Ability to seamlessly integrate all phases
China National Offshore
The world's first technology advancement for multilaterals (TAML) level 6 intelli-gent completion could help steer the industry to develop similar installations for new wells and reentry applications.
A pilot log from well NE Intan Alpha 22 was used before sidetrack.
The primary incentive to apply the multilateral well technology by Schlumberger and the China National Offshore Operating Co. on this project in the South Java Sea off Indonesia was minimizing further capital investment of installing an additional platform structure by maximizing usage of existing slots. Costing less than two horizontal wells, not including fishing operations, this intelligent completion allowed for optimized setting of the downhole choke and delayed water influx.
Over the years, the understanding of field structure has evolved and revealed some significant amount of oil left between existing wells in smaller isolated targets. Multilateral technology makes it possible to penetrate more of the reservoir while using only one slot and to achieve greater flow rates from individual parent wellbores, increasing total field production by drilling all infill wells in the development program and combining the targets.
As a result of savings in rig time, the level 6 multilateral overall well cost was about the same as a level 4 completion, including rig time. The level 6 operation minimized risks, trips, and downhole intervention. The RapidSeal junction allows for selective re-entry for any future workovers.
Advanced completion equipment installed above the junction included downhole hydraulic flow control valves to control production in each leg and sensors to measure pressure, temperature, and flow rate for each well branch. Based on this first successful RapidSeal intelligent completion, Cnooc intends to install another level 6 intelligent multilateral completion in 2Q 2004.
The two-leg multilateral well was drilled and completed from the NE Intan Alpha platform in 75 ft of water. Utilizing artificial lift, the NE Intan A-24 well is the first TAML level 6 multilateral well drilled in Indonesia and the world's first intelligent completion in a multilateral level 6 well.
North East Intan field
The producing horizon for both legs of the NE Intan A-24 multilateral well is the 31-1 reservoir, which is the main productive sand body in this and adjacent fields and is part of the uppermost early Miocene Talang Akar formation deposited just prior to marine transgression across the Asri basin area. A strong water drive exists in this field as a result of widespread sands pinching out laterally in the direction of the Intan field.
A poorly consolidated quartz sandstone (medium grained and coarser in parts) with high porosities (27% to 33%) and permeabilities in the multi-darcy range, the 31-1 sand is fluvial, but deposited low on the delta plain. Stacked sand bodies are present; units range from 20 to 80-ft thick in parts. This and other producing sands are sealed by 50 to 100 ft of shale, coal and limestone interbeds.
Key project considerations
Considered a mature field located in the Cnooc Southeast Sumatra Production Sharing Concession, the North East Intan A field was first developed in 1990. After drilling several pilot-disposal wells, it was decided to develop seven bottomhole targets. However, only three slots from existing platforms were available for use. Two slots had 30-in. conductors and the other a 20-in. conductor. The development was planned as follows:
- Slot A (30 in.) consisting of two single wells using conductor-shared technology to address two targets
- Slot B (30 in.) consisting of one level 4 multilateral well and one single well using conductor-shared technology to address three targets
- Slot C (20 in.) consisting of one level 6 multilateral to address two targets.
The plan was considered the most practical and economically feasible way to develop the reserves because, in addition to the cost of an additional caisson (a savings of about $3.5 million), the minimum lead-time for construction and installation would have been six months, resulting in deferred production and cash flow. It was also anticipated that the cost of the multilateral well should be less than twice that of an average single horizontal well, and production should be about twice that of an average single horizontal well.
These factors were then entered into a proprietary well economics program to generate acceptable net present value and rate of return for the project. To minimize completion cost, a business model was developed that facilitated the leasing of the intelligent completion (IC) assembly, considered an operating expense rather than a capital expense.
Equally critical to overall well performance and optimization of the production, IC components included:
- An electrical submersible pump to optimize oil flow from the wellbore
- Two downhole flow control valves to minimize water influx and selectively control the flow from either leg
- Downhole well sensors that provide real-time temperature and pressure readings from both legs of the well
- A multiphase flowmeter at surface to monitor well performance.
After careful consideration of target locations, well planning, and collision issues, it was decided to drill and develop NE Intan Alpha 22ST and NE Intan Alpha 24 as the multilateral. To ensure the desired return rate, the combined reserves of the well (around 900 MMbbl of oil) were a significant contributing factor in the decision-making process. Reservoir properties were not an issue as both wells were located in the same reservoir and possessed similar characteristics.
Drilling and junction deployment
The hole was drilled in four sections with 13 3/8-in. surface casing, 9 5/8-in. main bore and two laterals out of the RapidSeal symmetrical dual-leg level 6 junction. Objectives were to construct a 9 5/8 x 7 x 7-in. dual-lateral well using the multilateral junction, drill and successfully complete the laterals to produce the 31-1 sands, and successfully install the intelligent completion with an electrical submersible pump. The junction optimizes the branch-parent diameter ratio while providing a predictable mechanical integrity based on a known geometry. During manufacture, the junction legs are compressed so that the overall junction outer diameter is comparable to that of the parent casing, reducing casing size loss in the transition between the parent casing and the two symmetrical outlets.
The first two sections were drilled as planned and incident free. The 12 1/4-in. section was touched down and the bottom 200 ft was underreamed to 17 1/2 in. to facilitate junction expansion. The junction was run on the bottom of the 9 5/8-in. casing to a depth of 2,770 ft measured depth at an inclination of 24.3°.
The Modular Junction Expansion Tool was then deployed into the junction on wireline. With the casing set in the slips, the assembly and casing string were then rotated 130° so that the orientation slot of the Indexing Casing Coupling was at a tool face of 358° as planned.
Within 45 min, the junction was successfully expanded and its final geometry was rechecked by expanding the MJET a second time. Both times, the internal drift of the reformed legs were measured by downhole calipers on the MJET and transmitted to surface via electronic sensors. With the API drift confirmed, the casing was cemented using a drillable bridge plug and an inner string. The retainer was then drilled out and the drilling deflector installed to access outlet/leg 1 to drill the first lateral.
With the junction cemented in place, both lateral branches were drilled using synthetic oil-based mud, a 6 1/8-in. polycrystalline diamond compact (PDC) bit, and a bicenter PDC bit. The junction created a transition from 9 5/8-in. casing to two 6 1/8-in. laterals. The laterals were completed with premium screens, employing an external casing packer to isolate the producing zones.
The level 6 intelligent multilateral completion now in the ground for Cnooc represents a significant milestone in the fast developing area of multilateral and intelligent completion technology. Inherent advantages of the level 6 system earns it the distinction of being a preferred alternative to a level 3 or 4 in new wells since the cost differential is negligible.
Implementation of sophisticated completion components in tandem with advanced multilateral technology has enabled the company to optimize the use of a limited number of well slots on its existing platforms, improving production, oil recovery and field economics. As a result of this success, Cnooc plans to run more level 6 multilaterals in 2003.