EXTENDING REACH DRILLING: Replacing oil-based with water-based drilling fluids

An extended-reach-drilling case study

R. Stawaisz
S. Taylor
Britannia Operating Ltd.

T. Hemphill
U. Tare
Baroid Product Service Line, Halliburton Energy Services

K. Morton
ChevronTexaco

T. Valentine
Flowco

An extended-reach well was planned and successfully drilled in the North Sea Central Graben with a water-based drilling fluid in an area where wells have histor-ically been drilled with mineral oil-based and ester-based drilling fluids.

Water-based drilling fluids were used because of the high costs of shipping cuttings drilled with inverts to shore for processing and disposal, coupled with the potential shutdown of drilling operations in times of bad weather when cuttings could not be transferred from the rig to supply vessels. These issues are especially important in the large-diameter hole section where high amounts of cuttings are generated.

Many water-based systems have been used previously with little success in the Central Graben. The conditions indicated a need to develop a stable, shale-inhibiting water-based mud system capable of performing well in extended-reach drilling (ERD) applications.

The drilling program for this development well called for the intermediate hole interval to be drilled from the 20-in. casing shoe at ± 3,500 ft MD (± 3,400 ft TVD) to a TD of ± 8,900 ft MD (± 7,600 ft TVD) with a final hole angle of 63°. Additionally, the program included a suitable water-based system for this ERD based on shale inhibition and formation analysis, mud weight optimization, and hole-cleaning efficiency.

Shale inhibition

Mineral oil-based drilling fluids have traditionally provided the most inhibitive drilling fluid in this area. The challenge was to design a water-based drilling fluid system with performance near that of the mineral oil-based fluids. Shales encountered in the Central Graben area of the North Sea are some of the most chemically reactive found in any drilling area. They contain high percentages of mixed layer illite/smectite clays, which are prone to swelling and dispersion and can lead to diminishing control over mud system rheological properties. Laboratory work on shale samples collected from a nearby well included x-ray analysis, cation exchange capacity, exchangeable cations characterization, and the use of the dielectric constant measurement (DCM) technique.

The DCM technique provides a quantitative determination of rock properties by measuring the specific surface area per unit weight, thus representing the total hydratable surface area of a sample. The DCM is dominated by the presence of smectite and is also strongly influenced by the other hydratable clays. The DCM measurements indicated increases from 250 m2/g initially to a maximum of 580 m2/g at 7,500 ft TVD in the Alba and Lothian formations. The DCM showed where the most reactive shales would be encountered and at what point increased fluid inhibition properties would be required.

Mud weight optimization

Previous wells had trajectories of 82° in the deeper 12.25-in. hole interval. The high angle of attack into problematic shale at 12,800-13,000 ft TVD has been a source of several hole instability occurrences. These instabilities were mainly in the form of hole pack-offs and collapse primarily due to weak rock strength and laminated structure of the shale.

As a consequence, a field wellbore stability analysis resulted in the following conclusions:

• The higher hole angles drilled perpendicular to the orientation of the maximum horizontal stress required higher mud weights compared to those drilled parallel to the maximum horizontal stress
• The higher deviation angles required higher mud weights, which served to increase mud pressure penetration into the weak shale laminations and hasten wellbore instability
• The laminations in the shale limited the angle of attack due to weak bedding plane-related rock strength anisotropy. Consequently, when the well trajectory was drilled parallel to the bedding planes, more hole-collapse problems were observed.

Accordingly, the well trajectory design for the ERD well was changed so that the problem shale encountered in the 12.25-in interval would be drilled at a lower angle. This change required a shallower kick-off depth for the large-diameter interval. A borehole stability analysis was performed for the large diameter interval. Based upon anticipated hole angles with depth, a mud weight schedule with an initial mud weight of 11.5 ppg and a final mud weight of 12.5 ppg was developed. The minimum mud weights necessary to initiate hole collapse, breakdown, and formation fracture pressures were analyzed. Caution was given that any increases in mud weight above 12.5 ppg level should be carefully considered in order to avoid differential sticking across any permeable zones encountered while drilling the section.

Hole cleaning

Once the mud weight schedule was identified, pre-well planning focused on hole cleaning in the large-diameter interval. The hole-cleaning methodology was developed from earlier steady-state hole cleaning modeling work. Key parameters included mud density, fluid rheological parameters, cuttings size and shape, pump rate, hole geometry and angle, drill pipe eccentricity, drill pipe rotation speed, and rate of penetration (ROP).