Compensation of Surge and Swab Pressures in Floating Drilling Operations
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 108346, "Evaluation of MPD Methods for Compensation of Surge and Swab Pressures in Floating Drilling Operations," by Olve Sunde Rasmussen, SPE, and Sigbjorn Sangesland, SPE, Norwegian U. of Science and Technology, prepared for the 2007 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Galveston, Texas, 28–29 March.
One of the main concerns in deepwater drilling and drilling in depleted reservoirs is the low margin between fracture- and pore-pressure gradients. Drillstring tripping causes pressure variations in the borehole. Use of a mobile offshore drilling unit (MODU) in deep water results in even more-severe surge and swab pressures that are more difficult to control because of MODU heave motion. The full-length paper details the magnitude of surge and swab pressures that can occur in typical drilling operations, but focuses on through-tubing rotary-drilling (TTRD) operations.

Introduction
Through-tubing drilling using coiled tubing has gained acceptance as a cost-effective method for short-drainhole drilling. TTRD is more expensive but allows longer sections to be drilled, primarily because of drillstring rotation that reduces drillstring drag forces and improves hole cleaning. TTRD has been performed from fixed platforms, but rarely from MODUs. Only a few of these operations have been conducted in the North Sea. The first TTRD well drilled from a floating platform in the North Sea was drilled by Norsk Hydro in 2005. The limitation of TTRD in most wells is the small clearance between the borehole and drillstring, which causes high annular-pressure losses during drilling.
Maintaining the bottomhole pressure (BHP) within acceptable limits when drilling from a MODU is more complicated because of the motion of the MODU. The heave compensator on the MODU controls the position of the drillstring in drilling and tripping modes. However, during makeup and breakout of the connections, the drillstring is suspended in slips in the rotary table and the entire drillstring moves up and down with the heave of the MODU. This can cause high surge and swab pressures that may lead to lost circulation or influx of formation fluid, respectively. These consequences may limit the drilling length of the borehole section and may result in well-control challenges.

Surge and Swab Pressures
Surge and swab pressures take place as a result of fluid displacement caused by drillstring movement in a fluid-filled borehole. In long low-annular-clearance directional wells, mechanical friction and fluid drag will reduce and slow the motion of the lower end of the drillstring; as a result, the surge and swab pressures will be smaller than in shorter vertical wells. Calculations in the full-length paper are conservative because a closed-pipe model was used and drillpipe stretch is not considered. In a low-clearance wellbore, tripping speeds must be monitored and con-trolled to ensure that excessive surge or swab pressures are not generated.

Pressure Fluctuation in a Closed-Hole Volume Caused By Drillstring Displacement
Some managed-pressure-drilling (MPD) methods can be considered closed-loop systems. A rotating control head (RCH) allows the annulus to be pressurized while the returning fluid volume from the borehole is bled off through a choke manifold.