Underbalanced drilling (UBD) in becoming increasingly used as a technique to reduce significant invasive formation damage in vertical and horizontal wells to improve production rates of oil and gas, and enhance injectivity in gas and water injection situations. UBD may be a technically demanding procedure to execute in certain reservoirs and much of the benefit with respect to mitigation of formation damage may be compromised if the underbalanced drilling operation is not screened, designed and conducted in an appropriate fashion. This paper reviews common formation damage mechanisms which may occur in reservoirs and how, in certain situations, these types of damage may be reduced or eliminated through the appropriate use of underbalanced drilling technology. Various situations in which underbalanced drilling technology may result in potential significant formation damage are also discussed.
Underbalanced drilling is utilized worldwide for the drilling of horizontally and vertically oriented wells to increase rates of penetration, reduce invasive formation damage and reduce significant problems with drilling due to lost circulation and differential sticking. Many successful implementation stories of underbalanced drilling are evident in the literature1,2, however, underbalanced drilling is not a panacea for all formation damage problems in that inappropriately designed underbalanced drilling jobs may actually result in more formation damage than if a well-designed and contemplated overbalanced job had been used in the same situation in some circumstances.
This paper reviews some of the current technology in use at the present time in underbalanced drilling and illustrates some of the points which operators should be aware of before embarking on an underbalanced drilling operation.
A rigorous definition of UBD is the situation in which the exerted circulating pressure of the drilling fluid in contact with the formation is less than the effective pore pressure in the adjacent section of the matrix. The desirable course of action is to have this occur along the entire exposed section of the net productive pay of the reservoir under consideration, resulting in a net inflow of oil, water or gas (which may be contained in the matrix) into the wellbore. These produced fluids are then returned to the surface along with the circulating drilling fluid. A number of nomenclatures for a description of UBD exist in the literature. They can be defined as follows:
A situation in which the equivalent circulating density of the drilling mud is sufficient that, at bottom hole conditions, the drilling fluid pressure is greater than the formation pressure, resulting in an effectively killed state of the well (where no inflow of formation of fluids occurs) during a conventional drilling operation. This was the most ommon technique utilized to drill wells in the past, and is still the dominant technology utilized in many reservoir situations today.
Low head drilling refers to a situation where an overbalanced pressure condition, similar to that described above, is maintained with the use of lower density oil-based fluids or possibly aerated or gasified fluids to reduce the effective overbalance pressure exerted on the formation.