Balance between Formation Damage and Wellbore Damage: What Is the Controlling Factor in UBD Operations?

摘要

Underbalanced drilling (UBD) has been considered as aneffective means of formation damage control during drilling.However, there is a trade-off between formation damage andwellbore damage in UBD. The wellbore damage includesborehole collapse, cave-in and washout associated with thelowered bottom hole pressure. The damaged wellbore resultsin the increased drilling cost and also brings in completionproblems such as poor cement bonding quality and/or cementcollar being too thick to be penetrated during perforating job.While the highest bottom hole pressure during normaldrilling condition affects the degree of formation damage, thelowest bottom hole pressure during pipe connection controlsseverity of the wellbore damage. Both the highest bottom holepressure and the lowest bottom hole pressure are controlled bythe combination of the liquid and gas flow rates. In order tominimize both the formation damage and wellbore damageproblems, it is highly desirable to have an easy-to-useapproach to generate balanced design of combinations ofliquid and gas flow rates under given drilling conditions. Thecontent of this paper fills the gap.This paper presents an innovative procedure to approachthe optimum design of liquid and gas flow rates combinationsfor UBD under given geological constraints. The optimumflow rate combinations are selected from a liquid-gas ratewindow. The liquid-gas rate window is an envelope in theliquid flow rate vs. gas flow rate plot. In developing the liquidgasrate window, formation fluid pressure limits the upperbound of the flowing bottom hole pressure and wellborecollapse pressure serves the lower bound of the circulationbreakingbottom hole pressure. The window is closed byfluid’s cutting carrying capacity and wellbore washout criteria.This paper first time provides drilling engineers with aneasy-to-understand procedure to optimize mud and gas flowrates in UBD.