A client in the Bualuang Field in the Gulf of Thailand faced a typical problem of economics vs. investment. The client operated two offshore platforms, one with 10 slots and one with 16 slots, but all slots were full. Although the field held additional production potential, the client was reluctant to invest in a third platform.

The client decided on a series of multilaterals in multiple wells on the 16-slot platform, which would enable additional production without sacrificing production from existing wells. Among the requirements for the multilaterals were the following:
• Minimal risk to existing production from the mainbore;
• Ability to install the multilateral in a high-inclination or horizontal position (to stay in the top portion of the reservoir to avoid water coning issues);
• A TAML Level 4 cemented junction, providing the ability to kick off the lateral in a soft sandstone formation without risking hole collapse;
• Compatibility with rotary steerable systems (RSSs) and polycrystalline diamond compact bit drilling programs used in the lateral drilling program;
• Full isolation of the existing mainbore reservoir during drilling and completion of the lateral leg;
• Fullbore access into the lateral bore to drill out the lateral shoe and drill into the target zone; and
• Ability to run screens through the junction without damage.

In addition to the technical requirements, the client required both equipment and personnel to be mobilized rapidly to meet the non-negotiable end-of-contract date for the drilling rig.

Following a collaborative planning phase, the client selected the Weatherford OneTrip StarBurst Multilateral System. This technology uses a hollow whipstock assembly and QuickCut milling technology to mill a casing window. The 2-degree angle of the concave ensures a longer window length and full compatibility with RSS. Once the lateral wellbore is drilled, standard casing is run in the lateral bore and tied back into the mainbore casing above the junction point. The casing is then cemented, stimulated as desired and perforated with charges through both the lateral liner and the hollow whipstock assembly.

Installing multilaterals
The client selected a seven-well project, and Weatherford quickly mobilized people and equipment for a late May 2014 start date. Though mobilizing equipment was seamless, importing the needed perforating charges proved problematic because government regulations prevent the movement of most explosive products. As a result of this delay, the first multilateral well installation began in August 2014.

After pulling the existing completion and prepping the wellbore, the drilling team made up and ran the packer, hollow whipstock and window milling assembly in a single trip. The packer was then hydraulically set in 95⁄8-in., 43.5-lb/ft casing at a measured depth of 1,387 m (4,549 ft), where the wellbore presented an approximate 73-degree inclination.

Prior to setting the packer, a standard MWD system was used to determine and alter the orientation of the whipstock assembly. Once wellbore fluid had been circulated over to a 9.2-lb/gal mud, the milling assembly was sheared from the whipstock, and the entire window, including a 5-m (16-ft) rathole, was milled in approximately 4.5 hours.

The team drilled approximately 1,300 m (4,264 ft) of 8½-in.-diameter hole using an RSS with MWD and LWD systems. Seven-in. liner was then run, hung back in the mainbore and cemented in place. The 7-in. shoe was drilled out, and approximately 800 m (2,624 ft) of 61⁄8-in.-diameter hole were drilled into the reservoir, again using geosteering. This lateral reservoir was then completed with 4½-in. standalone screens.

Regaining mainbore access
With the lateral bore completed and producing, the team began an operation to regain mainbore production by perforating through the 7-in. liner and hollow whipstock. A perforating gun with 2.7 m (9 ft) of charges at four shots per 0.3 m (1 ft) and aligned along a single axis line was run downhole.

The perforating charges were developed for the OneTrip StarBurst hollow concave system to provide short penetration lengths, maximum hole size and minimum debris compared to typical perforating charges that seek long penetration depths. The perforation gun was run below an MWD system to orient the charges toward the open pocket of the hollow whipstock. The MWD assembly also provided the gamma ray logging necessary to identify the radioactive tags run at the top and bottom of the whipstock face to confirm proper setting depth of the perforating gun.

The team pressured up the assembly to fire the guns. Pressure immediately dropped by 40 psi, and losses of 450 bbl/hr were recorded, which confirmed that access to the mainbore had been achieved. After retrieving the perforating assembly, an electric submersible pump was run and set just above the junction to provide commingled production from both wellbores.

Creating multilaterals in remaining target wells
During the next two months, the multilateral system was installed in each of three additional existing wells at measured depths of between 944 m and 1,000 m (3,096 ft and 3,280 ft) and at well inclinations between 53 degrees and 62 degrees at the junction depth. In all three installations, the hollow concaves were set without difficulty in the 95⁄8-in. casing, and the casing exits and rathole were milled in single trips in less than seven hours.

Lateral completions consisting of cemented 7-in. intermediate casing and 4½-in. screens in the production zone were used in all three remaining wells. Finally, in two of the remaining three wells, the concaves were located and perforated without difficulty, and successful access into the mainbore was confirmed by a change in fluid losses after perforating.

On the final multilateral of the project, a malfunctioning firing pin cartridge resulted in an extra tubing trip to perforate back into the mainbore. The second perforating run was a success. Although fluid losses weren’t initially seen, a follow-up 200-psi injection test and resulting pressure drop verified the desired communication with the mainbore. In all four multilateral installations, production rates will be monitored over the life of the well to determine the level of reservoir/production benefit achieved with these installations.

Demonstrated viability
Because of the late start on the project and contractual obligations on the rig, the multilateral project was restricted to four installations rather than the original seven. The client considered the overall project a success, however, and it demonstrated how future projects could convert existing wellbores into multilaterals quickly and with low risk.

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