ExxonMobil, operator for the Sakhalin-1 project, planned an additional drilling campaign at the Chayvo field, Sakhalin Island, Russia. The focus of this campaign was to develop a thin-oil-column reservoir by use of extended-reach drilling (ERD). This overview describes the planning of these wells at the edge of the ERD envelope, the associated technical and operational challenges, and the complexity of targeting a thin oil column in an ERD environment. Planning of these wells involved building upon the operator’s previous experience and the use of additional tools and methods to address unique challenges.
Introduction
The Sakhalin-1 project comprises the Chayvo, Odoptu, and Arkutun Dagi fields off the east coast of Sakhalin Island, Russian Federation, as shown in Fig. 1. Development drilling at the Chayvo field started in 2003 with extended-reach wells drilled from an onshore wellsite pad with the Yastreb drilling rig. In 2005, further development drilling commenced from the offshore Orlan platform. In 2008, the Yastreb rig was moved approximately 75km north to the Odoptu onshore wellsite pad and drilled nine ERD wells as part of the initial-stage development of Odoptu. Following the Odoptu campaign, the Yastreb rig was moved back to the Chayvo onshore wellsite in 2011 for further development and infill drilling of the Chayvo reservoirs.
As of October 2012, 15 of the 20 longest ERD wells in the world had been drilled in the Sakhalin-1 project, with the recently completed Well Z-44 at Chayvo field setting a new measured-depth (MD) record at 12 376 m. Significant well-design parameters, tools, and techniques that enabled these challenging wells are detailed in the complete paper. The technical challenges included uncertainties in the location of the oil column and fluid contacts, the need for very accurate well placement for effective reservoir drainage and for preventing early gas/water breakthrough, completing long horizontal intervals, and high-torque/drag effects. The geologic uncertainties and wellbore-positioning challenges were addressed with a combination of a single near-horizontal pilot hole to locate the fluid contacts and a novel technique to minimize vertical-position uncertainty by use of formation-pressure measurements while drilling. Extensive torque/drag modeling and wellbore-stability modeling were performed to design the wells and equipment and define operational parameters. Completion designs were optimized to ensure successful placement in long horizontal intervals.
Chayvo-Zone-16 Challenges
Well Z-44 was the first well drilled to develop the Zone-16 reservoir at Chayvo. Fig. 2 shows the planned development of the northern portion of this reservoir from the existing onshore wellsite pad. Initially, four oil-producing wells and one gas-injector well were planned from this wellsite pad. The reservoir is part of a northwest/southeast-trending anticlinal structure, with the western flank closer to the shore than the eastern flank. The oil producers were planned to be horizontal wells drilled through the western flank, crossing the structural axis of the anticline into the eastern flank. The gas injector was planned to be placed horizontally in the gas cap. The distance of the reservoir targets from the onshore wellsite pad resulted in the planned wells being at the edge of the ERD envelope in terms of MD and horizontal reach. Also, the openhole-completion lengths were estimated to be greater than 3000 m in several of the wells.
An additional challenge was that the oil-column thickness (based on exploration-well data) was estimated to be approximately 20 m. The vertical position of the oil/water contact (OWC) and gas/oil contact (GOC) had an uncertainty of ±4–5 m. Results from reservoir-simulation studies indicated the best recovery would be obtained when the horizontal oil producers were placed at the middle of the oil column, vertically, with a small tolerance for deviation because of the risk of early gas or water breakthrough. Given this objective, and considering the thin oil column and the fluid-contact uncertainties, the wellbore-placement challenge was clear because wellbore-position uncertainty grows with increasing MD. To the operator’s knowledge, this type of wellbore-positioning challenge at the edge of the ERD envelope had not been faced before.
Planning for Challenges
A multifunctional team of drilling, reservoir, and geoscience personnel worked together for almost 1 year to plan the Zone-16 development wells from the Chayvo onshore wellsite pad. The successful experience of drilling ERD wells at Odoptu and the use of downhole formation-pressure-while-drilling tools were brought to bear on the planning of the Zone-16 wells. The wellbore-positioning challenge was broken into two components. The first challenge was establishing the fluid contacts and oil-column thickness with a single pilot hole in the first development well. The second challenge was developing a novel technique to use formation-pressure-while-drilling data to minimize vertical-position uncertainty, in addition to the use of state-of-the-art surveying tools and techniques. The ERD challenge was addressed by extending the successful well designs, tools, and practices used previously at Sakhalin-1. The Yastreb rig was upgraded during the rig move from Odoptu back to the Chayvo location, emphasizing equipment that would further enhance its ability to drill and complete long-reach wells efficiently.
