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 against the walls of the bore causing the drill pipe to become stuck from the pressure differential across its length at that depth, and becomes "differentially stuck." The fracture pressure of the wellbore has been exceeded.
4. The drilling industry has recognized that the heavier the drilling fluid and cuttings, the lower the rate of penetration (ROP) of the drill bit, adding significant cost to drilling.
5. As more of the world's hydrocarbon zones have become pressure depleted and as the desire to drill in ever deeper waters grows, tools and technology have been developed that permit a lighter-than-conventional drilling fluid to be used with good well control.
6. A lighter-than-conventional drilling fluid or mud can be achieved by adding less weighting material when mixing the mud. Engineers also use foams, mists, air, and fluids, where measured amounts of nitrogen are added to achieve a preestimated degree of under balance that is a safety factor within the pressure and volume limits of surface equipment being used at the moment.
7. Drilling with a fluid (mud) that imparts less hydrostatic head pressure than the natural pore pressure of the formation being drilled is "underbalanced drilling." Hydrocarbons, if present, will flow to the surface while drilling.
8. A well being drilled underbalanced is controlled by the weight of the column of lighter-than-conventional mud and cuttings in the wellbore, aided by rotating control heads, choke manifolds, and other specialized surface equipment that permits a backpressure to be placed upon the annulus of the wellbore as dictated by the well's tendency to kick or lose circulation.
9. Conventional overbalanced drilling relies primarily upon a heavy mud within an open-to-the-atmosphere annulus returns system to overbalance or control the well.
10. Underbalanced drilling (UBD) relies upon the weight of a lighter drilling fluid in the wellbore plus a closed annulus returns system that permits the operator to choke off the annulus returns. This adds the effective wellbore pressure, as required while drilling ahead, to control the amount of hydrocarbons that may tend to flow up the annulus of the well to the surface.
11. UBD's closed-system drilling enables the safe use of a lighter-than-conventional drilling mud, thus avoiding loss circulation and differentially stuck pipe issues. Wells are better producers because production zone porosity is not plugged with heavy mud and cuttings. Wells are drilled faster because of the higher ROP of the drillbit.
12. The surface equipment that enables "backpressure" is critical to safely drilling into a zone that has greater pressure than the head pressure of the mud being used at the moment.
13. Equipment developed for the safe practice of underbalanced drilling in the marine environment includes several offshore variations of rotating control heads applicable to rigs with surface and subsea blowout prevention stacks, choke manifolds, surface separators, means of handling produced hydrocarbons, membrane nitrogen generation systems, foams and drilling fluids chemistry, downhole deployment valves, wireline retrievable drillstring check valves, ECD reduction tool, continuous circulation systems, and flow modeling required to estimate pressure and flow rates.
14. UBD technology adds recoverable hydrocarbon assets.
15. Depletion of natural formation pressures dictate that the world's last economically viable well will have to be drilled or intervened using underbalanced drilling technology. Fortunately, and unlike the world's first well, tools and technology have been developed that provide positive HSE consequences.