Balance Between Formation Damage and Wellbore Damage: What is the Controlling Factor in UBD Operations?

Abstract

Abstract Underbalanced drilling (UBD) has been considered as an effective means of formation damage control during drilling. However, there is a trade-off between formation damage and wellbore damage in UBD. The wellbore damage includes borehole collapse, cave-in and washout associated with the lowered bottom hole pressure. The damaged wellbore results in the increased drilling cost and also brings in completion problems such as poor cement bonding quality and/or cement collar being too thick to be penetrated during perforating job. While the highest bottom hole pressure during normal drilling condition affects the degree of formation damage, the lowest bottom hole pressure during pipe connection controls severity of the wellbore damage. Both the highest bottom hole pressure and the lowest bottom hole pressure are controlled by the combination of the liquid and gas flow rates. In order to minimize both the formation damage and wellbore damage problems, it is highly desirable to have an easy-to-use approach to generate balanced design of combinations of liquid and gas flow rates under given drilling conditions. The content of this paper fills the gap. This paper presents an innovative procedure to approach the optimum design of liquid and gas flow rates combinations for UBD under given geological constraints. The optimum flow rate combinations are selected from a liquid-gas rate window. The liquid-gas rate window is an envelope in the liquid flow rate vs. gas flow rate plot. In developing the liquidgas rate window, formation fluid pressure limits the upper bound of the flowing bottom hole pressure and wellbore collapse pressure serves the lower bound of the circulation breaking bottom hole pressure. The window is closed by fluid's cutting carrying capacity and wellbore washout criteria. This paper first time provides drilling engineers with an easy-to-understand procedure to optimize mud and gas flow rates in UBD.