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In conventional, or “overbalanced”, drilling fluid is pumped into the well shaft at pressure higher than in the reservoir. This keeps the oil and gas in the reservoir during drilling, but the high pressure can damage the rock around  the wellbore. Underbalanced drilling – a type of managed pressure drilling – uses lighter fluids or gases so pressure is lower than in the reservoir so that fluids can enter the well shaft. This prevents damage to the reservoir rock and allows oil and gas to flow freely, increasing production.

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Shell uses underbalanced drilling to improve the flow of oil and gas at some of its operations, and to help gather more accurate data on underground reservoirs.

Removing uncertainty from tight gas reservoirs

Accessing gas trapped in rock pores, known as tight gas, is becoming increasingly important to meet rising energy demand. But finding out more about a tight gas well can be difficult.

In underbalanced drilling the circulating drilling fluid lifts the drilled cuttings and gas to the surface. We analyse this mixture and, along with other data such as seismic surveys, learn more about the flow characteristics and geology of the reservoir. We are able to identify the most promising production areas and so drill fewer wells to find oil.

We also use the data to identify areas where water may enter the reservoir and, for example, use expandable seals, called swellable elastomers, to keep any water out.

Innovative application

Underbalanced drilling stated in the 1970s but is now more widely applied. It can take longer to plan drilling a well in this way, and may raise costs. But it also has the potential to improve well production by up to 800%. Thanks to the improved flow of oil and gas, fewer wells are needed to drain reservoirs and the overall environmental footprint is smaller.

We use underbalanced drilling at some stage in all our tight gas reservoirs. In 2011 we drilled about 80% of North America tight gas wells using this technique and saved millions of dollars compared to conventional methods. 

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