A complete multilateral system provides two to five laterals from one new or existing wellbore. Applications for this type of multilateral system are similar to the limited-isolation/access design, but a complete multilateral system allows project designs for deepwater or subsea environments. In new wells, the exit portal must be capable of being placed in the vertical, inclined, or horizontal plane, and the window exit azimuth must be capable of being oriented after the primary casing string has been installed. The system must also be compatible with cementing operations for liners and/or slotted liners and prepacked screens for sand control.
When a complete multilateral system is used, the lateral wellbore is cased back to the primary bore exit, and the liner casing string is mechanically connected to the primary bore casing; the lateral- to main-wellbore junction must be hydraulically sealed. Any complete lateral bore or portions of any lateral can be isolated as needed to control the production inflow profile.
Each lateral must also be accessible for re-entry without rig intervention.
The system should allow for washdown methods that help transport long liners to the bottom, and it should accommodate high build rates (45 to 60°/100 ft) after tools exit the lateral window junction.
Reservoir Considerations
Multilateral systems capable of delivering additional reservoir management options increase the choice of target zones. Again, proper reservoir modeling and target selection must occur during the project planning phase, and a stable, nonsloughing, impermeable shale or hard-rock formation is desirable at the exit site. If, however, target selection requires exit in unconsolidated sands or in the producing interval itself, the unconsolidated sand can be stabilized with cement or plasticized material. Since low to medium build rates will be used to simplify casing installation, engineers must select targets and plan drill paths with such considerations in mind.
Installation Considerations
During installation of a complete multilateral system, the primary bore is drilled, and the primary production casing string is cemented in place across all anticipated lateral-bore exit points. The primary bore is normally drilled into a producing zone and completed for final production.
Temporary plugs are normally set above this lower zone to isolate it from fluids used to drill the upper lateral bores. If required, the lateral exit area can be pretreated through the use of oriented perforating and squeeze techniques, without damaging the lower primary zone.
As shown in Fig. 3-23, a combination assembly consisting of a whipstock packer anchor, a hollow whipstock temporarily filled with a composite plugging material, and a starter mill are run to depth, oriented to the correct azimuth, and set hydraulically.
Figure 3-23 Running hollow whipstock and packer; orienting, and setting packer The starter mill is sheared loose from the now-anchored whipstock, and the first section of exit window is rotary-milled. Additional window and watermelon mills are run to open the exit portal to a full-gauge dimension (Fig. 3-24).
Figure 3-24 Window and watermelon mills used to ream/feather window Oil-based or special salt-sized drilling fluids are circulated to the surface in preparation for directional drilling of the lateral bore. Lower-bore isolation plugs and the internal plug contained in the whipstock anchor packer prevent contamination of the lower, completed producing zone.
Directional-drilling bottomhole assemblies (BHAs) consisting of steerable drilling motors, MWD tools, and LWD tools are used for drilling the lateral bore to casing setting depth (Fig. 3-25).
Figure 3-25 Formation Bit and Directional Drilling Assembly Used to Drill Angle-Build Section of Hole
Drilling-fluid additives and maintenance of mud weight are the primary concerns in regards to maintaining a stable borehole and preventing uncontrolled flow as pressured zones are encountered.
The hole size must be consistent so that the casing can be properly set and cemented. Therefore, when drilling is complete, openhole caliper logs are run and analyzed. If necessary, hole-opening and reaming steps are then performed.
A liner casing string assembly is made up and run to depth. This string consists of (1) float equipment for cement circulation, (2) applicable mechanical centralizers, (3) sufficient casing to extend 150 to 300 ft back into the primary bore, and (4) a hydraulically set liner hanger with a cement-compatible running tool (Fig. 3-26).
Figure 3-26 Run Lateral Liner and Cement in Hole
Typical primary casing/lateral liner sizes are listed in Table 3-3.
Table 3-3 Typical Primary Casing/Lateral Liner Sizes
Primary Casing (in.) Lateral Liner (in.)
13 3/8 9 5/8
10 3/4 7
9 5/8 7
7 5/8 5
7 4 1/2
5 1/2 3 1/2
4 1/2 2 7/8
Unlike slimhole well designs, multilateral applications require larger casing and tubing diameters. For maximum benefit, the optimum primary casing size should be 9 5/8 in. or larger, allowing for the installation of a 7-in. liner.
