Shallow Horizontal Drilling in Unconsolidated Sands Offshore California
- J.D. Payne (Unocal Corp.) | C.W. Huston (Smith Intl. Inc.) | M.J. Bunyak (Unocal Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1993
- Document Type
- Journal Paper
- 233 - 238
- 1993. Society of Petroleum Engineers
- 1.8 Formation Damage, 1.6.2 Technical Limit Drilling, 1.6.1 Drilling Operation Management, 2 Well completion, 1.4.3 Torque and drag analysis, 1.14 Casing and Cementing, 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials, 2.4.3 Sand/Solids Control, 1.1 Well Planning, 1.12.1 Measurement While Drilling, 1.6.1 Drilling Operation Management, 1.6 Drilling Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.12.3 Mud logging / Surface Measurements, 5.1.2 Faults and Fracture Characterisation, 5.6.1 Open hole/cased hole log analysis, 3.1.1 Beam and related pumping techniques, 1.6.6 Directional Drilling, 7.2.1 Risk, Uncertainty and Risk Assessment, 2.4.5 Gravel pack design & evaluation, 4.6 Natural Gas, 1.6.10 Running and Setting Casing
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Four shallow horizontal wells were drilled from Platform C in Dos Cuadrasfield offshore California to recover reserves inaccessible with conventionaldrilling techniques. The wells had true vertical depths (TVD's) ranging from746 to 989 ft with total horizontal displacements from 1,613 to 3,788 ft. Thewells had horizontal displacement TVD ratios up to 3.95. The targets wereunconsolidated, high-permeability sands. This paper details well planning,drilling, and completion.
Dos Cuadras field is a doubly plunging, east/west trending anticline about 71/2 miles southeast of Santa Barbara, CA, under Outer Continental Shelf (OCS)Leases P-0240 and P-0241. The primary producing horizon is the Repettoformation, an early Pliocene Age alternating sandstone/siltstone sequence. Thetop of the shallowest main reservoir, the Brown zone, is less than 300 ft belowthe mudline. The Brown zone includes two main markers, CP and DP, and numerousindividual markers designated C1P, C2P, etc. Original oil in place in the Brownzone was 205 million bbl, and recovery to date is 17 million bbl. Unocaloperates three platforms on OCS Lease P-0241: Platforms A, B, and C, set in 190ft of water. Fig. 1 shows the platform sites relative to the California coast.Commercial production from the field began in 1968 from Platform A, in 1969from Platform B, and in 1977 from Platform C. Production peaked at 60,000 BOPDin mid-1970 and declined to 6,500 BOPD by mid-1990.
Because producing horizons were very shallow, these platforms were equippedwith rigs that could drill with the masts slanted 30 from vertical.Conventional directional drilling techniques limited completions in theshallowest sands, even with the slanted masts, to less than 500 ft fromplatform centers. Fig. 2 shows the limits of completions in Sand C1P in theBrown zone. Because political and economic restrictions preclude settingadditional platforms, horizontal wells were drilled to confirm the ability todevelop previously inaccessible reserves economically. Well C-50 was drilledfirst to demonstrate the viability of the drilling and completion processes.Results from three subsequent wells verified the results of Well C-50 andextended completion limits.
Well C-50 Planning
Well Location. Sand C1P was chosen as the target for the first horizontalwell drilled in the Dos Cuadras field. This sand is one of the shallowest,thickest, and cleanest sands in the Brown zone. A well course was chosen in aline from Platforms C to B near Exploratory Well OCS P-0241 No. 2. This wellcourse is not the most attractive because of pressure depletion but wouldprovide the best geologic control. Good geologic control is important becauseDos Cuadras field is cut by numerous faults with small displacements. Thesefaults would not affect conventional wells greatly but could place a horizontalwellbore outside the target sand if intersected unexpectedly. The well would bedrilled from Platform C because of slot availability.
General Considerations. The preliminary well plan required setting 500 ft of13 %-in. casing with the shoe 200 ft below the mudline. The plan also requiredsetting 9%-in. casing through the build section to eliminate possible keyseatsin the soft formation and to allow use of 6 1/2-in. motors in the horizontalhole section. A 1,500-ft completion interval would complete the undepletedinterval between Platforms C and B.
Directional Planning. Minimum build rates were required for the followingreasons. 1. The ability to achieve high doglegs in soft formations and 121/4-in. hole was questionable. 2. Rod pump installation was required becauselow gross production rates and low reservoir pressure were anticipated. 3. Wewanted to save the most aggressive motor setup for a backup in case actualbuild rates were lower than predicted. To reduce build-rate requirements, aslanted slot was selected. With the rig in the slanted position, the hole anglewas 30 at the rotary table. The plan called for no build in the 17 1/2-in.surface hole because of a perceived inability to predict build rates accuratelyin the 17 1/2-in. hole and the desire to avoid opening the hole and possiblylosing angle, With the kickoff point restricted by the surface casing point, abuild rate of 8.5/100 ft was required to place the well at a 90 inclinationatop Sand C1P at 815 ft TVD. 9 5/8-in. casing would be set at this point. An81/2-in. hole then would be drilled to 2,802 ft measured depth (MD) whilemaintaining a 90 angle. This well course would fully penetrate the target sandas a result of a gradual upward dip from Platforms C to B. A 25-ft-high by50-ft-wide target that extended the length of the horizontal wellbore wasadopted.
BHA Planning. A build-rate-prediction program was used to determine downholemotor configurations. A bottomhole assembly (BHA) with a 7 3/4-in. 1 1/4bent-housing steerable mud motor was selected for the 12 1/4-in. hole. Thepredicted build rates were very sensitive to washout. Fig. 3 shows predictedbuild rates for various actual hole sizes. Actual hole sizes were not measured.A 1 3/4 bent-housing motor with a 1 1/4 bent sub was used as a backup BHAassembly. A 6 1/2 -in. 1 steerable mud-motor assembly was used for the 81/2-in.hole where minor correction runs were anticipated. Extensive drag-riskanalysis, as Mueller et al described, was done to select heavy-weight drillpipeand drill collar placement and to predict running-in hook loads for casing.High friction factors were used for preliminary analysis, and no problems werepredicted.
Logging Program. Openhole logs were omitted from the program because thefield geologist and reservoir engineer thought that they were unnecessary.However, mud logs and a measurement-while-drilling (MWD) gamma ray log wereused to determine sand/siltstone interfaces. An MWD tool also was chosen toprovide downhole weight on bit and torque.
Completion. Almost all Dos Cuadras well completions involve a sand controlmethod. Sand control methods vary from slotted liners to full openhole gravelpacks. A previously completed statistical analysis of all completion methodsindicated that maximum production was achieved with wire-wrapped stainlesssteel liner completions Without gravel packs.
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