Well Control: Issues of Under Balanced Drilling
Authors: Tangedahl Michael J. 
Published: December 2007

Horizontal or vertical wells drilled underbalanced or while flowing must have surface equipment and a blowout preventer stack specially designed for circulating operations.
Horizontal wells can be drilled overbalanced, near balanced or underbalanced. In certain cases, conventional well control techniques do not always apply. During drilling of the lateral section of a horizontal well, the primary means of well control (hydrostatic fluid head) may be ineffective because of abnormal pressures, formation seepage, lost circulation, or underbalanced drilling practices.
The calculated risks in horizontal drilling can be overcome by using advanced well control techniques. The best safety insurance is the use of the safest well control equipment and methods available.
Some horizontal wells have had drilling fluid losses at rates up to 100 gpm with 800 psi or more well bore pressure at the surface. On some of these wells, the operators saved up to $200,000 in drilling fluid costs because they used a rotating blowout preventer (RBOP) and drilled underbalanced.
Horizontal flow drilling and underbalanced drilling are different methods of drilling. The lateral section of a horizontal well can act as a gas separator. Although the vertical column of drilling fluid may provide a calculated overbalanced or near-balanced head, migrating gas can push large slugs of oil at significant velocities to the surface. The horizontal driller must evaluate the possible dynamics of a well during planning for well control and blowout contingency.
Functional well control methods for drilling horizontal wells have been developed in specific regions worldwide. Special safety equipment and procedures, however, are still required in most horizontal development applications.
The challenge for underbalanced drilling development is to overcome the obstacles of government regulation, reduce pollution dangers, and improve personnel and equipment safety. Well control techniques tailored to the demands of each field can help overcome these challenges.
Several well control elements must be addressed carefully on each underbalanced well:–Drilling fluid requirements–Well control procedures and equipment–Surface equipment and special considerations for handling hydrocarbons produced while drilling.
Additionally, personnel training and on-site supervision are critical aspects of a successful underbalanced well.

Under Balanced Drilling (UBD)
Underbalanced drilling has become one of the most economical ways to drill fractured formations. The method called flow drilling, developed in the South Texas fields, has proven successful worldwide, in areas such as the Midale beds in southern Canada, fractured limestone in southern North America in the U.S. and Mexico, the Barrow Islands of Australia, the mature depleted field of Western Argentina and the fractured formations in China.
In some horizontal wells, vertical fractures are penetrated as the horizontal section of the well is drilled. The drilling fluid exiting the bit is lost into newly drilled fractures, and the annular column of drilling fluid is simultaneously lost into earlier drilled fractures. This reduction in hydrostatic head results in oil and gas influx into the annulus, further decreasing the hydrostatic head. The well can then flow up the annulus naturally underbalanced.
In a well with a high gas/oil ratio (GOR) and a 10-ppg mud, underbalanced drilling can easily cut the equivalent mud weight to 6 ppg. The net effect can be a significant pressure gain at the wellhead. The resulting intermittent and unpredictable surface pressures require reliable surface equipment to ensure safety and adequate well control.

Drilling Fluids
One of the major considerations in planning an underbalanced well is the type of drilling fluid used.
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