Teamwork and technology contributed to the successful installation of three offshore multilateral wells for Burlington Resources.

Multilateral completion technology and close collaboration created a winning formula that allowed Burlington Resources to develop its Millom West field in the Irish Sea successfully and economically using technology advancement for multi-laterals (TAML) Level 2 completions. Crews carried out the completion phase without incident, met all objectives, brought a relatively tight natural gas accumulation onstream, ran and commissioned multilateral completions virtually trouble-free and came in ahead of the AFE's "most likely" timings. Even more importantly, the project suffered no accidents. Critical factors in this success were project planning, proven technology for multilateral open-hole completions and a consistent team - including the operator and service suppliers - that worked together during all phases of the project.
Millom West field
Burlington Resources is the operator for the Millom West field, located in Block 113/26a of the Irish Sea about 25 miles (40 km) west of Barrow. The field was to be developed utilizing multilateral drilling and completion technology via three multilateral wells (two dual-lateral and one tri-lateral). One conventional vertical well was also drilled. The wells were designed to access the Ormskirk Sandstone reservoir, exiting the casing and drilling lateral sections, each with about 3,000 ft (915 m) of 6-in. diameter hole.
The Triassic reservoir of the Millom West structure is composed of the Ormskirk Sandstone formation of the Sherwood Sandstone Group, similar to the Keuper Waterstones and Keuper Sandstone formations of the nearby South Morecambe field. The discovery well, 113/26-1, encountered 778 ft (237 m) of Ormskirk Sandstone formation from 4,122 ft to 4,900 ft (1,257 m to 1,495 m) TVDSS above the St. Bees Sandstone formation. A free water level was identified at 4,420 ft (1,348 m) TVDSS.
Five wells previously had been drilled through the Upper Sherwood Sandstone Group in the Millom field: three near vertical, one inclined and one horizontal well. About 1,115 ft (340 m) of core samples were taken. The reservoir is characterized by a complex, highly layered suite of lacustrine, sabkha, aeolian and fluvial facies deposited in an erg margin setting. Reservoir quality is significantly reduced (compared to the other east Irish Sea fields) by the presence of pore-blocking diagenetic cements. It is for this reason that multilateral technology was employed to maximize the wellbore length. The Millom West reservoir contains dry gas with a low percentage of CO2 content and no traces of H2S.
Project parameters
Initial objectives for the Millom West completions called for: exiting 95/8-in., 53.5-lb/ft casing; providing multilateral TAML Level 2 and Level 4 systems with options to access one or two bores; providing selective isolation of either bore; allowing commingled flow to the surface; and providing a 6-in. open hole through the reservoir. Overall project objectives included the following.
Maximize well productivity by:
• placing the well a sufficient distance from the central fault;
• placing the well in a clearly defined target;
• ensuring the well has a maximum deviation through the reservoir to maximize the pay and allow wireline work;
• minimizing skin damage by mud compatibility and optimization;
• logging to identify the free water level for perforating;
• obtaining zonal isolation through quality cementation of the production liner in lateral bores;
• optimizing the tubing size for well life; and
• ensuring full cleanup of each lateral.
Ensure 25-year life of field by:
• proper conductor design and corrosion protection;
• casing design and materials to ensure well integrity;
• leaving non-corrosive mud in well annuli;
• minimal installation of completion equipment; and
• minimizing well maintenance and interventions during the life of the field.
Establish reserves and reserves uncertainty for commercial and fiscal requirements by:
• logging and establishing the exact depth of the free water level in the well.
Establish long-term productivity and reservoir quality data by:
• acquiring quality wireline logs; and
• conducting a contingent vertical seismic profile at TD if top reservoir depth is significantly different from the expected program.
Acquire accurate geological data by:
• gathering geological data from cuttings and mud logging during the drilling phases; and
• obtaining geological control point at the toe of well by penetrating overlying formations.
The bulk of the project involved a jackup drilling rig cantilevered over a newly installed, normally unmanned, minimum-facilities platform. The final platform work program included one tri-lateral, two dual-laterals and one near-vertical single-bore well. In addition, a single, standalone well was drilled in Block 113/27a using "finder well" principles to provide geological well control for a further subsea completed tri-lateral well.
After detailed design review, the junctions were all of the TAML Level 2 design. Due to the requirements for future access into the lateral legs, the multilateral junctions were placed at less than 60 degrees to allow wireline operations during completion and testing. Crews gained experience on a wide range of multilateral issues, Lateral Entry Nipple (LEN) installation, External Casing Packer (ECP) inflation, WindowMaster whipstock setting and recovery and window milling. The development was particularly noteworthy with respect to the fractured nature of the reservoir. The extremely hard formation required proper selection of bit and motor for optimum drilling. Mud losses, both static and dynamic, were encountered in several of the legs - more than 14 instances in all. The methods of dealing with the loss problems ranged from conventional particulate lost circulation materials (LCM) to a floating mud cap solution. Ultimately, only cement squeezes could cure the problems.
