Tripping operations are the source of possible serious drilling incidents caused by swab and surge pressures exceeding the geo-pressure margins. Yet, it is difficult to monitor the actual downhole pressure during tripping because of telemetry limitations but also because there are no available methods to measure downhole pressure below the bit. Hydraulic models are therefore necessary to estimate the downhole pressure along the whole open hole section, also below the bit. It is therefore important to understand the factors influencing downhole pressure estimations when the drill-string is only subject to axial displacement.
In most cases tripping procedures consist in a series of drill-string movements intertwined with connection procedures. While tripping, the drill-string needs to accelerate from a stand-still position and to stop at another position. As a result of these accelerations and decelerations, hydraulic calculations must be transient. Furthermore, the movement of the bottom of the string is not synchronous with the top of string displacement because of the elasticity of the string, meaning that transient torque and drag estimations are also necessary. Actually, when using a floater, the top of string movement is influenced by heave also during connection. When the drill-string is displaced, drilling fluid needs to be added or is returned to the trip tank, depending on the direction. Consequently, the drilling fluid columns in the string and in the annulus change position affecting the downhole temperature conditions. As the fluid density and viscous behavior depends much on the local condition of pressure and temperature, heat transfer estimations also play a role on downhole pressure estimations. The evaluation of downhole pressure requires the combined estimations of transient hydraulic, mechanical and heat transfer models.
The drilling fluid is also subject to gelling in a non-uniform manner along the borehole. Drilling fluid with high viscosity at low shear rate may take several minutes to return to hydrostatic conditions like for example during connection. Depending on the static and kinetic friction factors, part of the drill-string may remain stretched also after the top of string movement has stopped. Consequently, connection time influences the swab and surge variations for the next stand. Therefore, a tripping operation is typically a non-holonomic process which requires a continuous follow-up of the time evolution of the hydraulic, mechanical and heat transfer variables to be able to predict accurately the downhole pressure.
Through several examples from actual tripping operations, the importance of the mutual interaction between transient hydraulic, mechanic and heat transfer, as well as the non-holonomy of tripping operations is shown. Understanding the factors that influence swab and surge estimations is of paramount importance to progress with the automation of tripping operations.
