On April 4th. while on "vacation", encountered a sand storm west of Midland, texas, all the way thru New Mexico, that evening after arriving at Tucson, Arizona in shorts and short sleeve shirt felt confident I had beat the elemnts. Wrong again, next day it snowed in Tucson. Moral of the story even the best plans go wrong. ========== This story lead me in to your question: "what is under-balanced drilling?". ========== In conventional drilling you conventional hardware and a mud system designed "to avoid well blow outs". ========================== In underbalanced drilling "a rotating blow-out preventor, and "a special particle separator" are used,
"in planning ahead for a bow out to take place". ================================================ Thus minimizing formation damage. ================================= Conventional methods specially in matured areas where formation pressures are low due to depletion, formation damage can be quite extensive. Often fractures are filled with mud, and clays swellings cuts down on the potential productivity of the well, and needs further chemical treatments (acids, inhibitors, etc.) to try to clean the damage caused by the drilling system, also often fracturing of the reservoir is needed to re-open the formation and more work-over-cost are incurred to the well operator. Hope this over simplified explanation helps...!