DRILLING & COMPLETION Drilling and completing multiplelateral sections from one borehole

Cased, uncased, supported, unsupported multi-lateral technology
offers opportunities for re-entering existing wells

Mark Brockman
Kevin Jones
Baker Oil Tools

• Figure 1: Barefoot sidetract in cased and cemented mainbores.
  
• Figure 2: Completed openhole sidetrack in cased and cemented mainbores.
  
• Figure 3: Dual completion for re-entry wells.
  
• Figure 4: Selective entry for re-entry wells.
  
• Figure 5: Limited entry for completed and cased sidetrack well.
  
• Figure 6: Cemented completed and cased sidetrack well.
  
• Figure 7: Commingled production in open-hole sidetrack well.
  
• Figure 8: Open-hole sidetrack well without entry.
  
• Figure 9: Selective entry in open-hole mainbore.
  

The concept of drilling and completing multiple lateral sections from a common borehole is gaining technical acceptance for many reservoir production scenarios. Although the application technology is still in its infancy, more reservoir planning teams readily assume multiple branches emanating from fit-for-purpose boreholes to optimize production.

From all indications, the development and application of multi-lateral technology will constitute an appreciable event in the technological history of the oilfield, analogous to the development of horizontal technology. Many major operators and service companies have now fully integrated horizontal technology for all applicable opportunities. It appears that in the next three to five years, most applicable reservoir opportunities will use multi-laterals as a given method for optimizing production.

The economic impact of exposing many parts of a reservoir to near-wellbore production or of tapping into multiple reservoir targets from one main borehole is such a logical progression that it seems timely to start investigating specific categories for multi-lateral applications. Just as the basic economic impact of designing future reservoir development schemes seems obvious, so, too, does exploiting re-entries.

Early market research indicates that re-entries will comprise a significant part of the mature multi-lateral development market. The opportunity to bypass major capital expenditures associated with drilling and developing a new wellbore by tapping into a previously paid-for wellbore to search for new or additional hydrocarbon reserves certainly seems a worthy focus area. The primary attributes of a multi-lateral completion are:

• Lateral entry: The ability to re-enter the lateral section with either coiled tubing or drill pipe
• Isolation: Controlling the flow from laterals to the main production conduit
• Selectivity: The capacity to selectively re-enter the lateral wellbores for remedial purposes
• Flexibility: The versatility of the system, as measured by its ability to adapt to a changing reservoir
• Lateral stability: The ability to avoid lost production due to collapse of the lateral wellbore.

The general analysis that follows addresses applications for multi-lateral re-entries, classifies available systems for re-entry multi-laterals, and compares them with regard to the five attributes listed above.

Re-entry applications

There are numerous reasons to utilize multi-lateral completion techniques for well re-entry. All focus on the same ultimate objective, which is to increase or enhance current production from an existing wellbore, thereby increasing total revenue potential.

Currently, the most popular applications for multi-lateral re-entry technology are to produce additional reservoirs, to enhance drainage patterns, and to increase fracture contact.

Many wellbores intersect multiple thin producing reservoirs, but only one reservoir can be produced with a horizontal wellbore. With current multi-lateral completion techniques, multiple zones can be independently produced and drained, each with its own horizontal wellbore.

Multi-lateral drilling and completion methods can enhance drainage patterns by dramatically increasing reservoir contact through selective isolation as individual zones and/or lateral sections lose production.

Multi-lateral drilling and completion techniques can also increase fracture contact. In the span of just a few years, horizontal drilling and completion methods have helped make highly fractured reservoirs attractive by increasing the contact area several orders of magnitude over conventional vertical wellbores. The emergence of multi-lateral technology will increase contact area even more.

Thru-tubing re-entry

When planning an enhancement of an existing wellbore using multi-lateral completion technology, consideration must be given to the existing completion hardware. If it is functioning properly and/or if removal is not economical, a thru-tubing lateral drilled with state-of-the-art coiled tubing drilling techniques can be considered. Thru-tubing drilling with coiled tubing provides an economical means of increasing reservoir exposure by significantly reducing or eliminating the topside costs associated with rig utilization.

Two significant advances in current casing exit technology have made thru-tubing re-entry possible.

1. A whipstock that can be deployed through the tubing, exit out the tailpipe, and set and oriented in the larger diameter casing allows for easy access to the reservoir
2. Recent tests have shown that it is possible to exit through the tubing and the casing.

While thru-tubing re-entry multi-laterals are in their developmental infancy, the possibilities long term are enticing.

Cased, cemented main bore

If the smaller diameter thru-tubing laterals do not meet the need of predicted reservoir drainage capacity, the existing completion hardware must be removed and larger, full-bore laterals drilled and completed.

