The Situation

The Offshore Black Sea area is well known for drilling difficulties like wellbore instability, bit balling and tight hole while tripping, caused by the highly reactive shale formation. Other challenges for this Deep Water exploration well were considered to be Gas Hydrates, narrow Mud Weight window, hole cleaning, fluid and waste volume handling. The Operator required formulating a high performance inhibitive Water Based Mud not only to mitigate all technical challenges but also to minimize any environmental risk.

The Solution

The High Performance Water-Based drilling fluid ULTRADRIL was selected to drill the most difficult sections of this well, knowing that this system can be formulated to inhibit both dispersive and swelling clay formations using polyamine technology. Addition of Mono Ethylene Glycol in the fluid formulation was required and detailed procedures were prepared to insure Gas Hydrate suppression. Also POROSEAL, a unique copolymer suitable for shale sealing was added in order to enhance inhibition properties and provide wellbore stability when drilling micro-fractured and dispersive shales.

The Results

The well was completed in only 80 days, which includes a 118m (387 ft) supplementary interval (exploration reasons). Actually, the initial planned Total Depth was reached in only 75 days, a 35% reduction of the “Best Case” scheduled time (115 days) or a 44% reduction compared to AFE scheduled time (135 days). Considering the daily cost of a Deep Water Operation, the time reduction was considered a very good achievement.

The average Rate of Penetration (ROP) was ranging between 9.6 m/hr (31.5 ft/hr) in the 8.5-in. hole to 18.7 m/hr (61.4 ft/hr) in the16.5-in. hole, whilst the instant ROP was intentionally limited to 20 m/hr (65.5 ft/hr) due to gas kicks and losses potential. The drilled cuttings were always firm and dry with clear PDC bit marks, proving an inhibition level comparable with Oil Based Mud.

The only problems encountered during drilling were some seepage or partial losses (quickly solved by placing Lost Circulation Materials pills and reduced flow rates) and few gas kicks, but these were expected due to narrow Mud Weight windows.

The Details

When designing the ULTRADRIL system for this well, several key aspects were considered:

• Provide proper inhibition level; monitor and adjust inhibitor concentrations while drilling based on depletion rate, hole and cuttings condition
• Insure Gas Hydrates suppression during each and every operational situation (static and dynamic condition)
• Adjust fluid rheology to provide proper hole cleaning without exceeding formation fracture pressure
• Avoid drilling fluid related problems (hole instability, stuck pipe, circulation losses)
• Plan and coordinate logistics for all materials and equipment to ensure smooth operation
• 100% environmental compliance.

Due to the very large volumes required, most of the ULTRADRIL system was prepared in the land mud plant and transferred to the rig, only minimal dilution volumes being built on the drilling platform.

Proper inhibition was provided by several dedicated products: ULTRAHIB – the amine hydration suppressant: 2-4% by volume; ULTRACAP PLUS – Encapsulator to prevent dispersion: 1.25-2.5 ppb (3.5-7.0 kg/m3); and KCl – ionic inhibitor: 7-9% by weight.

Gas Hydrate suppression was very challenging to obtain, especially for the top hole sections, where Mud Weight was very low limiting brine content; as a result, fully inhibited pills were tested and prepared to be placed during static conditions in the areas susceptible to plugging by hydrates formation.

Low end rheology was optimized each interval by DUOVIS biopolymer addition to provide adequate hole cleaning; Virtual Hydraulics software simulations together with pump pressure/ECD monitoring were used to ensure cuttings transport to surface and weighting material suspension.

Questions? We’ll be glad to answer them.

If you’d like to know more about ULTRADRIL system, and how it’s performing for our other customers, please call the M-I SWACO office nearest you.