Multilateral wells increase reserves, lower costs on Troll Olje field

Within the Troll gas field, Norsk Hydro must develop two areas that have a thin oil rim: a 22-26 m thick oil zone in the Troll Olje oil province and a 13-m thick oil zone in the Troll Olje gas province (Fig. 1). From the two areas combined, the company expects to recover 1.35 billion bbl of oil.

The Troll field wells produce to one of two floating production platforms, the Troll B and Troll C, and by January 2001 the company had produced a total of 585 million bbl of oil from the field.

Norsk Hydro has drilled and completed 80 wells, which included 13 dual-branched multilateral wells. Continued development of Troll Olje will require 15 additional multilateral wells, which will include two three-branched multilateral completions.

Multilateral development

Norsk Hydro began the multilateral well development for the Troll Olje field with three TAML (Technology Advancement for Multilaterals) Level-4 wells, based on the Halliburton Sperry-Sun System 4503, a milled casing exit system.

The wells achieved junction isolation with special cement and a formation resin squeeze treatment. Two of the wells, however, experienced lumps of resin flowing back and plugging surface equipment.

Crews had completed rig operations to install the Level 4 multilaterals in the three wells in 22, 17.9, and 14.6 days, respectively.

All of the wells were technically successful and the installation time trended downward. Using the milled casing exit system, however, the companies estimated that they would reach a lower limit of 12 days to install a lateral.

This was due to the extensive milling operations required to establish the initial casing exit and to regain entry to the main wellbore. To improve the junction reliability and the installation time, Norsk Hydro would require a new system.

The company accepted Halliburton Sperry-Sun's proposal to develop further its existing prototype TAML Level-5 Isolated Tie-Back System (ITBS) for use on Troll Olje.

The ITBS provides both hydraulic and mechanical isolation, a combined lateral flow area that is larger than that of the tubing above, and an acceptable solution for access to main bore and lateral.

Engineers created an early prototype of the system in 1998-99 and developed the system further in 2000. Norsk Hydro completed the first TAML Level-5 multilateral installation in the Troll Olje field in January 2001.

The ITBS is a multilateral system based on a premilled window and latch coupling placed in the 95/8-in. liner section. After installation of screens in the main bore reservoir section, the rig installs a whipstock in the latch coupling, then opens the window to drill the lateral.

To complete the well, the rig retrieves the whipstock and replaces it with a deflector and installs a multilateral junction system as an integral part of the lateral bore completion.

The main component is a flexible hanger with D-shaped legs. One of the legs attaches to the lateral liner and the other has a stinger that orients and lands in a seal stack in the main bore deflector.

Norsk Hydro installs screens in both the main and lateral boreholes in the Troll Olje wells and without cementing the lateral liner across the junction.

Project objectives

Based on the field operating requirements and multilateral installation experience using Level-4 System 4503, Norsk Hydro defined the following objectives for the ITBS project development for Troll Olje:

• Reduce installation time and cost, targeting a potential 3-day system installation time.
• Create mechanical seal to isolate the junction (TAML Level 5).
• Make the installation process robust and simple, from floating rigs, with no milling of steel.
• Optimize the lateral junction's flow area.
• Achieve access to main bore and lateral.
• Maintain ability to plug both the main and lateral bore above the junction.

Field installation results

The semisubmersible drilling rig West Vanguard drilled and completed the well Y-22, which was the first ITBS well in Troll Olje field.

The drilling rigs Polar Pioneer, West Venture, and West Vanguard subsequently completed nine additional installations.

The company made considerable progress in achieving its TAML Level-5 system installation goals.

Compared with a new slot with single lateral well, Norsk Hydro can reach a targeted reservoir completion with a preplanned branch from an existing well, as a rule of thumb, for one third of the cost.

The company can now justify drilling reservoir targets with use of the new multilateral technology, which it could not have drilled with conventional technology due to the economic risk and geological uncertainty.

The lower investment cost needed to explore and drain new areas of the field makes the multilateral technology wells an important contributor in increasing the recoverable reserves in the Troll field.

Main wellbore

The rig drills the top section of the wellbore according to normal proced ures, with 13 3/8-in. casing set just above the reservoir in the range 1,420-1,520 m TVD and 1,450-2,110 m MD (Fig. 2).

After the rig sets the 13 3/8-in. casing, crews drill the 12 1/4-in. hole into the reservoir and in the same operation under-reams it to 13 1/2 in., while building inclination from 40°-64° at the 13 3/8-in. shoe to horizontal or 90° within the reservoir (Fig. 3a).

This section of the wellbore has ranged 1,579-1,593 m TVD and 1,980-2,500 m MD, with the bottom 110 m of the 12 1/4-in. hole (milled to 13 1/2 in.) drilled horizontally in the reservoir, to allow ITBS placement in this area.

