Figure F20. Pressure versus depth plot illustrating the effect of drilling with weighted mud. Normally the JOIDES Resolution drills with seawater. Thus, the borehole pressure will follow the hydrostatic pressure. If the formation is overpressured (the pore pressure is greater than the hydrostatic pressure at any depth), permeable, and unconsolidated, there is danger that there will be flow of water and sand into the borehole. To counter this effect, weighted mud was used during drilling. A 10.5 ppg mud weight raised the formation pressure to approximately one-third of the way between hydrostatic and lithostatic, whereas a 12.5 ppg mud would raise the borehole pressure to more than half of the distance from hydrostatic to lithostatic pressure.
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