Dynamic modeling of hydraulics and well control

Successful drilling of challenging wells requires in-depth understanding of the hydraulics during all phases of the operation. The drilling process is highly dynamic and complicated to model, thus dynamics have traditionally been neglected. However, with diminishing operational margins the importance of dynamic effects is growing. Coupled with increasing well construction cost from increasing water depth or total well depth, software tools capable of modeling the dynamics have become strategically important.

In addition to the highly dynamic processes involved in challenging drilling operations, proper operational responses are also crucial in controlling the wellbore pressures within the operational window. Dynamic effects associated with pipe movement (e.g. surge and swab) effects must be taken into consideration. If the clearance between the moving pipe and the hole is small (for example, when running casing or liner), short-term pressure peaks may be generated that exceed the operational window.

Changing drilling rate, fluid density or circulating viscous pills can also lead to situations where the operational window is exceeded. These changes are not as fast as surge and swab, but may be as challenging to handle. Circulating a volume of heavy mud through the wellbore may be straightforward most of the time, but if circulated through a narrow geometry the hydrostatic head will temporarily increase and it may be enough to go above the fracture pressure.

Another important dynamic effect is wellbore temperature. This is of particular importance in HPHT wells, deepwater wells and also in deep gas wells. These offer huge potential, but at the same time represent special operational challenges. The wellbore temperature varies significantly between a static (geothermal) condition and a circulating situation. However, temperature effects may be quite slow. This can be illustrated by the time for the well to go from circulating temperature profile back to a static profile, a process that can easily take more than 24 hours. Temperature also has a major impact on fluid properties like fluid rheology and PVT properties.

Thermal effects may also complicate kick detection. As the fluid is heated up there will be flow from the well due to the thermal expansion. In cases where there is a choke for maintaining a constant surface pressure, an increase in wellbore pressure may be experienced.

All these examples can easily be modeled and handled operationally if proper planning is performed. This requires a dynamic analysis.

Basic single bubble models are frequently used in wells with reasonable margins. These models are basic and not capable of a very realistic simulation, but as long as the margins are available, they are normally conservative. When the wells become more advanced, using a basic model would result in a situation where the operation is impossible to perform. For example, the risk of well control incidents will increase with more difficult wells. Controlling such incidents may be challenging as the well conditions become more extreme. A proper multiphase model can accurately predict complex operations and also give an accurate picture of reality. From a thorough planning phase (including proper risk evaluation), it is possible to extract vital information to verify that the well can be drilled and that its gas-handling capacity and kick tolerance are within the limits of the operation.

Scandpower Petroleum Technology’s Drillbench software complies with increasingly complex and challenging demands by introducing accurate dynamic modeling, with a focus on advanced wells – inter alia extended reach drilling, deepwater drilling, managed pressure drilling, underbalanced drilling and extreme HTHP wells. Two decades of drilling research and development, and more than $20 million invested in modeling of the drilling process (including small and full scale experiments together with verification projects) has formed the basis of this industry-leading technology.

With a user-friendly interface it is easy to set up a session file in Drillbench. Input parameters are logically grouped, and graphical previews of the entered information guide the user in the specification process. Basic inputs can be found in the daily drilling report. Advanced inputs can be specified in properties windows, or the built-in software suggestions can be utilized.

Operational planning can be further improved by applying the same tools online during the operation to provide optimum decision support. E-field solutions with real time monitoring and control have grown in importance; tighter margins require close monitoring of well conditions and applying software in combination with available downhole instrumentation is the predominant means of achieving control. The models in Drillbench provide a unique basis for E-field applications. Accurate dynamic modeling of the wellbore flow process yields reliable results for monitoring and a basis for any automatic control of the operation.

By applying dynamic modeling, the drilling engineer will be able to monitor and forecast the processes with high accuracy, thus allowing the user to ensure well conditions meet the design requirements when planning and operating the most challenging and complex wells.