The reservoir sands of the Troll Olje have permeability in excess of 10 darcies, and the wells produce at 19,000-25,000 b/d of liquid with only 1-2 bar of pressure drawdown.

The company, therefore, must place the multilateral junction horizontally in the reservoir. Hydrostatic pressure effects from gas or water ingress into one of the laterals may kill the other lateral, if the multilateral junction is positioned more vertically, above the reservoir.

Liner, premilled window

Under-reaming the wellbore section below the 13 3/8-in. casing to 13 1/2 in. allows technicians easily to orient the 10 3/4-in. by 9 5/8-in. liner that contains the premilled ITBS window (Fig. 3a).

The 9 5/8-in. liner section consists of:

• Shoe and float collar joints with double floats.
• Three joints of 9 5/8-in. casing.
• A drillable alignment bushing for orienting the inner string with MWD (measurement while drilling).
• A latch coupling for setting the whipstock and deflector.
• A premilled window section, for the drill bit to exit and drill the lateral.
• A casing-alignment sub for lining up the premilled window to the alignment crossover.
• A 10 3/4-in. alignment crossover, with 10 3/4-in. casing joints above.

The rig runs the liner on an inner string of pipe that contains an MWD and an orienting key sub, stung into a drillable alignment bushing, which enables window orientation.

Once the liner is at TD, technicians orient the ITBS window to 4°-16° right or left of the high side and the rig sets the liner hanger.

Crews then cement the liner and run a 10 3/4-in. tieback string that is installed back to the wellhead (Fig. 3a).

Main, lateral boreholes

After running, orienting, and cementing the 9 5/8-in. ITBS liner in place, the rig drills the 8 1/2-in. main borehole section. Using a rotary steerable system, crews drill the main borehole horizontally to TD, which has ranged 4,364-5,208 m MD.

The company then completes the main borehole with 5 1/2-in. and 6 5/8-in. combination excluder screens.

After drilling and completing the main borehole section, crews install the ITBS drilling whipstock, which they make up to the milling assembly and then run and set in the latch coupling below the window (Fig. 3b).

The rig shears off the running bolt with a combination of torque and weight down. Using a rotary steerable system, crews drill the 8 1/2-in. lateral section horizontally to TD, which has ranged 4,060-5,238 m MD (Fig. 3c).

The rig runs the whipstock with an open throat so there is always hydrostatic communication to the main bore, preventing the typical fluid loss operations experience after pulling plugs on the Troll Olje field.

Lateral completion

Once the rig has drilled the lateral borehole, crews retrieve the ITBS drilling whipstock with a spear that engages the whipstock's 4-in. open throat (Fig. 3d).

No apparent fluid losses have occurred in any of the wells after the rig had pulled the whipstock.

On the first wells, the rig had performed a latch cleanup run after pulling the whipstock.

After two wells, however, the company eliminated the latch cleanup run to save operational time.

After pulling the whipstock, crews ran ITBS deflectors and set them in the latch coupling without problems. The tail pipe below the deflector included a seal stinger (Fig. 3e).

The hydraulic running tool releases the deflector by pressure drop over a nozzle. Bypass holes allow for circulation after the seal assembly stings into the PBR.

The company had included swivels in the tail pipe below the deflector in the first seven wells to prevent the seal from rotating in the PBR when the deflector was turned to engage the latch coupling.

Engineers later eliminated the swivels to optimize flow area for the main wellbore.

Lateral completion

The lateral completion consisted of the following main components:

• Bullnose-deflects off of the deflector assembly and into the lateral wellbore.
• Screens-excludes debris with 5 1/2-in. and 6 5/8-in. combination screens.
• Lateral safety sub-allows straight pull to shear contingency.
• Swivel sub-enables orientation of junction independent of the screen section.
• Flexible hanger-connects main wellbore with continuous lateral leg and main bore stinger.
• Y-block with round bores-avoids possible plugging of main bore or lateral.
• Selective access block-allows deflector set to enter lateral with plugs or tools.
• Orientation key sub-orients flexible hanger when stung into the 103/4-in. orienting crossover.
• Screen hanger and packer-allows final completion.

The rig runs the 5 1/2-in. by 6 5/8-in. screens, which the TIBS deflector diverts into the lateral. Crews make up the ITBS components, with a 43/4-in. MWD hung off below the screen-hanger running tool and scribed to the flexible-hanger orienting key.

The rig runs the screen liner into the hole on a landing string that crews turn to orient the ITBS flexible hanger. The key enters the 10 3/4-in. crossover alignment slot, confirming correct orientation and completing the multilateral well completion (Fig. 4).

Tubing completion

The rig installs a middle completion with a pressure-operated isolation valve to secure the well and allow the rig to pull the BOP.

Crews installs the horizontal christmas tree and rerun the BOP, prior to installing the upper completion.

On two of the multilateral wells, the company installed a middle completion with gas lift valves to extend well life when the water cut increases.

