Only recently drilled laterally in Kingfisher County, Okla., the lower Mississippian series features beds of shale, limestone, and cherty dolomite prone to bouts with lost circulation. Farther north, in southern Kansas, the Mississippian is encountered around 1,829 m (6,000ft) true vertical depth (TVD) and is traditionally drilled with water using polymer and lubricant sweeps for hole cleaning and lubricity.
In Kingfisher County, the Mississippian series lies deeper, at about 2,438 m (8,000 ft) TVD, and operational demands are more complex. The Oswego shale formation immediately above the Mississippian and the Morrow shale immediately below that target zone can prove troublesome if drilled with water. Problems typically associated with drilling the production interval include lost circulation in the Mississippian’s carbonate beds, stuckpipe, and wellbore instability in the adjacent shale formations. These issues have frequently resulted in significant and expensive nonproductive time (NPT) and costly remedial operations.
Offset horizontal well data is virtually unavailable as extended-reach lateral drilling is new in the area, only recently attracting operators’ collective interest as they transition from natural gas to liquids-focused production. In Oklahoma, only the distant Kay and Grant counties had seen the Mississippian drilled horizontally before planning this well.
Drilling fluid selection criteria
The operator sought a lubricious and inhibitive fluid system to efficiently drill the lateral production interval. Targeted drilling fluid performance attributes critical to this application included the ability to provide wellbore lubricity and stability to foster efficient drilling or sliding and trouble-free tripping while avoiding stuckpipe and lost circulation experienced on offset Mississippian horizontal wells in Kay and Grant counties. After setting 7-in. intermediate casing through the build section to horizontal in the Mississippian limestone, the specified fluid would be used to drill a 6.25-in. hole from the casing shoe at about 2,469 m (8,100 ft) measured depth (MD)/2,347 m (7,700 ft) TVD to a total depth (TD) of about 3,810 m (12,500 ft) with a final hole angle of 90 degrees. Directional control would be critical as a small target was defined and tight wellbore path required. The system would be unweighted and fluid density minimized to avoid fluid losses in the Mississippian.
Potential lost circulation zones through anticipated carbonate beds in the Mississippian made oil-based mud a less than ideal fluid choice for this application. An economical water-based fluid was sought to minimize fluid losses. Anticipated bottomhole temperature would be approximately 49°C (120°F). Minimizing environmental impact at the well site also was prioritized.
Drilling fluid system description
The Evolution drilling fluid system, a proprietary waterbased polymer system developed by Newpark Drilling Fluids, was specified for this interval and was formulated to address its unique geological demands. The system combines a rugged polymeric viscosifier/shale stabilizer with an effective drilling performance enhancer to provide efficient drilling penetration rates through hydraulics and lubricating optimization. It is effective over a wide thermal range to 218°C (425°F) and is not adversely affected by common wellbore contaminants. Fluid may efficiently be built on location, and its environmentally sound formulation requires no special wellsite fluid and cuttings handling equipment or process, avoiding transportation cost associated with oil-based fluid.
The system’s inherent lubricious capability yields a significant reduction in drilling torque and drag, a critical requirement to efficiently drill extended-lateral sections. The shear thinning characteristics provide low effective viscosity at the bit to improve penetration rates, with optimized low shear rate viscosity in the annulus to ensure adequate hole cleaning. This water-based formulation is significantly less prone to lost circulation than oil-based mud. It contains no commercial clays, providing minimal solids content to protect production zones from solids blockage.
For this application, secondary components included NewPhalt, a sulfonated asphalt shale stabilizer, and potassium hydroxide for alkalinity control. As a source of potassium ions to increase osmotic pressure, it also effectively enhanced shale stability through dehydration.
The 6.25-in. lateral section through the Mississippian proved to be hard and abrasive, and weight on the bit was limited to improve PDC bit life. Penetration rates averaged 4.6 m to 7.6 m (15 ft to 25 ft) per hour while rotating in the abrasive limestone formation. Rotating torque ranged from 10 ft-lbs to 12,000 ft-lbs. Several bit trips were required to drill the interval. All trips were without incident, with overpull weights at 10,000 lbs or less, and no reaming was required returning to bottom.
Fluid lubricity was monitored at the well site throughout the interval, and lubricity coefficient values held steady at .08 to .09 throughout the interval following displacement, comparing quite favorably to those typical of oil-based mud.
The unweighted fluid was maintained at 8.5 lb/gal throughout the interval, and the low solids formulation helped minimize density. Rig shakers were fitted with 120 API screens, and a high-speed centrifuge was employed to maintain low gravity solids below 3% by volume.
The horizontal section of the well was drilled to 3,810 m (12,500 ft) MD without incident. Shuttle logs were run to bottom and revealed a gauge hole throughout the approximate 1,341-m (4,400-ft) lateral section. Logs also indicated that 133 m (436 ft) of Oswego shale above the target zone had been penetrated when the Mississippian was exited and reentered, with no wellbore issues observed.
A 4.5-in. liner was run to bottom and set without excessive drag. No tight spots were encountered in or out of the well bore during tripping and casing operations, and the horizontal interval was drilled in 21 days without incident.
The water-based drilling fluid system provided ample lubricity and stable gauge well bore throughout the horizontal production interval to a precise target, enabling trouble-free drilling, tripping, logging, and running casing to TD. No fluid losses were experienced. Trip time and time required to run casing were improved versus offsets.
Fluid rheology and filtration rates were efficiently controlled within programmed parameters using only basic system formulation and products to promote penetration rates and adequate hole cleaning throughout the interval, satisfying operator goals for efficiency and total well cost.
This first horizontal application of the high performance water-based drilling fluid system here in Kingfisher County resulted in virtually zero NPT or fluid losses while drilling/sliding, tripping, logging, and ultimately running and setting production casing efficiently to TD. Overall operational efficiency achieved by the operator immediately established a rigorous standard by which future Lower Mississippian horizontal projects will be measured.