Case Study: ULTRAFREE NS ROP Enhancer Delivers Sustained Improvement in ROP

Challenge: An operator in the UK North Sea used a standard‐class semisubmersible rig with a limited pit space of 1,500 bbl to drill a well in winter conditions. Normally, the operator would have chosen oil‐base mud for this well; however, because there was a risk of bad weather that could have prevented mud transfer from the supply boat and would have disrupted “skip and ship” operations, the operator decided to use the M‐I SWACO ULTRADRIL high‐performance, water‐base system. Until this well, the ULTRADRIL system was typically run without ULTRFREE ROP enhancer included in the standard formulation for wells drilled in the UK sector. The 12¼‐inch section was drilled through the 5,000‐ft (1,524‐m) thick Shetland Marl, a relatively young (Cretaceous) plastic formation with high clay content. This formation is known to be prone to bit balling with water‐base muds and PDC bits. The rate of penetration (ROP) fell to an average of 8‐10 ft/hr (2.4‐3 m/hr) through the Shetland C group despite the use of seawater pills to clear the bit.

Solution: ULTRAFREE NS ROP enhancer added at 2‐3% by volume immediately improved ROP, which was sustained for the remainder of the section. Prior lab testing had shown that this product would reduce accretion and improve lubricity.

Result: The ROP had slowed to 15 ft/hr (4.6 m/hr), and bit torque was very low, indicating a potential bit‐balling problem. The drilling team decided to pump a 240‐bbl pill with 3% by volume ULTRAFREE NS ROP enhancer in active mud. The ROP increased to 30 ft/hr (9.1 m/hr), and the torque increased from 2,000 ft‐lbs (2.71 kN/m) to 10,000 ft‐lbs (13.56 kN/m). The drilling team then increased the concentration of ULTRAFREE NS ROP enhancer to 2% by volume in the active system by adding several 80‐bbl pills containing 6.25% enhancer. This resulted in a sustained ROP improvement. Finally, the concentration of ULTRAFREE NS ROP enhancer was increased to 3% by volume, after which, the ROP increased to 70‐100 ft/hr (21.3‐30.5 m/hr). This rate exceeded the anticipated ROP of 40‐60 ft/hr (12.2‐18.3 m/hr) based on a nearby offset well drilled three months previously with the same mud system without ULTRAFREE NS ROP enhancer.

Well Information
Location Block 211, UK Northern North Sea
Well Type Exploration/Oil Producer
Date February 2006
Maximum Deviation
Bottomhole Temperature 150°F (66°C)
Well Depth 11,120 ft MD (3,389 m)

The Details

The drilling data show the effect adding ULTRAFREE NS ROP enhancer has on ROP and torque.

1.75% by volume 5% by volume 10% by volume 15% by volume
R 600 85 86 90 94
R 300 61 61 62 64
R 200 51 49 50 51
R 100 36 35 35 35
R 6 10 10 11 11
R 3 8 8 8 8
PV (cP) 24 25 28 30
YP (lb/100 ft2) 37 36 34 34
Gels (lb/100 ft2) 8/13/16 8/13/16 8/14/16 9/14/16
pH 120°F 9.01 8.99 8.74 8.65
Effect on field mud rheology with addition of ULTRAFREE NS ROP enhancer

Pilot testing showed no adverse effect on rheology or incompatibility at high concentrations (up to 15% by volume) in the ULTRADRIL fluid. The wellsite drilling fluids engineer commented that the coating mechanism of the ULTRAFREE NS ROP enhancer was progressively more evident as the concentration was increased. According to the drilling fluids engineer, the sleeve of the Fann‐35 viscometer was free of mud after removing the thermo cup, “as if it had been sprayed with WD‐40.”

The graph below shows the ULTRADRIL system’s performance once the concentration of ULTRAFREE NS ROP enhancer was added to the system.

Download: ULTRAFREE NS ROP Enhancer Delivers Sustained Improvement in ROP (0.82 MB PDF)

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