At this point, the state of the main borehole must be considered. If the existing wellbore has been cased, cemented and perforated, exit windows must be milled in the production casing before a cased and cemented mainbore multi-lateral completion system can be utilized.

• Barefoot sidetracks: By far the simplest lateral, and the method with the most case histories, is one that is drilled out of the exit window and left open-hole and unsupported (barefoot) (Figure 1). This requires a substantially competent formation that will not fill or collapse during the life of the wellbore.
  Advantages:

  (1) Selective isolation is accomplished with standard packers and sliding sleeves.
  (2) Economic due to a minimum of completion equipment required.

  Disadvantages:

  (1) Lateral entry is difficult if not impossible.
  (2) Hole stability is always questionable.

• Supported sidetracks: The open-hole sidetrack can be supported with pre-packed screens or a slotted liner (Figure 2). The liner (or screens) is run into the sidetrack but not tied back to the main bore casing. Again, because there is a short section of unsupported open hole, a competent formation is required.
  Advantages:

  (1) Selective isolation is accomplished with standard packers and sliding sleeves.
  (2) Uses standard completion equipment.

  Disadvantages:

  (1) Lateral entry is difficult if not impossible.
  (2) Hole stability is questionable at the junction to the main casing.

Dual completion,supported sidetrack

Utilizing the same directional datum point that was used to orient the retrievable whipstock, the scoophead/diverter will divert the lateral liner (or screens) into the previously drilled open-hole lateral (Figure 3). The diverter is connected to the scoophead, which allows for a sealing juncture for both the mainbore tubing and the lateral liner. The completion above the scoophead uses standard dual completion hardware.
Advantages:

(1) Pressure integrity for two independent zones or reservoirs.
(2) Utilizes conventional dual completion equipment.

(1) Relatively small production tubulars.
(2) Requires significant amount of hardware

Selective re-entry system

The cased hole selective re-entry system is ideal for all well completions that require increased reservoir contact but demand independent selective isolation because of predicted future gas or water coning (Figure 4). As in the dual completion above, the lateral liner is diverted into the open-hole lateral by the scoophead/diverter, leaving the dual sealing points in the scoophead section.

The heart of this system is an inverted Y nipple called the Selective Re-Entry Tool (SRT). The SRT is sealed onto the scoophead, thus commingling production from both the lateral liner and the mainbore tubing. Selectivity is accomplished via coiled tubing. If the shifting profile inside the SRT is shifted up, the coiled tubing will be directed into the lateral liner, if the shifting profile is shifted down, access will be given to the mainbore tubing.
Advantages:

(1) SRT allows independent re-entry.
(2) All lateral sections are isolated with permanent packers.
(3) All production is commingled and transported to the surface via a single production tubing.
(4) Allows lateral wellbores to be stabilized with liners and screens.

(1) High degree of complexity.
(2) Possible economic impact.

Limited entry completed,cased open-hole sidetrack

Utilized mostly in long, thin reservoirs, primarily in the Austin Chalk region of Texas, this system offers hole stability and entry to at least one of the lateral sections. A special liner is installed in the open-hole mainbore section which has a pre-cut exit window (Figure 5). A perforated diverter or kick plate is installed below the window and oriented such that anything run into the liner is diverted through the window. The lateral wellbore is then drilled off, using the kick plate for orientation. Finally, the tubing string/slotted liner is run in normally, using a production packer. The tubing is diverted off the kick plate into the open-hole lateral.
Advantages:

(1) Can be installed in new or existing wellbores.
(2) Increased drainage area exposed.
(3) Allows boreholes to be stabilized with liner or screens.
(4) Second liner is run as part of the tubing and can be retrieved with the tubing.
(5) Standard completion technology.

(1) Re-entry to main liner is difficult.
(2) No isolation options.

Cased, cemented lateral connection system

This revolutionary system offers the advantage of having cased and cemented lateral wellbores, which become advantageous when developing highly fractured reservoirs that demand strict zonal isolation (Figure 6). Based on existing liner technology, this system can be installed in either new or existing wells. Isolation is achieved by utilizing standard completion and flow control devices. Selective entry may be accomplished using the SRT or using methods currently under development.
Advantages:

(1) Lateral sections are cased and cemented.
(2) Increased drainage area exposed.
(3) Established data point used for re-entry.

(1) Significant risk with high number of branches.
(2) Existing re-entry methods result in small production tubulars.

Open-hole main bore

When considering re-entry wells for expansion to multi-lateral completions, open-hole mainbore completions offer an economical solution in competent formations. This becomes a very attractive solution since time and money is saved by not running the production casing and/or liner. Here are three examples of current open-hole multi-lateral systems.

