Dynamically Overbalanced Coiled-Tubing Drilling on the North Slope of Alaska
- D.T. Kara (BP Exploration, Alaska) | D.D. Hearn (BP Exploration, Alaska) | L.L. Gantt (Phillips Alaska Inc.) | C.G. Blount (Phillips Alaska Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2001
- Document Type
- Journal Paper
- 91 - 97
- 2001. Society of Petroleum Engineers
- 1.6.6 Directional Drilling, 5.4.1 Waterflooding, 1.6.1 Drilling Operation Management, 2.2.2 Perforating, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 3 Production and Well Operations, 1.6 Drilling Operations, 3.1.6 Gas Lift, 1.11 Drilling Fluids and Materials, 2.2.3 Fluid Loss Control, 1.7.5 Well Control, 1.8 Formation Damage, 1.2.7 Geosteering / reservoir navigation, 1.5 Drill Bits, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.7 Pressure Management, 5.1.2 Faults and Fracture Characterisation, 5.2 Reservoir Fluid Dynamics, 1.6.10 Running and Setting Casing, 1.7.1 Underbalanced Drilling, 1.10 Drilling Equipment, 5.4.2 Gas Injection Methods, 4.2 Pipelines, Flowlines and Risers, 1.14 Casing and Cementing, 2.4.3 Sand/Solids Control
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Coiled-tubing drilling (CTD) continues to play a dominant role in through-tubing sidetracking of wells on the North Slope of Alaska. Recently several CTD candidates were targeted in an overpressured reservoir. Three wells were successfully CT sidetracked as a pilot project in this overpressured reservoir. The ultraslimhole directional-drilling system with a 2.75-in. nominal openhole diameter developed at Prudhoe Bay1 was used in combination with an unconventional approach to maintain overbalanced conditions when sidetracking these wells.
Hydraulic modeling showed that with a kill-weight drilling fluid, the calculated equivalent circulating density (ECD) at the sand face could approach the regional fracture gradient. In addition, kill-weight mud in the formulation required for CTD operations would be a significant expense. To address these concerns, the wells were CT sidetracked using a less expensive underbalanced drilling-fluid system. Although the drilling-fluid hydrostatic pressure was less than reservoir pressure, overbalanced drilling conditions were maintained dynamically by the high ECD while circulating, and controlled by increasing the surface pressure using a choke when circulating at less than drilling rates.
Horizontal sidetracks drilled through 4.5-in. nominal diameter (OD) or larger tubulars using CT have become an integral part of the development drilling program on the North Slope of Alaska.2-4 The standard through 4.5-in. production tubing CTD sidetrack program uses an orientor/mud pulser/motor bottomhole assembly (BHA) that is a 27/8-in. OD to drill a 33/4-in. inside diameter (ID) hole. All the drilling operations are performed with a solids-free polymer-mud system that is approximately 1.5 lbm/gal overbalanced to the formation.
The primary objective of the CTD re-entry program is to convert noncompetitive, low-rate slant-hole completions into horizontal completions to enhance production rate and to access additional drainage acreage. To date, more than 130 CTD sidetracks have been drilled in Alaska with this system. However, it was estimated that the candidate population could be increased by more than 30% if CTD operations through 3.5-in. tubing were possible. In an effort to capture this additional market, this BHA has been downsized to allow for CTD operations through 3.5-in. OD production tubing.
The primary requirement for this ultraslimhole CTD equipment is that the system must pass through a minimum restriction of 2.8-in. ID. A 2-in. OD by 0.156-in. wall CT string is used as the drillstring. This provides a 1.688-in. ID flow path inside the drillstring and a 2-in. OD by 2.992-in. ID annular flow area for returns. Bicentered bits typically are used to drill a 3-in. ID open hole for horizontal distances reaching over 2000 ft. A 23/8-in. mud-pulse-telemetry BHA is used for directional and gamma ray (GR) information.
One of the constraints experienced with the ultraslimhole drilling system is the higher ECD caused by the smaller annular flow areas. With some CTD sidetrack candidates, the extra pressure exerted by the ECD can approach the regional fracture gradient and result in excessive fluid losses to the formation. This problem was compounded because the reservoir targets had become overpressured by waterflood operations in the three CTD drilling operations discussed in the paper. The higher costs associated with the kill-weight muds required and the potential for excessive losses compromised the economics of the projects. A different approach was needed to exploit these opportunities.
The conventional mud system for CT drilling in the larger, 33/4-in. diameter CTD openhole on the North Slope is a shear thinning solids-free Xanthan polymer fluid typically weighted to 8.9 lbm/gal with potassium chloride and sodium chloride (KCl/NaCl).5 In current Alaskan CTD operations, the salt is primarily for polymer yield and shale stability rather than hydrostatic-density requirements. Polymer concentration is maintained at a level to yield an elevated, low-shear-rate viscosity (LSRV=30-80K) for hole cleaning and fluid-loss control to matrix permeability. Introduction of this drilling fluid system to the CTD operation has resulted in increased horizontal reach, improved hole cleaning, and formation stability. However, the fluid is viscous by nature and high circulating pressures coupled with high annular-pressure losses are experienced routinely. Past experience in the large-diameter CTD program using 2 3/8-in. or 2 5/8-in. coil through 4 1/2-in. production tubing has shown that bottomhole pressures (BHP's) can exceed 1,000 psi over hydrostatic pressure while circulating. Drilling through 3 1/2-in. production tubing presents an additional challenge as the annular-friction pressure increases significantly with the reduction in annular clearance.
The BHA was downsized to allow for CT sidetrack applications through 3 1/2-in. production tubing. Two-inch coil is used with a nominal 2 3/8-in. BHA to drill a 2 3/4-in. open hole. The 2-in.×0.156-in. wall CT had been selected for ultraslimhole CTD application based on available weight to bit, pressure drop through the CT, BHA, the CT by production tubing annulus at flow rates required for hole cleaning, and the ability to maintain a decodable mud-pulse signal. From the onset of this ultraslim CTD re-entry project, annular pressure drop between the 2-in. OD CT and the typical 3 1/2-in., 9.3 lbf/ft production tubing was a major concern.1 The total annular-pressure loss from circulating is predominately from the typical 10,000 ft of 3 1/2-in. production tubing by 2-in. CT annulus and to a much lesser degree, the 2 3/4-in. open hole by 2-in. coil annulus.
The fluid for slimhole drilling needed changes to prevent excessive matrix losses or reservoir fracturing. Several changes that successfully mitigated a significant portion of the lost-circulation problem were made simultaneously. These changes included decreasing mud weight from 8.9 lbm/gal to 8.5 lbm/gal by decreasing salt content. The salt content was decreased to 2% KCl, which was thought to be the minimum salinity required for shale stability and desirable polymer yield. In addition, polymer concentration, and therefore viscosity, was reduced to what was considered a minimum for good hole cleaning and matrix-fluid-loss control. However, when drilling with a solids-free fluid which relies on viscosity for fluid-loss control, there is a trade-off between using a high-viscosity mud which minimizes fluid loss but increases ECD and pressure drop, and a low-viscosity fluid which minimizes ECD but may result in increased fluid losses.
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