RIASSUNTO
Summary
Two wells extending more than 2 miles were drilled from Platform Irene in the Pt. Pedernales field, offshore California. These wells are the two longest extended-reach wells on the U.S. west coast. Although several wells around the world have been drilled beyond two miles, none extend at such shallow true vertical depths (TVD's) as the two from Platform Irene. The average angles from rotary table to total depth (TD) for Platform Irene Wells A-16 and A-17 are 70.9° and 68.2°, respectively. This paper presents the concepts of a ""negative-weight well"" and ""drag-risk analysis,"" focusing on the innovative technology used to drill and complete a high-drag well. The successful ,drilling and completion of these wells will allow recovery of millions of barrels of oil originally expected to be produced from a second platform.
Introduction
Unocal Corp., Chevron Corp., and Mobil Oil Corp. acquired Federal Lease OCS P-0441 in OCS Sale No. 53 in May 1981. The Pt. Pedernales field was discovered in Nov. 1982 with the drilling of Well Unoca1 OCS P-0441 No.1. Platform Irene was set in Aug. 1985, and the first well was spudded in April 1986. In March 1987, Tracts P-0437, P-0438, P-0440, P-0441, P-0444, and P-051O were combined to form the Pt. Pedernales unit, consisting of seven partners.
Platform Irene is located in 242 ft of water roughly 4.5 miles from nearest land and 60 miles west-northwest of Santa Barbara, CA. Fig. 1 shows Platform Irene's location relative to the California coast.
Field Development
A viscous, 15.8° API crude oil and gas are produced from the Miocene epoch Monterey formation. The Monterey varies in thickness by several hundred feet and consists of vertically fractured, thin-bedded, and interlaminated cherts, dolomites, and shales.
Production began in April 1987 at the rate of 10,500 BOPD, 0% water cut. Peak production of 23 ,500 BOPD was achieved in Aug. 1987 after 11 development wells were acidized.
The original plan was to develop the southeast portion of the reservoir with Platform Irene and set another structure, Platform Iris, roughly 2 miles to the northwest to develop the rest of the field. The setting of Platform Iris became uneconomic when oil prices plummeted.
Objectives
The high-angle, extended-reach wells were drilled from Platform Irene to achieve three objectives: to eliminate the high capital costs of a second platform, to intersect more vertical fractures and cut more formation with the near-horizontal wellbores, and to prove that a negative-weight well could be drilled and completed economically.
It was proved that most of the field could be economically developed from Platform Irene.
Negative-Weight Well
Because differential sticking was not a major problem in Platform Irene wells, the key obstacle was drag. Extensive drag data from previously drilled wells gave significant insight into the severity of drag that. would be encountered in Well A-16. The majority of Well A-16's path would surpass the critical, drag angle determined with previous data. A critical drag angle is the wellbore inclination at which a pipe element, with no other forces than gravity and drag, will no longer slide down the hole. The critical angle is reached when a pipe element's axial force equals the drag created by its normal force. In simple 2D terms,
Equation
If the typical friction factors for a field are known, the approximate critical angle at which pipe elements stop contributing a positive weight and start contributing a negative weight is also known.
When the majority of the elements of a constant-weight string of pipe lie at inclinations greater than the critical angle, the entire string will not slide in the hole. An additional compressive (i.e., negative) force is required to push the string in the hole; hence the name negative-weight well.
An understanding of the severity of drag in a negative-weight wellbore is necessary to understand the difficulties of planning Well A-16.
Directional Planning of Well A-18
Each hole section was examined individually and assigned specific well-path design criteria.
Conductor Hole.
Because of the proximity of existing wells, the need to acquire section as quickly as possible, and efficient slot utilization, an 8° ""nudge"" was planned for the surface hole.
17 1/2-in. Surface Hole.
During the drilling of previous high-angle wells of shorter reach, heavyweight drillpipe (HWDP) was run on top of the drill string to overcome weight-on-bit (WOB) problems. Consequently, severe drillpipe fatigue stress cracking resulted from buckling and eventually led to a costly twist-off. Therefore, the inclination of the large surface hole was minimized and WOB was kept under 50,000 lbf.
The plan called for a build rate of 4.5° /100 ft to 65°, with a 50ft radius target at 65° on the mathematical line. From there, a gradual build rate of 0.51°/100 ft was planned to an angle of 79° at casing point, with another 50-ft radius target. The multiple-target criteria were assigned to ensure that the proposed path was followed. Staying on the path was essential because the accompanying casing string would be nearing its drag limit.
12 1/4-in. Hole.
This section was designed to hold angle at 79° until the producing formation was encountered, and then to build 3°/100 ft to 85.5°. As a guideline, the actual well path was not to drop more than 50 ft below the line or below 75° inclination.
Drag analysis showed that the 9 5/8 in. casing could not be run conventionally to the desired setting depth because of excessive drag forces, so an experimental casing running technique was planned. Because of the unconfirmed running technique, the hole angle was minimized to permit the majority of the hole to be cased conventionally if the new technology failed.
8 1/2-in. Hole.
The plan called for holding an inclination of 85.5° to the mid-Monterey target, the 9,033-ft section at 3,520 ft TVD, with a 200-ft radius target. The path would then drop at a rate of 0.3°/100 ft out of the Monterey. Because of drilling and geologic uncertainties, a preliminary target, the 11,473-ft section at 3,870 ft TVD, was assigned at TD.
Conductor Hole.
Because of the proximity of existing wells, the need to acquire section as quickly as possible, and efficient slot utilization, an 8° ""nudge"" was planned for the surface hole.
17 1/2-in. Surface Hole.
During the drilling of previous high-angle wells of shorter reach, heavyweight drillpipe (HWDP) was run on top of the drill string to overcome weight-on-bit (WOB) problems. Consequently, severe drillpipe fatigue stress cracking resulted from buckling and eventually led to a costly twist-off. Therefore, the inclination of the large surface hole was minimized and WOB was kept under 50,000 lbf.
The plan called for a build rate of 4.5° /100 ft to 65°, with a 50ft radius target at 65° on the mathematical line. From there, a gradual build rate of 0.51°/100 ft was planned to an angle of 79° at casing point, with another 50-ft radius target. The multiple-target criteria were assigned to ensure that the proposed path was followed. Staying on the path was essential because the accompanying casing string would be nearing its drag limit.
12 1/4-in. Hole.
This section was designed to hold angle at 79° until the producing formation was encountered, and then to build 3°/100 ft to 85.5°. As a guideline, the actual well path was not to drop more than 50 ft below the line or below 75° inclination.
Drag analysis showed that the 9 5/8 in. casing could not be run conventionally to the desired setting depth because of excessive drag forces, so an experimental casing running technique was planned. Because of the unconfirmed running technique, the hole angle was minimized to permit the majority of the hole to be cased conventionally if the new technology failed.
8 1/2-in. Hole.
The plan called for holding an inclination of 85.5° to the mid-Monterey target, the 9,033-ft section at 3,520 ft TVD, with a 200-ft radius target. The path would then drop at a rate of 0.3°/100 ft out of the Monterey. Because of drilling and geologic uncertainties, a preliminary target, the 11,473-ft section at 3,870 ft TVD, was assigned at TD.