Thin Oil Column and Fluid-Contact Uncertainty. Accurate vertical placement of the horizontal production wells within the oil column was essential to avoid early water or gas breakthrough and to maximize ultimate oil recovery, but neither the OWC nor the GOC was well constrained by offset wells. Fluid-contact uncertainty was addressed by planning a pilot hole in the first development well before drilling the reservoir section. The first well would be landed horizontally at the predrill-estimated middle of the oil column, and then the intermediate/production liner would be run. As shown in Fig. 3, the pilot hole was designed to then plunge below highest known water before being turned upstructure to penetrate the water, oil, and gas zones. Once confident that water, oil, and gas gradients had been established, the pilot hole was designed to turn up farther and penetrate the top of the reservoir, providing a second structural-control point and helping to reduce structural uncertainty.
After the pilot hole was complete, it was plugged and abandoned with cement in preparation for the subsequent openhole production sidetrack. Because of the unconventional wellbore profile and the importance of ensuring wellbore isolation, the team cemented a sacrificial liner with swell packers in place to ensure proper isolation. An openhole wellbore anchor was used to set the liner in place, ensuring the liner remained stationary throughout the cementing job. Next, a kickoff cement plug was placed on top of the sacrificial liner using a modified balanced-plug procedure. This operation built upon learnings from setting openhole kickoff plugs in ERD wells at Odoptu in that the team underdisplaced the cement in the drillstring by the amount of drillpipe-metal displacement expected during drillstring retrieval. The team then slowly rotated and pumped the drillstring out of hole until the end of the drillstring was out of the cement column.
Wellbore Placement. The combination of a thin oil column and the uncertainty inherent in the measurement-while-drilling (MWD) surveys in such long-reach wells meant that even after applying a newly developed best-practice MWD total-vertical-depth (TVD) assurance, the predrill-estimated vertical uncertainty at the reservoir-entry point on the first well was ±6.6 m, which represented approximately 66% of the estimated column height. The effect of vertical well placement was tested in full-field and fine-scale models. In the full Zone-16 development plan, placing wells 6 m from the middle of the oil-column target reduced overall recovery by 30% over the life of the field. In the fine-scale model, a difference of 4 m affected single-well recovery by up to 10% over the first 10 years. Therefore, there was a need to reduce the vertical uncertainty further than was possible with surveys alone, to make the development feasible.
Well Designs and ERD Challenge
The well schematic shown in Fig. 4 is for the first well, Well Z-44, with the pilot hole. The typical design for subsequent Zone-16 oil producers was similar, except without the pilot hole. The design included 30-, 18⅝-, and 13⅝-in. casing strings; a 9⅝-in. liner; and an 8½-in. open hole to total depth (TD). The openhole completion used a 6⅝-×5½-in. design consisting of sand-control screens, inflow-control devices, predrilled liner, and swell packers for zonal isolation. The upper completion used 5½-in. tubing.
The plan is to drive the 30-in. casing to a depth of approximately 80 m. Then, the 24-in. hole is drilled to approximately 800 m with a final inclination of 27 to 37°, and the 18⅝-in. casing is set. The 17½-in. hole is drilled, building to a tangent angle of approximately 80°, and the 13⅝-in. casing is set. The 12¼-in. hole is drilled at the tangent angle and then built to a final angle of 90°, and the section TD is reached before entering the reservoir interval. This plan allows the well to be positioned to drill the 8½-in. production hole horizontally through the reservoir targets. To reduce the equivalent circulating density of the subsequent 8½-in.-hole section, the wells are designed to run the 9⅝-in. string as a liner. To run the liner all the way to the planned depth, the liner must be selectively floated (i.e., air inside the liner and mud inside the running string). The 8½-in. production hole is drilled horizontally through the reservoir to the objective TD. The drilling mud is displaced with a filtered mud to allow running the lower completion with sand-control screens without plugging. The lower completion is run into the open hole with a swivel tool that allows the drillpipe landing string to be rotated without rotating the lower completion. The upper completion, consisting of 5½-in. tubing, downhole pressure/temperature gauges, gas lift mandrels, and surface-controlled subsurface safety valve, is then run.
A key consideration to ensure success in this drilling campaign was maximizing drill-bit and bottomhole-assembly (BHA) life expectancy by reducing damaging vibrations. This effort also helps minimize wellbore patterning that would result in higher openhole friction factors. The best bit designs from the previous drilling campaign were carried over and redesigned to extend the drilling limiters. Emphasis was placed on vibrational stability of the bit, rather than solely maximizing rate-of-penetration (ROP) capability. The BHA-vibration tendencies were modeled by the operator and the directional-drilling-service provider to find component configurations that would minimize the potential for downhole vibrations. Both vibration models recommended the same BHA configurations, which provided assurance that the team had optimized the BHA designsuccessfully.
