Balance between Formation Damage and Wellbore Damage: What Is the Controlling Factor in UBD Operations?
- Boyun Guo (University of Louisiana at Lafayette)
- Document ID
- Society of Petroleum Engineers
- International Symposium and Exhibition on Formation Damage Control, 20-21 February, Lafayette, Louisiana
- Publication Date
- Document Type
- Conference Paper
- 2002. Society of Petroleum Engineers
- 5.4 Enhanced Recovery, 1.10 Drilling Equipment, 1.2.1 Wellbore integrity, 1.7.7 Cuttings Transport, 1.7.1 Underbalanced Drilling, 2.2.2 Perforating, 5.4.2 Gas Injection Methods, 1.8 Formation Damage, 1.6 Drilling Operations, 1.14 Casing and Cementing, 5.6.1 Open hole/cased hole log analysis, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 2 Well Completion, 1.11 Drilling Fluids and Materials
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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 liquid-gas 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.
Underbalanced drilling (UBD) is defined as the drilling operations where the drilling fluid pressure in the borehole is less than the pore pressure in the formation rock in the open-hole section. The borehole pressure is intentionally maintained not to balance formation pore fluid pressure by using light drilling fluids. The light fluids used in UBD are usually air, gas, foam, and aerated water. However, un-aerated oil, water, even weighted mud can be used for UBD in areas where formation pore pressure gradients are higher than hydrostatic pressure gradient of water.
The increasing coverage of wells being drilled with the UBD is due to many advantages of the technology. These advantages include increased penetration rate, minimized lost circulation, prolonged bit life, minimized differential sticking, improved formation evaluation, reduced formation damage (reduced stimulation requirements), earlier oil production, larger wellbore available to production in offshore, and environmental benefits.
The successfulness of UBD applications relies on good UBD designs. Sever wellbore damages, or failures, due to wellbore collapse and hole-wall caving-in problems can result from poor UBD designs. The combination of mud flow rate and gas injection rate plays a very import role in balancing the benefit and possible failure of the UBD. If the combination is chosen such that it gives too high bottom hole pressures, the degree of underbalance is reduced and the benefit of the UBD will be marginal. In the other hand, if the combination is chosen such that it gives too low bottom hole pressures, wellbore collapse problem will fail the UBD operation.
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