Pressure compensation and rotary seal system for measurement while drilling instrumentation

A pressure compensation system for a wellbore instrument coupled to a drill string, the instrument including a shaft rotatably mounted with respect to an instrument housing, the system comprising:

• a lubrication chamber disposed in an annular space between the housing and the shaft, the lubrication chamber including therein at least one bearing for rotatably supporting the shaft, the lubrication chamber including a face seal coupled on one face to the shaft and on another face to the housing;
• a pressure compensator in hydraulic communication between the lubrication chamber and the interior of the drill string, the compensator including a barrier to fluid movement between the lubrication chamber and the interior of the drill string, the barrier enabling pressure communication therebetween, the compensator including a pressure communication port extending between the barrier and a portion of the shaft exposed to the interior of the drill string, wherein the pressure compensator comprises a removable enclosure configured around the shaft and configured to be removed from around the shaft when the shaft is disassembled from the housing such that access to the removable enclosure is obtained without removing the shaft from the housing, the removable enclosure having a bladder;
• and
• a valve disposed in an interior passage of the shaft, wherein the valve closes upon removal of an upper portion of the shaft.