The project benefited from close collaboration among all parties, enabling a complex drilling and completion program to be executed efficiently, while simultaneously constructing and commissioning a platform - all with an exceptional HSE performance.
Multidisciplinary team approach
Multilateral wells are not difficult, but they are more intricate than other wells. For this reason, all parties were involved from day one to ensure each operational phase would not compromise any future operations. Dog-leg severities were controlled to enable liner and completion running; drilling motor angles were predetermined and checked for compatibility with equipment prior to running. Cleanliness within the well bore was greatly emphasized as well as well control and safety during all operations.
The fact that this was a multi-well project gave both operator and service companies the opportunity to utilize the same people for the same operations and operational phases on each well in the project. A team of personnel, operating through a single point of contact, was dedicated to the project. The single-point contact helped manage risk by cutting down on miscommunication and protecting interests of both the operator and Baker Oil Tools. This also became very important during the coordination of stack-up tests and subassembly work to ensure that fully endorsed coiled tubing and wireline bottomhole assemblies were field tested and optimized for traversing the junction.
Detailed training courses at the Baker Oil Tools facility relayed all aspects of the multilateral completion equipment. The operator's senior management and the third-party service personnel attended these courses, which raised the awareness of this new hardware and gave participants additional comfort and understanding of its applications.
Management found immeasurable added value with consistent selection of personnel. The interfaces were numerous, but with the team remaining in place for the duration, lessons learned early in the project were carried onto the next phase and beyond.
As problems or unplanned operations occurred during the installation phase (lost circulation, for example), quick-turn manufacture items and well-design changes were not uncommon. Due to the design flexibility in the multilateral and the team-based approach, last-minute changes required to the completion design were relatively simple to achieve, allowing change-out of one component for another to meet the new requirement. The close-working, multidisciplinary team approach and risk assessments that were carried out ensured that all effects of the change were reviewed, discussed, approved and implemented within a relatively short time frame.
In another instance, spaceout changes in the open-hole section to place the isolation packers in different locations meant unplanned sub-assembly breakouts and make-ups were necessary. Teamwork and dedicated personnel ensured these operations were carried out accurately, safely and in a timely manner.
Flexibility and challenges
One of the challenges of this project was the placement of the casing junction within the reservoir section. In the dual-lateral wells, the measured depth from the top of the reservoir to the 95/8-in. casing shoe was about 160 ft (49 m). The first (lowest) joint was a conventional shoe joint. The second joint had a float collar on the lower connection (to create a single joint shoetrack) and was also the joint that housed the orientation datum packer/hanger system, which subsequently anchored the whipstock (and later the LEN). The third joint housed the whipstock from which the casing exit was made, and the fourth was a contingency joint for the casing exit. This resulted in a vertical displacement from the top of the reservoir to the 95/8-in. casing shoe of 80 ft to 100 ft (24 m to 31 m), as the junction had to be positioned within wireline capability (<60 degrees). Due to the relatively short liner available for tieback, the lower casing joints were hand-selected for optimum spaceout.
As the multi-well project continued, it was possible to use equipment backups from previous wells, resulting in cost benefits to the project. Each casing exit was created using the Baker Oil Tools one-trip WindowMaster casing exit system in combination with the orientation datum packer/hanger system. For this reason, it was possible to provide some flexibility in the junction completion design system based on findings during the drilling construction phase.
In the tri-lateral well, both casing exits were created from the same orientation datum packer/hanger system. The casing exit junctions were positioned about two joints apart. A detailed critical spaceout and alignment procedure onshore prior to shipment ensured accuracy and a safe working operation during makeup and alignment once the equipment arrived offshore. Prior to commencing the milling and recovery of the casing exit and whipstock system, these operations were perceived as being the most risky parts of the entire program. Although these operations went relatively smoothly, during the initial drilling of the first lateral on this well, mud losses were encountered in the main bore. This meant that the prime whipstock had to be recovered, contingency well control procedures taken and the backup whipstock installed. These operations were completed trouble-free, allowing the remaining drilling operation to continue. In each wellbore, the exact spaceout reference dimensions, casing exit geometry, top of window, bottom of window, whipstock face alignment and orientation of the datum packer/hanger
system were accurately checked. These were also checked and approved by all relevant parties and recorded for future operations to ensure accurate placement of the junction completion equipment.
Winning formula
There was no non-productive time allocated to any of the multilateral construction phases in any of the Millom West wells, indicating that although the procedures are intricate, multilaterals can be completed successfully and economically through dedicated teamwork. All lessons learned during a specific operation were captured for use in the next well completion program.

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