On the Influence of the Equivalent Circulent Density Fluctuations on the Development of the Wellbore Natural Fracture

Abstract Wellbore instability is one of the most important causes of Non-Productive Time during drilling operations causing billions of dollars of losses every year. During the drilling stage, the Equivalent Circulent Density (ECD) is subjected to fluctuations caused by some factors such as the drill string vibrations cyclic loads. The fluctuating ECD applied on the fractured formation progressively modifies the initial parameters of the fractured formation such as its length and its width and this process finally results into wellbore instability. In this research, a poroelastic model based on a finite element method has been established to analyze the influence of the drill string vibration cyclic loads on the development of the wellbore natural fracture. The analysis was conducted with a two-dimensional plane strain model. A traction-separation law based on energy has been proposed for the Cohesive Zone Model. A nonlinear finite element software ABAQUS was utilized as the numerical simulator. The numerical results showed that the profiles of the fracture width as a function of time follow a sinusoidal behavior similar to the behavior of the drill string vibration cyclic loads profile. For different values of the Weight On Bit (WOB) and constant drill string Revolution Per Minute (RPM), an increase of the fracture width with the fracture length is observed in the near wellbore region. In the region far away the wellbore, the fracture width globally decreases with an increase of the fracture length for each fracture profile. the investigation of the effect of some drilling operational parameters on the development of the wellbore natural fracture also demonstrated that the drillstring vibration cyclic loads lead to an increase of the fracture length, fracture width, the loss circulation and the Bottom Hole Pressure. This study couples the integration of the fracture rock development with the continuous cyclic load generated by drill string vibrations. This aspect has been rarely discussed in the literature. The study indicates that the cyclic loads significantly affect the development of the wellbore natural fracture during drilling operations, and therefore has an important impact on the wellbore stability analysis.