This paper reviews the outcome of the first-ever deployment of low-solids formate fluids for the managed pressure drilling (MPD) and overbalanced open-hole completion of two subsea gas wells in a highly-pressured carbonate reservoir. The field development plan required the use of low-solids well construction fluids to minimise formation damage and ensure maximum rates of gas production from the reservoir. The deployment of low-solids fluids was also required to minimise the amount of solids that would pass through the gas processing facilities on production start-up.

The high pressure of the reservoir required fluids of an equivalent mud weight of >15 pounds per gallon (ppg) - 1.80 Specific Gravity (SG) to maintain well control. Previous drilling experience in the area suggested there was a high probability of losing large volumes of fluid to the reservoir while drilling conventionally in overbalance. The strategy adopted to mitigate the risk of massive fluid losses was to use a 12.7 ppg (statically and dynamically underbalanced) potassium / sodium formate drilling fluid in combination with MPD technology to control the level of overbalance through backpressure. The reservoir section of each well was successfully and safely drilled with the low-solids potassium / sodium formate fluid at 150 psi overbalance in less than 2 days at an average ROP of 15-17 metres/hour with no measureable fluid losses to the formation.

On reaching Total Depth (TD) the potassium / sodium formate drilling fluid was displaced to kill weight (15.1 ppg) low-solids cesium formate brine as the tripping fluid before running the 7" pre-drilled liner. The tripping fluid was then displaced to 15.1 ppg clear cesium formate brine for the upper completion operation. These open-hole completion operations in cesium formate brine took 67 hours for the first well and 72 hours for the second. Both wells cleaned up naturally at start- up and tested at gas production rates of > 150 MMscfd, exceeding expectations by 50%.

The results of these trials confirm that low-solids formate brines used in combination with MPD can help deliver highly productive wells in fractured carbonate reservoir developments without incurring excessive fluid losses during drilling and without requiring acid stimulation. From an economic perspective this first-ever deployment of an underbalanced formate brine system for drilling a carbonate reservoir in MPD mode reduced costs by:

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    Eliminating lost circulation events and associated fluid losses

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    Allowing the use of a lower-cost (i.e. lower density) brine for drilling

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    Providing early kick and loss detection

It is concluded that the successful use of MPD with low-solids formate fluids in two subsea gas wells have provided a new cost-saving technique for safely drilling and completing high-pressure carbonate reservoirs that are prone to severe circulation losses.

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