100 Watt Design & Marketing - Client Blog

The main benefit of under balanced drilling is to minimize formation damage. Negative differential pressure between the formation and the wellbore also stimulates the production of formation fluids and gasses. Increased penetration rates is another main benefit in wells drilled under balanced. under balanced will not build a filter cake in the wellbore. During conventional drilling, this filter cake acts as a protective barrier, reducing damage to formation permeability from drilled cuttings. When drilling horizontal wellbores, drill cuttings are ground into fine powder. In conventional drilling, if the wellbore does not have a filter cake and becomes overbalanced, this powder is carried into the formation, greatly reducing near-wellbore permeability. Unlike typical cased-hole completions, which can easily be stimulated, if an under balanced extended-reach open-hole completion develops a near-wellbore region of reduced permeability, the damaged region can significantly reduce the potential production rate. This is the reason, to maintain under balanced conditions all through the drilling phase. That said under balanced drilling is not appropriate in all formations. For example a lot of shale formations, salt formations, shattered coal sections, unconsolidated sections. If these formations are present, it may be appropriate to drill them overbalanced and then change to under balanced conditions for the production formation. Choosing which type of Gas Phase to use (either Air, nitrogen or natural gas) depends on several factors. When natural gas is available and can be recovered and reinjected into the supply/sales line, it can be the very cost-effective method to achieve an under balanced condition. Drilling with air is common, but is used mostly in upper hole sections and due to the potential for downhole fires it should not be used in the production zones. For these reasons, nitrogen has become the gas of choice for under balanced drilling. Cryogenic nitrogen of high purity can be purchased, but price and logistics are unfavorable. Generally the best source is nitrogen that is generated onsite using a membrane unit. Membrane units typically produce up to 95% purity nitrogen. Purity is such that corrosion can usually be managed and downhole fires are not a problem. Membrane units are available in a wide range of capacities to meet most drilling requirements.

for more information about under balanced drilling visit Precision Air Drilling's website www.precisionairdrilling.com

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