This selection allows (1) the use of widely available drilling, MWD, and LWD
tool sets, (2) the installation of large-diameter production packers, and (3) the use of tubing strings that are 3 ½ in. and larger.
When the liner string has been positioned, circulation is established and fluids are changed as required for upcoming cementing operations. A special blend of cement and additives is then mixed and then pumped down and around the liner string. If long liners are used, multistage cement processes can be used to place general-purpose cement for the lower liner section and to place special sealant in the junction area. During cementing, casing rotation and reciprocation will help ensure complete cement coverage. Once proper displacement is achieved, wiper plugs are released from the surface, wiping the liner string of excess cement and providing a pressure block for setting the hydraulic liner hanger (Fig. 3-27).
Figure 3-27 Placing Specialized Sealant at Junction
The hydraulically sealed, cement-anchored lateral liner can now be perforated and completed with production packers, sliding sleeves, sand-control tools, or other completion tools that are required to manage and produce the target reservoir. Any completion design used in single-bore
vertical or horizontal wells can be installed in the lateral liner string (Fig. 3-28).
Figure 3-28 Setting Packer in Lateral
Additional openhole drilling out the lower end of the lateral liner may also occur to meet completion objectives requiring openhole prepacked screens or slotted liners (Fig. 3-29).
Figure 3-29 Washdown System for Screens Run in Open Hole (Sized-Salt Mud Displaced)
At this point, access to the lower producing bore is achieved. A milling assembly consisting of a retrievable downhole mill guide with an internal pilot mill is run into the well and positioned in the lateral liner across from the face of the plugged, hollow whipstock below the liner. The mill guide is oriented and depth-positioned by radioactive tags in the whipstock and conventional gamma ray and collar-locator electric-line logging practices.
When positioned properly, the mill guide is hydraulically set, and the pilot mill is released. Timed, limited weight milling is used to cut a pilot hole through the side of the lateral liner in the vertical plane of the primary casing alignment (Fig. 3-30).
Figure 3-30 Run Round-Nose Mill Assembly to Complete Pilot Hole through Hollow Whipstock
Large-diameter window and watermelon mills, used in follow-up runs, open the vertical access hole in the liner string and mill out the temporary plugging material in the hollow whipstock. This process creates a full-gauge hole that is dimensionally compatible with the drift diameter of the liner string (Fig. 3-31).
Figure 3-31 Run Packer Plug Mill Assembly to Remove Packer Plug
Completion fluids are circulated through the well, and the final plugs are milled in the base of the whipstock anchor packer. Temporary isolation plugs can now be retrieved from the lower primary bore, and the lower zones are open for completion and production. Other lateral wellbores are then drilled and completed farther up the primary wellbore.
After all installed lateral bores have vertical access to the primary bore, uphole primary bore completion equipment is installed. Completion options range from single-string, commingled flow designs to multistring, segregated production installations as shown in Figs. 3-32 and 3-33.
Figure 3-32 Advanced Multilateral Completion (Production Commingled)
Figure 3-33 Advanced Multilateral Completion (Production Segregated) Each lateral wellbore can be re-entered through the production tubing string with size-compatible tools or with a rig, if the tools required are larger than the tubing ID. Since all lateral exit bores are created from the "high" side of the primary bore, tools will track the low side of the primary bore, travel past the open lateral liners and remain in the primary bore unless they are mechanically forced into a selected lateral.
As a means of tool deflection, a diverter tool is installed in the orienting/anchor nipple profile in each primary-bore whipstock anchor packer. As the diverter is set and anchored in the profile, the orienting track rotates the diverter face to align in the direction of the lateral bore. The diverter fills in the milled-out section of the lateral liner and serves as a temporary base to the liner; tools strike the deflector face and are guided into the lateral casing string (Fig. 3-34).
Figure 3-34 Re-entry Access for Full-Gauge Tools
Slickline, coiled tubing, or jointed pipe-conveyed toolstrings can then be run into any selected lateral bore to shift sliding sleeves, set or retrieve plugs, perform production logging services, or pump stimulation chemicals or other chemicals into the lateral producing zone.
The same diverter tool is used at each lateral. Plugs and downhole chokes can also be set to isolate any portion of any lateral or primary bore to shut off unwanted water or gas production, allowing production control over each zone or segment.