7000psi mud pressures at 4.5km..., page-22

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You may be right but I refer to Pascal's Law.

Pressure on a confined fluid is transmitted undiminished in every direction,and acts with equal force on equal areas,and at right angles to the container walls.

I view the inside of the drill string that contains the pumping mud as a container.The hydraulic pump at the top on the drill its self and cutting head against the rock creating the seal.It is not open to the Atmosphere