Abstract

One of the main limitations in deep water drilling and drilling in depleted reservoirs is the low margin between the fracture and the pore pressure gradients. Drillstring tripping causes additional pressure variations in the borehole. The use of a Mobile Offshore Drilling Unit (MODU) in deep water results in even more severe surge and swab pressures, which are more difficult to control due to the MODU heave motion. Normally, no control of surge and swab pressure is possible during drillstring connections.

This paper presents the magnitude of surge and swab pressures that can occur in typical drilling operations, but will focus on Through Tubing Rotary Drilling (TTRD) operations. Most of the current methods for Managed Pressure Drilling (MPD) from MODUs do not have a functionality to compensate for both surge and swab pressures. A comparison and evaluation of selected MPD methods for compensation of surge and swab pressure are presented in the paper.

Introduction

In deep waters and depleted reservoirs, the margin between the pore and fracture gradient could be extremely narrow. This characteristic introduces a demand for accurate wellbore pressure prediction and control.

To date, Through Tubing Drilling (TTD) using Coiled Tubing (CT) has gained acceptance as a cost effective method for short drainhole drilling. Through Tubing Rotary Drilling (TTRD) is more expensive, but allows longer sections to be drilled principally due to drill string rotation which reduce drill string drag forces and improves hole cleaning 1,2. TTRD has been performed from fixed platforms but rarely from mobile offshore drilling units (MODU). Only a few of these operations have been conducted in the North Sea. The first well was drilled by Norsk Hydro in 20053. However, the limitation of TTRD in most wells is the small clearance between the borehole and drillstring which causes high annular pressure loss during drilling.

Maintaining the bottomhole pressure (BHP) within acceptable limits when drilling from a MODU is more complicated due to the motion of the MODU. The heave compensator located on the MODU controls the position of the drillstring in drilling mode and also in tripping mode. However, during make-up and break-out of the connections, the drill string is suspended in slips in the rotary table and the entire drillstring moves up and down in conjunction with the heave of the MODU. This can in turn cause high surge and swab pressures that may lead to lost circulation or the infux of formation fluid. These consequences may limit the drilling length of the borehole section and may also result in well control challenges.

Several methods for Managed Pressure Drilling (MPD) have been suggested and some of these are already commercially available to the industry 4,5. These methods allow a more precise control of the borehole pressure.

The paper presents a comparison and evaluation of the different MPD methods for compensation of surge- and swab pressure.

Surge & Swab Pressures

Surge and swab friction pressures take place due to displacement of fluid caused by drillstring movement (piston effect) in a fluid-filled borehole.

Typical well and drillstring parameter data for a TTRD and a conventional well are presented in Table 1 and Table 2. Mud rheology data are shown in Table 3. For simplicity, a vertical well has been used in the case example. All surge and swab pressure calculations are based on a steady state model 6.

Table 4 shows surge and swab pressures generated by heave motion of the MODU for a range of heave scenarios. The conventional well case provides only a slight reduction in pressure variation compared to the TTRD case. In a normal drilling operation, a float sub will prevent flow into the drill string from below. However, when the pipe is moved out of the borehole, some of the content of the drill string is likely to flow into the annulus. This gives slightly less swab pressure compared to a closed pipe.

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