Application of Bicenter Bits in Well-Deepening Operations
- Kent Myhre (Eastman Christensen)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- June 1991
- Document Type
- Journal Paper
- 105 - 110
- 1991. Society of Petroleum Engineers
- 1.6.1 Drilling Operation Management, 1.5.1 Bit Design, 2.4.3 Sand/Solids Control, 5.3.4 Integration of geomechanics in models, 1.10 Drilling Equipment, 1.6 Drilling Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.6.1 Open hole/cased hole log analysis, 1.1.6 Hole Openers & Under-reamers, 1.5.4 Bit hydraulics, 1.5 Drill Bits, 1.14 Casing and Cementing
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This paper describes the design and operational evolution of bicenter fixed cutter bits used to drill and to underream below existing casing for a well-deepening program in south Texas. These bits were required to pass through the casing ID and to drill a hole diameter large enough to allow adequate annular clearance for cementing a liner. The traditional method for deepening these wells required several roller-cone bits and hole openers. Use of bicenter bits decreased the cost of the well deepening by eliminating bit and underreamer trips and increasing rate of penetration (ROP).
Several wells in the West Ranch field, Jackson County, TX, were planned to be re-entered and deepened. The formations planned to be re-entered and deepened. The formations encountered in the deepening section consist of interbedded sand and shale sequences. The entire interval includes different members of the Frio formation. The sands are generally poor to moderately con-solidated, and the shales are soft and sticky to moderately firm. The interval of the deepening section typically is from about 5,600 to 8,200 ft.
These wells typically are deepened by drilling a 4 3/4-in. hole from the 5 1/2-in. casing shoe to total depth (TD) with milled-tooth roller-come bits (IADC 1-3-6). The interval is opened to a 6 1/2-in. diameter with underreamers. A 4-in. liner is then hung from the bottom of the 5 1/2-in. casing and cemented.
This method was used to deepen Well WR-A445 from 4,843 to 6,314 ft. Four roller-one bits and three underreamers were required to deepen this interval. Deepening and underreaming took more than 160 hours to complete for an effective ROP of about 9 ft/hr (Table 1).
To reduce the cost of well deepening, use of a polycrystalline-diamond-compact (PDC) cutter bicenter drill bit was polycrystalline-diamond-compact (PDC) cutter bicenter drill bit was proposed. The bicenter bit was designed to pass through 5 1/2-in., proposed. The bicenter bit was designed to pass through 5 1/2-in., 17-lbm/ft casing (4.767-in. drift ID) and to drill a 5 1/2-in. minimum hole diameter to produce adequate annular clearance for cementing the liner. It was determined that the PDC cutting elements would produce the best ROP in this lithology and would last long enough to drill the entire 2,600-ft interval.
Concept. A bicenter (and eccentric) bit is designed to pass through a tight hole or casing string and to drill a slightly larger hole when rotated (Fig. 1).
Bicenter and eccentric bits are commonly used to drill through fast-moving salt formations to reduce bit and bottomhole-assembly (BHA) sticking. These bits also have been used for the same reasons in swelling shale formations. Eccentric bits have been used to improve cementing of small, deep casing strings by increasing the annular space between casing and the hole to allow increased cement thickness.
Geometry. Bicenter or eccentric bit geometry is defined by four variables: drill-hole diameter, nominal pass-through diameter, pilot diameter, and the angle that defines the circumference of the pilot diameter, and the angle that defines the circumference of the reamer wing.
Fig. 2 is a schematic of the bicenter bit geometry. "Bicenter" refers to the bit's two center axes: the reamer/drill-hole center axis (Point a) and the nominal pass-through center axis (Point b). In Fig. 2, the four geometric variables are defined as follows. The drill-hole radius, rd, is the length from Point a to Point d, L . The pass-through radius, rn, is the length from Point b to Point d, L , pass-through radius, rn, is the length from Point b to Point d, L , or the length from Point b to Point e, L . The pilot radius, r , is the length from Point a to Point e, L . One-half the reamer arc is defined by the angle a.
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