Calculation and Analysis for Fracture Pressure of Deep Water Shallow Formation

摘要

The deep water shallow strata has the characteristics of low overburden pressure, poor cementation properties and low fracture pressure of the formation. Therefore, the wall rupture and collapse are prone to occur. Currently, there is no perfect theoretical model for shallow formation pressure prediction. This paper assumes that the shallow strata are composed of homogeneous and isotropic ideal elastoplastic materials. An elastoplastic mechanical model of a circular wellbore is established. By introducing the theory of excess pore pressure and rock damage degree, the theoretical formula of formation fracture pressure under uniform ground stress is corrected. Verification by using actual log data. Furthermore, the law of the rupture pressure of the borehole wall with the change of the super-pore pressure coefficient and the degree of fracture coefficient is further analyzed. The results show that the well wall will be damaged to a certain extent during the drilling process, and the drilling fluid column pressure will produce squeezing effect on the well wall to cause the super-pore pressure in the shallow stratum. The wall rupture pressure decreases with the increase of the super-pore pressure coefficient and the rock fragmentation coefficient. Compared with the actual results of the field engineering, the calculation results under the model are closer to the actual situation than the traditional model calculation results, which explains the fracture mechanism of shallow water formations more reasonably. Therefore, when considering the problem of the shallow strata rupture mechanism, the relevant theory in geotechnical mechanics should be introduced for a more comprehensive analysis.