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Under balanced Drilling

Underbalanced drilling operations reduce the hydrostatic pressure of the drilling fluid column so that the pressure in the well bore is less than the formation pressure.

Is defined as the practice of drilling a well with the wellbore fluid gradient less than the natural formation gradient. It differs from conventional drilling in that the bottomhole circulating pressure is lower than the formation pressure, thereby permitting the well to flow while drilling proceeds.

Besides minimizing lost circulation and increasing the penetration rate, this technique has a widely recognized benefit of minimizing the damage caused by invasion of drilling fluid into the formation. In many UBD applications, additional benefits are seen due to reduction in drilling time, increased bit life, and early detection and dynamic testing of productive intervals while drilling. It is critical to keep the well underbalanced at all times, if formation damage is to be minimized.

Underbalanced drilling technology is a valuable method for minimizing formation invasion related problems. Because the majority of hydrocarbons today are found in existing fields with depleting pressures, or in complex and low quality reservoirs, the economical use of UBD becomes more and more popular.

Most of the underbalanced drilling applications today are conducted through the use of coiled tubing systems. Forty percent of all the onshore wells drilled in the year 2000 were conducted through underbalanced conditions. Joint industry projects currently underway off the coast of Brazil will likely change the conventional drilling practices in offshore applications.

"Underbalanced drilling technology can save the industry millions of dollars by increasing the amount of recoverable oil within a shorter time frame".

Underbalanced drilling offers a number of important benefits:

Maintaining wellbore pressure below the reservoir pressure allows reservoir fluids to enter the wellbore, thus avoiding formation damage. Since significant formation damage is avoided, the stimulation requirements during well completion are also reduced, leading to considerable savings.

During underbalanced drilling there is no physical mechanism to force drilling fluid into the formation drilled. Therefore, lost circulation is kept to a minimum when fractured or high permeability zones are encountered.

Drilling underbalanced can help in detecting potential hydrocarbon zones, even identifying zones that would have been bypassed with conventional drilling methods.

Due to the decreased pressure at the bit head, UBD operations demonstrate superior penetration rates compared to conventional drilling techniques. Along with reduced drilling times, an increase in bit life is typically reported.

Since there is no filter cake around the wellbore wall, the chances of differential sticking are also reduced.

Since conventional drilling fluids are not used in underbalanced drilling applications, there is no need to worry about disposing potentially hazardous drilling mud.