• Commingled open-hole sidetracks: This is an example of the simplest application of multi-lateral technology (Figure 7). Essentially, the lateral branches are drilled below the production casing to increase the wellbore drainage contact area. Well control is attained by using a standard permanent or retrievable production packer. Because of its simplicity and the fact that it uses standard completion technology, this application requires a relatively small initial investment.
  Advantages:

  (1) Can be installed in new or existing wellbores.
  (2) Increased drainage area exposed.
  (3) Inexpensive and low maintenance.
  (4) Well cased and cemented to the reservoir.
  (5) Standard completion technology.

  
  Disadvantages:

  (1) No isolation options.
  (2) Lateral entry is difficult at best.
  (3) Hole stability problems.

• Isolated open-hole sidetracks, suported or barefoot: This multi-branch completion option is based on the use of a liner string to provide an isolation system to each of the pre-drilled lateral branches (Figure 8). Production from all lateral branches is commingled in the mainbore completion, but each branch may be individually produced or isolated by using sliding sleeve devices between each external casing packer (ECP). Every branch can either be supported by liners with ECP inflatable packers, or left unsupported (barefoot). This option requires a relatively stable formation since direct tie-back to the mainbore completion is not feasible.
  Advantages:

  (1) Can be installed in new or existing wellbores.
  (2) Provides isolation control to all lateral branches.
  (3) Increased drainage area exposed.
  (4) Uses conventional liner and inflatable technology.
  (5) Well is cased and cemented to the reservoir.

  
  Disadvantages:

  (1) Lateral entry is difficult.
  (2) Hole stability problems.

• Open-hole selective re-entry system: Like the cased hole selective re-entry system, the open-hole variant is ideal for all multi-laterals that require completed lateral wellbores and main bore due to unstable geology, but which also require independent selective isolation because of predicted future gas or water coning (Figure 9). This system brings together many facets of completion technology, including permanent packers, whipstock devices, flow control, liner hangers and inflatable external casing packers. As in the previously mentioned cased hole selective re-entry system, the heart of the system is the SRT, which allows for selective entry into any of the completed laterals.
  Advantages:

  (1) Can be installed in new or existing wellbores.
  (2) SRT allows independent re-entry.
  (3) Allows lateral wellbores to be stabilized with liners and screens.

  
  Disadvantages:

  (1) High degree of complexity.
  (2) Possible economic impact.

Critical areas

The most significant aspect of re-entries is dealing with the geometry in the existing wellbore. For example, some of the concepts for new wells employing subsurface splitters use 20-in. conductor for the top 1,000 ft to fit the desired subsurface geometry in the wellbore.

It quickly becomes obvious that there is a problem when the operator encounters a 7-in. string cemented to surface. Thus, one of the most consistent long-term challenges for multi-lateral re-entries will be fitting optimum sized tubing strings for high-volume potentials.

A second critical area relative to completion schemes for re-entry multi-laterals is that the operator can't rely on pre-spaced and cut casing exits or casing-mounted equipment to aid the actual exit of the main borehole. Current technology dictates a casing exit through as-is casing, accomplished with equipment introduced specifically for this purpose.

Presently, one of the most practical methods is to utilize a retrievable whipstock system. Assuming that it is desirable to maintain access to a lower zone, the likelihood of retrieving the whipstock becomes critical. The inability to retrieve the whipstock used for an upper branch would literally cancel the progress made on the wellbore construction below the lateral.

This means that the retrieving reliability needs to be in excess of 95%. When trade-offs are considered, reliable retrievability should receive the greatest concern. Retrievable whipstock systems such as two-trip whipstocks have been designed to provide potential redundancy for retrieval.

A third critical area within the re-entry multi-lateral arena is the need for cooperation from non-traditional disciplines. Most applications require the integration of measurement-while-drilling (MWD) and gyro systems to accurately place the completion equipment. This, in turn, requires effective multi-disciplinary communication among service personnel at the well site.

Future applications

The future for multi-lateral re-entry applications will likely be a high growth area within wellbore construction. The combination of the sheer number of existing wells and the reservoir opportunities represented by multi-laterals will make this a high-profile subject area. Future re-entry applications will likely focus on:

(1) Maximum tubular sizes
(2) Thru-tubing applications
(3) Coiled tubing and underbalanced applications.

Maximizing tubing sizes will be an issue because of the limitations imposed by working inside existing casing. Multi-laterals should enhance a well's production potential, so it becomes a priority to compensate for completion sizes that inhibit production.

Thru-tubing will most assuredly be a topic of future discussions. The potential for going through tubing to expose new parts of a reservoir without requiring a full-scale cleanout will definitely be a high-profile area of development. This focus will quite naturally accompany current developments of coiled tubing applications and underbalanced drilling for fully optimized wellbore construction. Systems for re-entry applications have been developed sufficiently to allow operators to test the waters prudently.

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