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doi:10.2204/iodp.proc.308.203.2008

Appendix C

Fluid pressure within the drilling pipe

To check tool performance and the pressure calibration, we made multiple 2–10 min tool stops to take the fluid pressure in the drill pipe prior to and after tool penetration. We stopped fluid circulation during tool stop to remove the effect of pump pressure on the measured pressure. Here, we present a discussion of the pressure state within the drill pipe based on the DVTPP pressure measurements.

Figure AF7 presents the fluid pressure taken at or above the seafloor. The measured fluid pressure is generally not equal to the calculated hydrostatic pressure. The tool pressure is either close to or higher than hydrostatic pressure for deployments with no drilling mud involved. However, the tool pressure can be either significantly higher or lower than hydrostatic pressure if drilling mud was used. In addition, the range of the offset value is larger than seen during deployments in water without mud. Thus, the tool-stop technique can not effectively check the pressure calibration.

Figure AF8 presents the fluid pressure taken at BOH prior to tool penetration. The fluid pressure at BOH is either close to or higher than the hydrostatic pressure for deployments with no drilling mud involved. The tool pressure is significantly higher than hydrostatic pressure if drilling mud was used. It shows a general trend where the pressure offset at the BOH increases with the depth of the borehole.

To understand the fluid pressure in the drill pipe, we present two ideal scenarios of the fluid conditions within the drill pipe and outside the drill pipe. The first scenario is one where no drilling mud was used and the seawater was not contaminated with drilling cuttings (Fig. AF9A). In this case, the fluid in the pipe is static and the pressure is equal to hydrostatic pressure everywhere. The second scenario is when drilling mud was used and the mud elevations inside and outside the pipe are at the seafloor. The fluid in the drill pipe is static and has hydrostatic pressure above the mud elevation (e.g., Stops 1 and 2 in Fig. AF9B). The fluid pressure below the mud elevation (e.g., Stop 3) is higher than hydrostatic pressure, follows the static pressure gradient of the drilling mud, and can be calculated using

P = ρwgHw + ρmg(zHw),

where

  • g = acceleration of gravity,
  • ρw = density of seawater,
  • ρm = density of drilling mud,
  • z = the target depth, and
  • Hw = depth of water at the location of the hole.

For almost all deployments, the tool pressure during tool stop reached a steady pressure within 1 min (see “Appendix A,” “Appendix B”). This suggests that fluid in the dill pipe was static during most of the time of the tool stop. It is reasonable to assume that the fluid pressure within the drill pipe was equal to the fluid pressure inside the annulus at BOH.

Figure AF10 presents three possible scenarios that could be encountered at tool stops. The first scenario is one in that no drilling mud was used; the elevation of fluid with cuttings inside the pipe is lower than that outside the pipe. The fluid elevation in the drill pipe must be above sea level to reach the static condition. ΔH can be calculated by

.

Once ΔH is determined, the fluid pressure in the drill pipe can be calculated everywhere. The offset from hydrostatic pressure is constant above the interface of seawater and the fluid with cuttings (e.g., Stops 1 and 2). Below the interface, fluid pressure follows the pressure gradient of the fluid with drilling cuttings (Fig. AF10A).

The second scenario is one where drilling mud was used to stabilize the borehole, and the mud elevation within the pipe is lower than that outside the pipe. The fluid elevation in the drill pipe must be above sea level to equilibrate with the fluid pressure at BOH. ΔH can be calculated by

.

The pressure within the pipe is higher than hydrostatic pressure everywhere. The offset from hydrostatic pressure is constant above the interface of seawater and the drilling mud (e.g., Stops 1 and 2). Below the interface, fluid pressure follows the pressure gradient of the drilling mud (Fig. AF10B).

The third scenario is one where drilling mud was used to stabilize the borehole and the mud elevation within the pipe is higher than the mud elevation outside the pipe. The fluid elevation in the drill pipe will be lower than sea level to equilibrate with the fluid pressure at BOH. ΔH can be calculated by

.

The pressure within the pipe could be either lower (e.g., Stops 1 and 2) or higher than hydrostatic pressure (e.g., Stop 3) depending on the location of the tool stop (Fig. AF10C).