A primer on underbalanced drilling

1. Drilling mud weighted with bentonite or other heavy material is used to create a hydrostatic head pressure that keeps the well under control by overbalancing the formation pore pressure. This is called "conventional drilling" today.2. In wellbore zones where the mud hydrostatic head pressure plus circulating friction overbalances the formation pore pressure, mud and cuttings flow not back to the surface but into a zone of depleted pressure and often into otherwise productive zones, plugging the porosity required for the well to be economically viable or a better producer. The driller recognizes this as "lost circulation." 3. Often, the mud escaping the wellbore presses the drillstring ...

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