Multilateral technology has seen major growth within the last 15 years. In 1998 TAML (Technology Advancement for Multilaterals) began as a JIP made up primarily of operators and oilfield service companies. TAML's stated mission is to promote efficient development and use of multilateral technology within the international petroleum industry. The organization has a stated classification system for multilateral wells. The system is made up of 2 tiers; Complexity Ranking and Functionality Classification. The complexity ranking consists of a single numeric character whilst the functionality classification includes a series of alpha and numeric characters that describe the critical functionality characteristics of the well.

The complexity ranking uses a number between 1 and 5, the complexity ranking gives a first-pass indication of the complexity of the well based on junction complexity. In a well with more than one junction, the most complex one would be referred to.

The organization defines a "Level 4" multilateral well as one in which the main bore and lateral are cased and cemented. Both bores are cemented at the junction.

Halliburton recently set a new world record for the deepest cemented junction in a Level 4 multilateral well.

Production optimization challenge

PetroChina's Tarim Oil Co. (TOC) recently was faced with the task of developing production optimization strategies to increase single vertical well production and efficiencies in deep reservoirs in the Xinjiang Province in Northwestern China. The company decided to trial a multilateral project as a viable strategy for achieving its production objectives, increasing reservoir exposure and production while lowering capital expenditures by reducing the need for extensive surface facilities.

The depth and remote location of the subject well presented great challenges; a milled casing exit with a cemented multilateral junction had never before been performed at this extreme depth. In addition, the location in the Tarim Desert presented a vast distance of 3,600 km (2,237 miles) to the nearest Halliburton repair and maintenance facility in Tanggu, Tianjin Province in Eastern China. This meant a four- or five-day trip via truck; so tools had to be ready and available and had to operate reliably once deployed. Timing of the project was important to TOC and accelerated the pace of planning and execution.

Halliburton's MillRite system was chosen to complete TOC's prospect. The system allows a precision-milled window exit to be machined downhole, eliminating the uncertainty of a conventionally milled window. With no internal diameter restrictions in the main bore of lateral(s), reliability is achieved through repeatable reentry to lateral and main bore.

According to Calvin Ponton, global technical advisor, Multilateral Technology, for Halliburton's Sperry Drilling business line, "This also allows for larger and longer tools to be run into and recovered from the lateral."

The MillRite system provides a longer, full-gauge window and also eliminates "roll-off" and dogleg severity problems associated with conventionally milled windows. With a geometrically controlled exit, the system has been proven to save time.

Improved recovery

Sperry Drilling services applied engineering expertise during planning and execution to overcome the numerous technical challenges associated with creating this milled casing exit with a cemented multilateral junction. The team deployed the MillRite milled exit multilateral system, a track-guided window milling system that creates a consistent window. The unique design of the Mill-Rite system's latch coupling allows multiple junctions to be created in new or existing wells, providing a permanent depth and orientation reference.

In operation, the window typically is positioned at an inclination; however, this was a vertical well, which posed an additional challenge. Halliburton successfully constructed a 7-in. x 4½-in. MillRite TAML Level 4 junction at 5,092 m (16,674 feet) TVD/MD. The main and lateral wellbores were cased and cemented, with both bores cemented at the junction.

The company delivered all customer objectives while doubling reservoir exposure, increasing production, and reducing drilling and completions costs.

In addition, the system's long, full-gauge window enables swell packers, external casing packers (ECPs), and screens to be run into the lateral. The MillRite's junction isolation (JI) system enables fracture of individual laterals while isolating the junction and main bore from the high fracture pressures and volumes.

After the successful installation of its premiere application of multilateral technology, TOC now regards multilateral technology (MLT) as a solution for additional wells of the same nature, possibly with more than one junction per well.

For the record

The TOC case allowed Sperry to achieve a world record for the deepest cemented (Level 4) junction with its Mill-Rite System. This was the first such application in China, but MLT technology is not new to the company. "Halliburton has completed more than 396 cemented Level 4 Junctions since 1993, and more than 734 MLT junction installations globally in 29 countries," Ponton said.

Improvements in reliability have continued to make MillRite's MLT junction installation a selected choice among many operators. "Operators want to reduce their capex and opex while drilling and completing increasingly complex and challenging wells," Ponton said.

Benefits for shale

The MillRite system used in conjunction with a JI System has a broad application for many of the shale plays in North America and abroad.

"In shale and tight gas plays, where hydraulic fracturing is required, reliable and flexible isolation systems have been designed and applied," Ponton said. Halliburton's MillRite track-guided precision milling system is applicable in new or existing wells.

The MillRite JI system is designed for use in wells that are to be developed as multilaterals specifically for the exploitation of shale and tight gas resources. The low permeability of shale and tight gas plays makes multiple stage fracture stimulation of the formation necessary for recovering hydrocarbons. With the MillRite JI system, the expensive fracturing stimulation treatment becomes more economical as multiple horizontal laterals can be drilled from one location. But, all multilateral junctions must be isolated from the stimulation fluids and pressure. To isolate the junction, a temporary Level 5 completion is installed using the junction isolation tool (JIT). The JIT requires no increase in hole/casing size and no limitations to casing while allowing selective access to multiple laterals for high-pressure stimulations, without restricting the high pump rates required.

The MillRite JI system uses SperryRite latch coupling technology, which is an important advantage in wells that must be reentered to be selectively stimulated. The latch coupling is a unique profile that is built into the ID of a well's casing, providing a repeatable reference for the depth and orientation of multilateral equipment. Whipstocks and other downhole tools can be consistently landed and oriented by latching into the coupling. It provides full-bore, unrestricted casing access to the lower mainbore and maintains casing pressure integrity, allowing the well to be completed and produced in any manner.

The MillRite JI system incorporates a special window-milling machine that allows the creation of a near-rectangular window at a defined depth and azimuth on a repeatable basis. Precise control of the window geometry and position makes the MillRite system especially useful for multilateral wells in which lateral reentry is required and is particularly important when using the custom designed JIT. The MillRite system windows are machined with an elongated full-gauge aperture along their entire length and are exactly in line with the axis of the casing. Problems often associated with conventionally milled windows, such as elliptical and spiraled window geometry with no control over precise depth, azimuth, and full-gauge section length, are eliminated. The straighter, longer window geometry delivered by the MillRite system eliminates the dogleg severity problems that can occur when running lateral liners or tools into the lateral bore through conventionally milled windows.