Simplifying the complex is not always easy; sometimes it takes a back-to-basics approach and a focus on the fundamentals. A case in point is the recent effort to re-invent the Level 4 multilateral junction as a simple, low-cost, low-risk installation.

Background

Traditional Level 4 multilateral technology has focused on high-end applications with the intent of creating a foundation for conversion to Level 5 functionality. Little attention has been spent on reducing installation risks, junction hardware and complexity. But that is exactly the approach taken to construct a cemented junction that utilized proven milling and completion techniques.

By leveraging experience from proven casing exit and mainbore access milling techniques, Weatherford developed a mill-through liner method to provide a simple Level 4 multilateral junction. Prior to the latest iteration, the company had previously teamed with Halliburton to develop and deploy a mill-through concept. Although the process required six runs to mill the mainbore access window, the mill-through liner concept proved to be viable. The liner was milled through, and the cemented liner remained intact and provided a strong cemented Level 4 junction. In addition, the use of a conventional liner minimized hardware and simplified installations by eliminating the need to properly place and orient the liner. After validating the concept, the next challenge became apparent - reduce the mill-through liner creation trips and reduce hardware requirements wherever possible.

Redefining the Level 4 multilateral system

The new method reduces hardware and minimizes trips to provide an easy installation with lower inherent risks. This mill-through liner multilateral system uses a standard, packer-based casing exit system to create the lateral window. After directionally drilling and conditioning the lateral, the whipstock is retrieved and a sand barrier is spotted in the packer and main bore. A conventional liner assembly is run out of the lateral, assisted by a bent joint, and landed without orientation or a specific liner overlap. A standard cement procedure is performed to ensure Level 4 functionality. The first mill-through liner milling assembly is then run to create an opening through the liner at the junction. Another mill-through liner milling assembly follows and completes the mainbore access window and cleans out the main bore to the permanent orientation packer. The bull nose below the packer, used for isolation, can then be milled out using a typical junk mill assembly to complete the process. Re-entry is achieved by using a deflector with a flow-activated latch to allow self-orientation of the deflector with the lateral well bore.

The Level 4 multilateral junction with liner support back into the junction is stronger than a cemented junction with the liner overlap removed.

Conventional Level 4 washover methods are more destructive to the cement around the liner than if the liner had simply been milled through. The lateral is drilled, the liner is run, and the whipstock is cemented in place. A washover shoe and sleeve swallows the liner overlap in the main bore while cutting it away from the liner extending into the lateral. With this process, washover tools have a tendency to track the liner transition path through the window as the washover assembly swallows the liner overlap. As a result, the washover shoe begins to damage the top of the whipstock as it tracks the liner through the window. Once damaged, the whipstock may prevent the washover assembly, and subsequent fishing assemblies, from recovering the whipstock. It may take several trips to complete the washover process, if at all.
Problems with the washover multilateral junction may continue beyond the initial installation. The washover process removes cement from around the liner in order to swallow the liner overlap. The destructive effects and vibrations of the process tend to break away cement from around the lateral liner and jeopardize mechanical integrity. The cemented liner overlap into the main bore is a source of mechanical support for the junction. When the entire liner overlap and cement is removed, the near-junction liner is unsupported and relieves the imposed stress by falling back into the main bore. Combining these two situations can often result in a collapsed junction that prevents re-entry into, and possibly production from, the contributing well bores.

Field-testing the solution

The solution to these issues was initially tested in a heavy oil field located in the Orinoco Belt of Venezuela on the MFB-666 well, first drilled in 1974, in PDVSA's Bare field. Wells in this field typically produce 8? to 16? API oil under solution gas drive with progressive cavity pumps and electric submersible pumps providing artificial lift. The mill-through milling operations were completed in 5% less time than planned at 35.5 hours including trip time, handling time at surface, and bull nose cleanout at a 3,400 ft (1,037 m) junction depth. As a result, the MFB-666 is currently the highest producing multilateral for PDVSA in the Bare field with 1,854 b/d compared to an active field average of 633 b/d for other producing multilaterals.

Although the process was completed under planned time and with surprisingly minimal mill wear, difficulties were encountered. While attempting to latch into the packer orientation slot, the packer moved downhole approximately one joint before setting again. Attempts to latch and determine the orientation of the packer slot profile were unsuccessful. A backup packer was run and set above the first packer. The second problem occurred when difficulties verifying the latch engagement and orientation with the packer slot profile were again encountered and exacerbated with measurement-while-drilling (MWD) reading difficulties. A standard, mechanical, one-trip casing exit system was MWD-oriented and tripped off the top of the packer to set the whipstock.

Milling the casing exit window was completed and the whipstock was retrieved following drilling operations. During the bull nose cleanout procedure, the bull noses on both packers were easily cleaned out on the same junk mill run. Cement particles discovered during the tear down of the packer running tool were determined to be remnants from inside the drill pipe of a cementing procedure on the previous well. Design changes were made to the packer and latch to address this possibility. The process was amended to include drifting the drill pipe and running a cleanout/gauge assembly prior to setting any packers or casing exit systems.

Turning challenges into benefits

The difficulties encountered in this field test were used as an opportunity to reduce trips and risk by continuing to simplify the process. Problems using the latch with the packer led to the use of the one-trip casing exit system with the proven mechanical trip anchor substituted for the latch. By reverting to the mechanical trip anchor, the stand-alone orientation trip can be eliminated and a few options become available. In one option, the packer can be set blind as before; therefore, the orientation of the slot profile can be deferred until re-entry is required by using MWD with the flow-activated latch. In another option, the packer may be oriented and set in the same trip. This option requires a sliding valve to allow circulation for MWD readings and allow for closure to the annulus during the packer setting sequence.

A typical washover procedure requires six trips to complete, excluding drilling or cleanout procedures, assuming that the washover is successful on the first attempt. This mill-through liner method initially required nine trips assuming the same basis. However, efficiencies discovered during the field trial will reduce the mill-through liner technique to six simple steps such as setting the packer blind or orientating it during the setting procedure with the addition of a sliding valve and MWD. In lieu of the sand barrier placement procedure, the packer may be preloaded with a diesel gel to protect the packer profiles, thereby eliminating this trip. The observed milling parameters from the mill-through liner procedures and the post-condition of the milling assemblies indicate that the two mill-through trips may be combined into one. This field trial demonstrated the mill-through liner technique as a viable method for constructing Level 4 multilateral junctions. The simplicity of the method lends itself as a feasible Level 3 multilateral replacement with the added functionality of a Level 4 junction. By minimizing junction hardware and using cost-effective casing exit and mill-through assemblies, the price of this new mill-through is comparable to conventional Level 3 multilateral installations available today.

Editor's Note. This paper was originally presented as SPE 92606 at the SPE/IADC Drilling Conference, held in Amsterdam, The Netherlands, February 23-25, 2005.

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