The operator implemented its fast-drill process early in the planning stages of this drilling campaign. Focusing on persistent redesign is the cornerstone of this process. A physics-based understanding of drilling operations is crucial for successful application of this process. From this scientific understanding, the operator identified limiters that, once extended, improved drilling performance. Focusing on one limiter at a time and working to eliminate it allowed the team to focus its efforts. One example of the process was use of step-rate tests at the beginning of each hole section. The bit-rotation speed, weight on bit, and mud-flow rate were varied to determine the combination of parameters that achieved the maximum ROP without incurring high vibrations. This method enabled use of the optimum drilling parameters.
Results
The Zone-16 development-drilling campaign at Chayvo began in early 2012 with Well Z-44. Learnings from the Odoptu campaign and previous Chayvo campaigns were incorporated in its design and execution. The well reached all the objectives, and resulted in a world-record 12 376-m MD (with a reach of 11371m), as shown in Fig. 4.
Successful application of the planned formation-pressure technique for wellbore positioning was achieved. The pilot hole established both of the fluid contacts and the oil-column thickness. The production hole was placed at the middle of the oil column per the plan. The data gathered in this first well were used in the design and execution of subsequent development wells. As of the writing of this paper, the second well, Well Z-45, had been drilled and completed to 11 277-m MD (reach of 10 251 m).
Batch Operations. The 30-in. conductor and the 18⅝-in. surface casing were set in a batch mode, gaining efficiency with repetitive operations and minimizing swap outs of the mud system from water-based to nonaqueous fluid. The 30-in. conductor was driven to a depth of 76 m, with an intermediate cleanout run at 39 m. A 10-lbm/gal water-based mud was used to drill the 24-in.-hole section to 800-m MD per plan. The well inclination at section TD was 27°. This hole was drilled with a single BHA consisting of a mill-tooth bit, mud motor with a bent housing, and both gyro- and magnetic-survey MWD tools. The 18⅝‑in. casing was run and cemented in place by use of 5⅞-in. drillpipe as an inner cementing string that was stung into the 18⅝-in. float shoe.
Completion. The 3750-m-long Well Z-44 lower completion was run successfully. Although a swivel tool was placed in the running string to allow rotation above the screens to decrease axial drag, it was not needed to get to liner-setting depth of 11 943 m. By use of a combination of 5⅞-in. drillpipe, 5⅞-in. heavyweight drillpipe, and 6⅝-in. drillpipe, the team was able to optimize string stiffness to mitigate severe drillstring buckling while running in hole. The optimized landing-string configuration played a large role in successful running of the lower completion. After the liner hanger was set and the running tool released, the well was displaced to 9.2-lbm/gal filtered mud above the liner to reduce the buoyant forces on the subsequent 5½-in. upper-completion run. After the upper completion was run, the wellbore was displaced with diesel, and the production packer was set hydrostatically. The well was completed 3 July 2012 after a 15-day completion operation.
Conclusions
The Chayvo Zone-16 reservoir development from an onshore location required planning record-length ERD wells and presented a unique well-placement challenge because of uncertainties in the vertical position of the thin oil column coupled with the requirement to place horizontal production holes with accuracy beyond that available with the latest MWD TVD-assurance techniques. To identify the vertical position of the oil column and fluid contacts accurately, a single near-horizontal pilot hole was planned. A unique approach to well-TVD positioning was developed that used a high-quality offset-well pressure-data set along with formation-pressure-data acquisition while drilling the ERD wells.
The operator drilled and completed the first Zone-16 development well, Well Z-44, to 12 376-m MD, establishing a new ERD MD record. The pilot hole identified the oil-column vertical position and both fluid contacts successfully. The well was landed and placed at the middle of the oil column successfully by use of the planned formation-pressure-while-drilling technique. The data gathered from the first well were used to drill and complete the second development well successfully (Well Z-45) and are being applied to planned wells.
Key contributors to successful drilling of this ERD well targeting a thin oil column were
- Multifunctional planning team with adequate planning time, leveraging state-of-the-art formation-evaluation capabilities.
- Up-front detailed modeling to develop a robust well design.
- A purpose-built drilling rig with equipment capable of delivering adequate mechanical power and hydraulics.
- Well-trained operating staff empowered to execute the plan while using proven workflows.
Learnings from this ERD well and the novel wellbore-positioning technique may enable development of other thin-oil-column reservoirs that may be technically or economically challenging in an ERD environment.
This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 163487, “Case History of a Challenging Thin-Oil-Column Extended-Reach-Drilling Development at Sakhalin,” by Vishwas P. Gupta and Shea R. Sanford, SPE, ExxonMobil Development Company; Randall S. Mathis and Erin K. DiPippo, SPE, Exxon Neftegas; and Michael J. Egan, AIPC, Consultant to Exxon Neftegas, prepared for the 2013 SPE/IADC Drilling Conference and Exhibition, Amsterdam, 5–7 March. The paper has not been peer reviewed.
