Underbalanced drilling, also called managed-pressure drilling, is a form of oil and gas drilling that is somewhat less common than other forms of drilling. This technique maintains a lower pressure in the well being dug than the pressure being exerted by the oil and gas reservoir. When the pressure in the well is lower than the pressure the oil and gas can exert, the formation of gases and fluids caused by the drilling can be taken to the surface and managed appropriately. Some consider underbalanced drilling to be a more effective technique, despite its higher initial cost.
Perhaps the most important benefit of such drilling is the reduction of damage to the natural oil and gas formation. In many other drilling techniques, the mud used for drilling and maintaining well pressure can be forced out of the well and into the reservoir deposit, possibly damaging the well and the operation overall. Underbalanced drilling, on the other hand, encourages the gas, fluids, and rock material from the well to the surface using a variety of methods.
One form of underbalanced drilling is dry air drilling, in which air pressure alone is used to cool the drill bit and encourage the flow of gas, liquid and rock out of the well. With dry air drilling, only a small amount of oil is necessary to lubricate the machinery and maintain optimal drilling conditions in addition to the air. Other forms of underbalanced drilling might include mist, foam or stable foam drilling, in which variable amounts of a foaming agent are used to pick up materials and encourage their flow to the surface, to help maintain the correct pressure. Of course, the specific form of underbalanced drilling usually depends on the conditions of the specific oil or gas formation.
Despite the many benefits of this type of drilling, many operations prefer overbalanced drilling and other drilling methods. Conditions vary widely between oil and gas formations, so underbalanced drilling might not be effective for all operations. The initial cost also may be higher for this type of drilling, though the long-term benefits — including longer bit life, less reservoir damage, and management of materials produced during drilling — might offset the cost. With proper planning, including intense pre-planning procedures and on-site engineers ready to manage drilling issues, underbalanced drilling can be more effective than many other forms of oil and gas drilling.
