Surge and swab pressure

Abstract

Shell is working on mono-diameter drilling (MOD). A MOD well, or MOD well section, has a (nearly) constant diameter over several sections. This is achieved by deploying expandable liners: a liner is lowered into position and is expanded by pulling an over-sized cone through it. This plasticly deforms the pipe, increasing its inner diameter to that of the previous section. MOD eliminates many of the constraints of conventional well design. Greater depths may be reached and zonal isolation may be performed without paying the penalty of reduced production rates.

While lowering an expandable assembly (expandable liner and expansion tools) into a well, caution is required. Displacement of the drilling mud causes a pressure drop due to fluid friction and acceleration. The running speed of the assembly into the well is limited by the surge (overpressure) or swab (underpressure) pressure. Pressure must remain within a predefined window to ensure well control.

The current understanding of the mechanisms governing surge/swab pressure, as well as the models used to predict it, are deemed insufficient. The effect of unorthodox MOD conditions is unknown. This study is conducted to overcome these obstacles.

Firstly, a model is programmed so that transient surge and swab pressures, induced by assembly movement, may be predicted. The drilling mud’s velocity and pressure are described by the water hammer equations, which are solved using the interpolated method of characteristics. One dimensional (1D), unsteady and nonuniform flow of a slightly compressible fluid in a conduit with linearly elastic walls is considered. The annular pressure drop for flow with a moving inner pipe is solved considering both laminar and turbulent flow of a Herschel-Bulkley fluid. Flow over the expansion tools is solved separately and is considered 1D and incompressible.

Secondly, the model is validated. The model is first validated under conventional conditions using field measurements presented in literature and performs better than existing models. The validity of the model while dealing with extremely small clearances, as found in MOD applications, is investigated through a full-scale experiment on Shell’s test rig. The surge and swab pressures are generally predictable. Some discrepancies between predictions and measurements are observed and are attributed to the relatively large uncertainty in clearance size and shape, which results from imperfect pipes. The surge/swab pressure’s sensitivity to the size and shape of the pipes means that predictions should be treated with care. The effect of local annular flow restriction over the expansion tools is investigated too. The tools are found to cause acceleration induced transient pressures not predicted by the model. This implies that the 1D and/or incompressible assumption at the interface does not describe the flow adequately.

Thirdly, the effect of MOD well design at true well depths and assembly lengths is investigated through a case study. The surge pressure is significant. The pressure drop generated in the narrow annulus between assembly and wellbore is too high to allow significant flow. Instead, all drilling mud is displaced upwards through the drillstring. Swabbing occurs behind the assembly. Circulation while tripping in expandable assemblies is therefore impossible. At these depth and length scales, the effect of fluid compression and borehole expansion becomes significant, causing steady state models to become increasingly inaccurate with depth.