Abstract
A new concept and process for real-time monitoring and control of wellbore stability establishes the drilling parameters required to optimize the drilling process and thereby reduce the potential for wellbore instability and subsequent unscheduled events or lost rig time. Surface and downhole measurements, recorded while drilling, are used to make regular updates to a model of the wellbore and to revise the drilling plan accordingly.
The first step in the process is the generation of a mechanical earth model (MEM) using information obtained in offset wells and field and regional data. The proposed well trajectory for a new well is projected into the MEM and a set of stability parameters is generated for a given initial drilling plan. The product identifies potential danger zones within a well plan.
During drilling, real-time data, including logging-while-drilling (LWD), measurements-while-drilling (MWD), surface mechanical measurements, and fluids and solids monitoring information, are used to diagnose the state of the wellbore. Any significant hole instability is detected and a warning is given to the driller. The state of the wellbore is compared to the model, and any revision required to align the predicted with the actual state is made. This real-time update of the mechanical model is then used to predict the future state of the wellbore, in front of and behind the bit, for the given drilling plan. If the drilling plan can be improved, a revision will be recommended; for instance, reduction in the rate of penetration, increase in mud weight and circulation, and change in hole direction. The drillers can independently evaluate their own recommendations for changes to the drilling plan and then decide on the best course of action. The process also provides a record of wellbore stability information that can be input to the field description for use in future wells and continuous improvement of the drilling process.
Use of this concept was validated on the Valhall field in the Norwegian sector of the North Sea. Extended-reach drilling (ERD) to downflank targets has been problematic in recent years; there is a high risk that wells will be suspended or abandoned because of problems associated with wellbore instability in this very weak overburden.
The Real-Time Wellbore Stability Control (RTWBSC) project team produced an MEM for the Valhall field, working closely with the drilling engineers to develop a well plan for a proposed ERD well. Implementation involved providing wellsite support to coordinate monitoring and detection of wellbore instability from real-time data, and on-line support in the drilling office to interpret data, update the MEM and revise the well plan. Through this process the team proposed and implemented a strategy of drilling the well in controlled states of failure—not a conventional drilling approach. The well successfully reached its target ahead of schedule and a planned string of intermediate casing was not required, mud losses (a previous problem contributing to instability and cost) were minimal and the well was cased to below the unstable overburden intervals.
