Analysis and Research on Influence Factors of Leakage Caused by Surge Pressure

Post-accident tracing is necessary for risk assessment and prevention in oil drilling industry. The blowouts of the wellbore were triggered by falling of broken drill string, which generated high surge pressure at the bottom hole and induced fractures with the formation. Because pre-kick condition of the wellbore is unclear, it is necessary to ascertain the condition by calculating key factors such as surge pressure and fracture width. Calculations in this research are based on Herschel-Bulkley’s model for non-Newtonian fluid and Lietard’s model for one-dimensional radial loss. It is concluded that the 4,500 m-long drill string may have generated more than 20 MPa of surge pressure as it fell to the bottom hole and could have caused over 20 m3 of drilling fluid loss over 7–8 min. In addition, the research also assessed risks for the wellbore and estimated time allowance for responses under different levels of surge pressure and fracture width. It finds that under high level of surge pressure, a fracture wider than 10 mm can instantly take in tens of cubic meters of drilling fluid and make the wellbore totally uncontrollable, but a mild fracture no wider than 0.25 mm may need about 10 h to consume 20 m3 of drilling fluid. At the width of 1 mm, a fracture will take in 20 m3 of mud over about 6 min, a situation that most matches the condition of the accident wellbore.