Improved Bottomhole Pressure Control for Underbalanced Drilling Operations

Abstract

Abstract Maintaining underbalanced conditions from the beginning to the end of the drilling process is necessary to guarantee that underbalanced drilling (UBD) operations successfully avoid formation damage and potential hazardous drilling problems such as lost circulation and differential sticking. However, maintaining these conditions during operations with jointed-pipe is an unmet challenge that continues motivating not only research but also technological developments. This paper proposes an improved UBD flow control procedure as an economical method for maintaining continuous underbalanced conditions in jointed-pipe UBD operations by maximizing the use of natural energy available from the reservoir through the proper manipulation of nitrogen and drilling fluid injection flow rates and choke pressure. It is applicable to wells that can flow without artificial lift and within appropriate safety limits. The flow control procedure is based on the results of a new comprehensive, mechanistic steady-state model, validated with both field data and full-scale experimental data, and on the results of a simplified, time dependent, mechanistic model, which numerically combines the accurate mechanistic, steady-state model, the conservation equations approximated by finite difference, and a well deliverability model. Introduction Even though the underbalanced drilling technique has proven itself to be successful in minimizing some drilling operating problems and reducing drilling time1, it has been recognized by the petroleum industry that its greatest advantage is to increase well productivity by avoiding formation damage during the drilling process. It is also becoming more recognized that the success of an UBD operation is dependent on maintaining underbalanced conditions during the entire drilling process. Unfortunately, during jointed-pipe UBD operations, the surface injection must be interrupted every time a connection or trip is needed. This stopping of injection causes a disruption of steady state conditions. Since this phenomenon occurs each time a connection takes place, and the time between drilling and connections is typically insufficient to regain steady state conditions, UBD pipe connection operations trigger a bottomhole pressure (BHP) fluctuation. If this fluctuation is not properly controlled to keep BHP below the formation pressure, the formation will be exposed to an overbalanced condition every time a connection or trip takes place. Additionally, this BHP fluctuation can mechanically destabilize the formation and degrade the performance of the rotating head rubbers2. Therefore, these unsteady state conditions can ruin or reduce the advantages obtained after making the efforts and expenses to drill the well underbalanced1–5. Literature Review The use of different drilling systems, such as snubbing and coiled tubing units, has been attempted as potential solutions to achieve 100% underbalanced conditions; however, their success has been limited to specific conditions6,7. Different gas injection techniques (parasite tubing string or parasite casing string) have only partially reduced the BHP fluctuation, but at a very high cost (additional gas injection, extra casing or tubing string, etc.)6. Also, there are some new and emerging technologies that could be used to better manage the wellbore pressure during jointed-pipe UBD operations (the Closed Loop Continuous Circulation System and the Equivalent Circulation Density Reduction Tool)8,9. However, these partially proven technologies are expensive, limiting their application to high productivity wells where the use of such technology is judged profitable.