Effect of Drillstring Vibration Cyclic Loads on Wellbore Stability

摘要

Wellbore instability resulted in the largest percentage of non-productive time (NPT) during drilling stage in oil industry. Wellbore instability during drilling includes: wellbore pack-off, excessive torque and drag, blowout, stuck pipe and other related well problems. Many studies assumed that wellbore instability problems were due to physical and chemical interactions between rocks and drilling fluid; and neglected impact of drillstring vibration on wellbore stability. Drillstring vibration is usually mitigated while drilling not for wellbore stability issues; but to prevent drillstring fatigue and downhole tools failure by not exceeding vibration operation limits for drillstring. This work aims to develop a new approach studies the impact of drillstring vibration on wellbore stability by analyzing different rock failure mechanisms can happen to wellbore due to drillstring vibration; and computing drillstring vibration limits (acceleration values) that can collapse wellbore. Rock failure mechanisms and drillstring dynamic behavior while vibration was studied to understand consequences happened due to drillstring collision with wellbore. Authors innovated a new model to predict drillstring vibration hotspots values that can collapse wellbore. Results show that drillstring vibration can collapse wellbore by three failure mechanisms: a- when drillstring vibration applies stresses above rock strength, rock compressive failure will take place; b- if drillstring vibration applies repeated cyclic loads on rock continuously; rock fatigue will occur; c- when cyclic loads on rock are not strong enough to cause rock failure; rock fatigue will reduce rock strength and lead to rock shear failure. Model computes drillstring acceleration values that can fail wellbore by these failure mechanisms; and calculates mud weight needed to prevent rock shear failure before and after reduction in rock strength due to rock fatigue. Model outputs must be compared with drillstring vibration operation limits; and the lower value must be used as a boundary limit for drillstring vibration while drilling to prevent wellbore collapse and drillstring failure. These will make model results used as an early detector for harmful drillstring vibration shocks on wellbore.