Multilateral Offshore Well Completed With Multistage Proppant Fracturing
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The complete paper discusses the successful execution of the first offshore multilateral well completed for multistage high-pressure proppant stimulation, according to the authors, in the Black Sea offshore Romania. The authors discuss the drivers that led the operator to trial a multilateral well; the selection, planning, and final completion solution; and the lessons learned.
Lebada Vest, located 95 km offshore northeast of the city of Constanţa, is the largest offshore Romanian oil field currently in production. It was discovered in 1984 and has been in production since 1993, and thus now faces the challenges and strong investment requirements associated with a mature field. Hydraulic fracturing has been used in the field’s low-permeability-formation wells since 1992, mainly in vertical or slanted wells.
With only one platform slot left and a significant undrained area of the reservoir, the operator had to maximize hydrocarbon recovery through a single well which, because of pressure to increase daily production, had to be finalized in 1 year. Building on field experience gained since 2008 in multistage proppant stimulation, a detailed screening and evaluation of multilateral completion technologies was performed. The focus was on developing a concept that would minimize risks during execution while meeting cost and lead-time objectives, which necessitated customizing the chosen Technology Advancement for Multilaterals (TAML) Level 3 completion design and installation methodology.
To maximize rig-time efficiency, the well was executed in two phases:
- Drilling and lower completion installation of both branches with a drilling rig
- Stimulation and upper completion installation with the platform’s workover rig
With six stages in each lateral, the high-pressure stimulation was executed by a converted supply vessel in four sailings, necessary to reload materials. To meet the delivery schedule, ensure simplicity, and leverage operator experience, the completion was undertaken with no dedicated multilateral hardware. Instead, the operator used standard, multistage, stimulation openhole completion equipment and appropriately engineered bent joints to exit the main bore.
With initial production rates higher than anticipated, the well proved considerably more economical than drilling two horizontal wells with equivalent reservoir coverage. The success of this well serves as a proof of concept and provides increased confidence in delivering reliable, cost-effective, multilateral wells, even under tight time constraints and in areas or with operators with no history of multilateral completions.
Identifying Multilateral Completion Requirements
The operator’s field-development team investigated single, dual, and trilateral well-design feasibility against agreed design criteria. The principal objective was to maximize the chance of success through an appropriately risk-minimized design using standard, field-proven tools and techniques. Completion-design criteria were carried from the previous (single-lateral wells) field-development campaign in terms of production objectives, intervention capabilities, and logistics. The complete paper discusses the effect of general well design, junction, stimulation, intervention, rig strategy, and logistics and experience on the completion requirements.
The ML01 well was planned to be completed as a dual lateral with two individual lower completions tied back to the production tubing for commingled production. The lower lateral, drilled out of the mainbore liner shoe, is referred to as Leg 1, while the upper lateral, which is sidetracked from the mainbore liner, is termed Leg 2, and the targets were planned to be reached within a 100°-azimuth difference.
To build on the development strategy already in place, both lower completions were to consist of 4.5-in. openhole multistage fracturing liner strings of six stages, with the intermediate and upper completion designs consisting of 3.5-in. tubing. The final lower completion spacing/staging was based on criteria and observation from the interpretation of logging performed while drilling the well. Packers and ports would be placed according to reservoir quality, rock strength and stress, and the extent of natural fractures.
With both laterals designed to produce from the same reservoir horizon, the operator wanted to sidetrack the upper lateral from the motherbore at up to 90° inclination to maximize production. To maximize accessibility, even in late well life, supporting the formation at the junction was deemed a fixed requirement. A TAML Level 3 cased and cemented main bore with a cased lateral was requested by the operator with full hydraulic isolation from the casing and formation during stimulation, production, and potential restimulation operations.
To increase production and maximize reservoir drainage from the low-permeability formation, the wells needed to be stimulated by multistage hydraulic fracturing. According to the operator’s best practices and historical experience, the stimulation was planned to be performed at 3.2 m3/min with all required equipment placed on a supply vessel and connected by flexible pipe to the platform. With this setup, three stages could be executed before returning to the harbor for reloading chemicals and proppant. The planned job design for six stages in each leg was between 80 to 100 t of 16/20 resin-coated proppant per stage at up to 7-ppa concentration with 300 to 400 m3 of crosslinked fluid.
Ensuring maximum recovery and enabling lower completion accessibility for the life of the well required coiled-tubing intervention for cleaning, restimulating, and closing each producing stage for water shutoff. Additionally, either or both laterals needed to produce or be tested selectively without heavy workover operations.
Several scenarios were conceived for the drilling rig, workover rig, and stimulation-vessel operation and standby time according to the drilling, completion, and stimulation sequence and strategy. With no adequate in-country stimulation spread available, the most-attractive option was to drill and complete Leg 1, drill and complete Leg 2, then stimulate Leg 2, and finally stimulate Leg 1. Time constraints made it imperative that the necessary equipment either be readily available or feasible for rapid engineering and manufacturing.
Selecting the Multilateral Completion System
The selected multilateral system used standard multistage openhole fracturing equipment and appropriately engineered bent joints to exit the main bore with the completion string. The system allowed for standard drilling techniques to be employed, with Leg 1 drilled and completed in the same manner as any other horizontal, openhole, multistage well. After isolating, setting, and testing the Leg 1 liner, a whipstock packer could be set and the Leg 2 lateral drilled to total depth in the same manner as a conventional sidetrack. In the event of continued failure to exit the window to Leg 2, the junction could still be isolated with an upper completion engaged in Leg 1 for stimulation and production of a single lateral only. Batched operations were executed in three phases to allow for the release of the drilling rig as early as possible and to use the platform’s light workover rig to its fullest extent, as shown in Fig. 1:
- Drilling and lower completion installation of both branches performed by a drilling rig
- Hydraulic proppant stimulation using a converted supply vessel and switching of the work string to the next lateral with the platform’s workover rig
- Upper completion installation performed using the platform’s workover rig
The complete paper describes in detail the design summary, multilateral completion installation, and stimulation process, as well as innovations and lessons learned from the project.
- The ML01 well was successfully drilled, completed, stimulated, and put on production on time and on budget, with initial rates exceeding expectations at more than double the original rate of an equivalent single lateral well in the field. No health, safety, or environmental incidents occurred.
- While the process of evaluating and understanding the production evolution of both laterals is ongoing, the strategy has generated significant cost savings over the comparable traditional development strategy.
- Successful installation of the offshore multilateral well completed for multistage high-pressure proppant stimulation validates that the system allowed for simple deployment according to the defined rig strategy and that using equipment from the operator’s inventory minimized equipment lead times adequately.
- This concept will pave the way for future applications. The authors believe that the appropriate application of the technology will have a positive economic effect in field-development projects in which rig and stimulation use and costs can be managed similarly.
Multilateral Offshore Well Completed With Multistage Proppant Fracturing
01 May 2020
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