As the reservoir requirements for zonal isolation and re-entry access become prerequisite for a successful long-term project, multilateral systems are available to address these needs. Figure 3-20 shows a system that isolates the flow entering the primary bore from the flow entering the lateral bore.
Figure 3-20 Limited-isolation multilateral completion
This type of installation is appropriate if one of the reservoirs will likely produce unwanted water or gas during the life of the well. This undesirable production can then be shut off from the producing bore. Expected pressure changes between the lateral-bore reservoir targets can also be managed through this "on/off" arrangement.
Reservoir Considerations
Proper reservoir analysis and target selection are required as with any multilateral well design. However, the reservoir management capability offered through this type of completion system allows some control over reservoir incompatibility or nonperformance. As a result, zonal homogeneity becomes less critical.
Installation Considerations
As with openhole designs, this limited-isolation/access system does not allow casing to be set in the lateral bore and mechanically reconnected to the parent bore. This system does, however, allow re-entry into the lateral borehole during the drilling and initial completion phases. A stable, nonsloughing, hard-rock formation is the best candidate for this well design.
When installed in the system, slotted liners hung on openhole inflatable packer anchors provide a means of hole-size maintenance and stability. If sand control problems are anticipated, prepacked screens can be installed in a similar manner.
Since insertable openhole liners and tools cannot be mechanically reconnected to the parent casing, a nonpermeable section of stable shale or hard rock should be selected for the lateral exit. Failures in this type of installation tend to occur at the unprotected, openhole section adjacent to the parent-bore casing (the heel of the lateral).
Before the system is installed (Fig. 3-20), the primary borehole should be drilled and cased below the point of the planned lateral exits. The primary bore may also be directionally drilled to penetrate lower target sands, serving as a producing bore. An alternative configuration involves placing the primary bore vertically through lower target sands for logging that will determine the exact sand location and thickness for enhanced accuracy of uphole lateral departures.
Once the borehole is cased and logged, a retrievable whipstock is installed at the required lateral exit point. This wedge-shaped diverter is oriented for both depth and azimuth, with the sloped face of the tool aimed in the general direction of the planned lateral bore path. Sometimes, the design includes an integral anchor or separate packer that provides a stable, nonrotating base for the whipstock to engage, which helps maintain placement accuracy during drilling. Pilot mills, window mills, and watermelon mills are run against the face of the whipstock to cut away the section of casing opposite the diverter face. When this process is used, a full-gauge window opening is cut and sized in the casing. This opening allows required directional drilling assemblies to pass.
For best results, the formation immediately adjacent to the exit window should be stable, nonsloughing, hard rock or shale. The window exit point should be selected while the directional drilling path is being planned, so that
the created bore track will allow for both target achievement and future liner or tool placement.
Directional drilling assemblies and MWD and/or LWD tools drill the lateral borehole through the desired targets. After completion of the drilling phase, liner, screen, or other tools may be installed, with the face of the whipstock once again being used as a diverter into the openhole lateral bore. Once completed, the whipstock is retrieved from the parent bore, which allows access to lower lateral or parent bore zones.
Additional lateral exits are created in a similar manner, with the process normally starting at the bottom of the parent wellbore and working up the hole. When lateral exits are started at the bottom, the previously created laterals can be temporarily plugged with retrievable bridge plugs as a means of preventing formation contamination from drilling or stimulation fluids used on subsequent laterals. Underbalanced drilling or other drilling and fluid systems can then be used on each lateral while well control is maintained, and unwanted zonal flow is prevented. Once all laterals have been drilled and stabilized, the plugs are removed, and the final phase of casing-completion equipment is installed.
Final completion scenarios for this type of well are numerous and offer more production management options than openhole multilateral wells. Typically, production packers are installed in the primary wellbore casing above and below each lateral exit point. Single or dual strings of production tubing could also be installed to commingle or segregate production as required.
Sometimes, sliding sleeves are placed at the lateral exit point to provide on/off production control from each lateral bore. Through-tubing access sleeves could also be used, adjacent to the lateral exit. These access sleeves contain a precut window that aims toward the lateral borehole. The window may be shifted open through the use of through-tubing slickline or coiled tubing-conveyed tools. Once open, an internal deflector is installed that diverts strings of tools into the openhole section for future logging, tool actuation, or borehole stimulation. Figures 3-21 and 3-22 show some of the completion options available with this type of installation.
Figure 3-21 Limited-isolation/access multilateral completion (production is commingled)
Figure 3-22 Limited-isolation/access multilateral completion (production is segregated through dual tubing strings)
When properly planned, limited-isolation/access designs pose fewer risks than openhole multilateral wells because additional production control options are available to adapt to changes in zonal inflow.