture pressure of the deep water shallow formation under non-uniform stress is affected by pore pressure coefficient,rock fragmentation coefficient,ellipse long and short axis ratio and other factors： the greater pore pressure coefficient and rock fragmentation coefficient brings the smaller fracture pressure;the greater ellipse long and short ratio brings the greater fracture pressure,and the fracture pressure is the smallest when the axis ratio is 1.0.