Using Underbalanced Drilling to Reduce Invasive Formation Damage and Improve Well Productivity-An Update
Authors: Bennion D.B. , Thomas F.B. , Jamaluddin A.M.M. , Ma T. 
Published: March 2010

Abstract
Underbalanced drilling (UBD) is 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.

Introduction
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 problems with drilling due to lost circulation and differential sticking. Many successful descriptions of underbalanced drilling are present in the literature(1–8). 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 circumstances.
This paper reviews some of the 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.

What Is Underbalanced Drilling (UBD)?
A rigorous definition of UBD is a condition where the circulating pressure of the drilling fluid in contact with the formation is less than the effective pore pressure in the adjacent matrix. The desirable course of action is to have this occur along the entire exposed section of the productive pay of the reservoir, resulting in an inflow of oil, water or gas (which may be contained in the matrix) into the wellbore. The produced fluids are returned to the surface along with the circulating drilling fluid. A number of descriptions of UBD exist in the literature. They can be described as follows:
Overbalanced Drilling
A situation in which the equivalent circulating density of the drilling mud is sufficient that, at bottomhole conditions, the drilling fluid pressure is greater than the formation pressure, resulting in an effectively "killed" state (where no inflow of formation of fluids occurs). This has been the most common technique utilized to drill wells in the past and is still the dominant technology currently used to drill many wells.
Low Head Drilling
Low head drilling refers to a situation where an overbalance pressure condition, similar to that described above, is maintained but with the use of lower density oil-based fluids or possibly gasified fluids to reduce the magnitude of the overbalance pressure exerted on the formation. The prime motivation is to reduce formation damage and the potential for severe invasive losses and to increase ROP. Low head drilling is still classified as a form of overbalanced drilling.