Total ITBS installation times have ranged from 15 days on the first installation (Y-22) to less than 3 days on well N-23, completed with the rig West Vanguard (Fig. 5).

Design, procedure changes

Engineers implemented several design changes following completion of the first ITBS wells.

The most critical change, for ensuring success, was reduction of the "D-to- round" OD from 8.45 in. to 7.40 in. and the swivel OD reduction from 8.3 in. to 7.4 in.

The "D-to-round" dimension change implied an acceptance of some additional risk, in that the tip of the flexible hanger main bore stinger would not be fully protected, while running in hole.

Engineers have looked at the well path out of the window, as another multilateral well design focus.

They have recommended a straight build well path, a minimum of 10 m out of the window, to limit the effects of double bending on equipment run through the window.

Out of the 10 ITBS installations, two of the hangers did not successfully sting into the main bore.

Well Y-22 was just 1.5 m from sting-in and the company ran a straddle to seal the junction.

Well M-13 was sitting too high to run a straddle, due to the lateral swivel becoming stuck in the window. The well was completed with sand screens above the junction.

No radioactive tracers were produced from the main branch during clean up indicating that some restrictions had occurred.

Production history

Norsk Hydro has drilled most of the multilateral wells in the Troll field within the past year.

The short production history and the fact that no production logs have been run in any of the ITBS wells makes it difficult to evaluate and draw conclusions on the contribution from individual multilateral well branches.

The company ran radioactive tracers at different depths in all of the multilateral well branches to give information on the contributions from the individual laterals during clean up.

Except for the main bore of well M-13, cleanup data indicated that both branches were contributing flow and from the entire length, as indicated by the well flow producing the tracers out of the well.

One of the main challenges on Troll is for Norsk Hydro to limit the draw down in the heel area of the well and avoid gas coning from the gas cap above.

Simulations of the inflow profile of the wells indicate that the multilateral wells have a better contribution from the entire well length than a single well, which is due to a lower flow rate from each branch compared to a single well, and therefore a lower drawdown in the heal area.

The company's production strategy for a single well is an initial oil rate of 9,400-12,600 b/d and for a multilateral well an initial oil rate of 15,700-18,900 b/d.

The multilateral wells produce longer on plateau, compared with a single well, with respect to gas break through.

An inflow control device, in addition to the screens, yields a smooth inflow profile with good contribution from the entire screen length.

The production history, to the beginning of 2002, indicated that the multilateral wells have had later gas break through than the single lateral wells. In combination with a high flow rate this indicates that the entire screen lengths are yielding good flow contribution.

Bibliography

1. Berge, F., Hepurn, N., Bowling, J., Grønås, T., "A New Generation Multilateral System for the Troll Olje Field - Development and Case History," SPE paper 71837, presented at the Offshore Europe Conference, Aberdeen, September 2001.
2. Storegjerde, D., and Soltvedt, R., "Troll West Oil Province Subsea Horizontal Completion Experience," SPE/IADC paper 35079, presented at IADC/SPE Drilling Conference, New Orleans, March 1996.
3. Henriksen, N., and Storegjerde, D., "Cost Effective Horizontal Drilling in the Troll Field Through use of State of the Art Technology and Optimal Operations," SPE/IADC paper 37577, presented at the SPE/IADC Drilling Conference, Amsterdam, March 1997.
4. Berge, F., Freeman, A., Gronas, T., Durst, D., and Luke, M., "First Successful Multilateral Well Installation From a Floating Rig," IADC/SPE paper 39369, presented at the SPE/IADC Drilling Conference, Dallas, March 1998.
5. Steele, D.J., and Edholm, S., "New Through-Tubing Junction-Isolation System Enables High-Pressure Stimulation in the Ekofisk X-02 North Sea Multilateral Well: Case History," SPE paper 63268, presented at the SPE Annual Technical Conference and Exhibition, Dallas, Oct. 1-4, 2000.
6. Smith, B., and Smith, R., "Developments with Multilateral Drilling and Completion Systems," Presented at the 7th International Conference on Horizontal Well and Emerging Technology Conference, Houston, Nov. 6-8, 1995.
7. Lowson, B., "Multi-Lateral Well Planning," SPE/IADC paper 39245, presented at the SPE/IADC Middle East Drilling Technology Conference, Bahrain, Nov. 23-25, 1997.

The authors

Tore Grønås is a drilling supervisor in Norsk Hydro's drilling and completion department, a position he has held since 1992. He was the project leader for Norsk Hydro's first successful multilateral technology well drilled from a floater in 1997 and is the current project leader for the ITBS project. He has an MS degree in petroleum technology.

Lovise Hundsnes is a senior production engineer with Norsk Hydro. Prior to Saga Petroleum's merger with Norsk Hydro in 1999, she worked in the company's drilling and production technology departments for 7 years. She has an MS degree in petroleum technology.