Abstract
Wellbore instability is one of the most serious issues faced in the drilling process. During drilling operations, the cyclic loads applied on the fractured formation progressively modify the initial parameters (i.e., length and width) of the fractured formation, thus resulting into undesirable wellbore instability. In this paper, using a nonlinear finite element software (ABAQUS) as the numerical simulator, a poro-elasto-plastic model has been established which aimed at analyzing the influence of drill string vibration cyclic loads on the development of the wellbore natural fracture. The numerical results showed that the fracture width as a function of time profiles followed a sinusoidal behavior similar to the drill string vibration cyclic load profiles. For different cyclic load magnitudes with constant number of cyclic loads, the highest percentage increase of the fracture width after integration of cyclic loads was 64.77%. Interestingly, the fracture width increased with the fracture length in the near wellbore region while it globally decreased in the region far away from the wellbore. But for constant cyclic load magnitude with different number of cyclic loads, the biggest percentage increase of the fracture width after integration of cyclic loads was slightly lower representing 63.12% while the oscillating period of the fracture width increased with the number of cyclic loads. The parametric study revealed that the drill string vibration cyclic loads were relatively independent of the fracture length and the bottom hole pressure. However, the fracture width and the loss circulation rates were considerably impacted by the drill string vibration and the highest percentage increase of the loss circulation rate after integration of cyclic loads was 14.3%. This study provides an insight into the coupling of the fracture rock development and the continuous cyclic loads generated by drill string vibrations which is an aspect that has been rarely discussed in the literature.
Highlights
This article is an open access articleThe conventional methods used to assess the wellbore stability analysis generally assume that the loads acting on the wellbore are in steady-state [1,2,3]
Ref. [8] after conducting fatigue tests experiments concluded that: (i) if the stress exerted by the cyclic loads on the rock formation was above 87% of the rock formation collapse strength, the rock collapsed within 100 cyclic loads, (ii) when the stress generated by the cyclic loads acting on the rock formation ranged between 75% to 87% of the rock formation collapse strength, the rock formation was weakened but the rock itself remained uncollapsed, and (iii) if the stress applied by the cyclic loads on the rock formation was below
For different values of the drilling operational parameters weight on bit (WOB) and revolutions per minute (RPM), the number of cyclic loads N and the magnitude of the stress vibration S hitting the wellbore surface can be determined respectively using equations (48) and (49), the stress exerted by the drill string vibration cyclic loads on the wellbore surface is totally defined since the initial phase of the vibration φ is assumed to be zero
Summary
The conventional methods used to assess the wellbore stability analysis generally assume that the loads acting on the wellbore are in steady-state [1,2,3]. [6] were among the first researchers to investigate the rock failure under dynamic conditions They found the critical rotary speed at which the fluctuating axial loads applied on the drill string will induce excessive wellbore enlargement. [12] investigated the effects of the drill string vibration cyclic loads on the wellbore stability and they obtained slightly similar results with those of [8]. To successfully seal the fractures and prevent loss circulation, it is vital to accurately predict the growth of the natural fractures especially when the wellbore pressure is subjected to fluctuations caused by some factors such as the drill string vibrations cyclic loads. Due to the aforementioned reasons, a plane strain poro-elasto-plastic finite element analysis is carried out in this paper to investigate the influence of the drill string vibrations cyclic loads on the development of the wellbore natural fracture rock. Where, ∆P is the pressure difference between P1 and P2 , Pa; ∆Phydro is the hydrostatic drilling fluid pressure between P1 and P2 , Pa; ∆Ploss is the viscous pressure loss, Pa; ρ is the drilling fluid density, kg/m3 ; g is the gravitational acceleration, m/s2 ; ∆Z is the elevation difference between P1 and P2 , m; V is the fluid velocity in the pipe, m /s; cl is the fluid loss coefficient
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
