DRILLING/WELL COMPLETION

Level 2 multilateral options expose hard-to-reach hydrocarbons

Field examples show how two multilateral systems provide access to oil and gas that otherwise would remain in place

Greg Nairn and Rocky Seale, Smith Services, Houston

lthough the concept of drilling and completing multilateral wells has been around for decades, it is only within the past ten years that technology has accelerated to meet industry needs. Before the early 1990s, the technology necessary for creating casing exits did not provide consistent performance. Without a reliable platform to build from, multilateral drilling remained stagnant.

During the development phases of retrievable whipstocks, several different systems emerged, each having unique benefits and applications. The wellbore requirements, during and after the drilling and completion phases, should be reviewed and referenced against the available systems to determine which system provides the greatest functionality. With proper planning, a more efficient wellbore, meeting all future requirements, can be provided. Prior to drilling the lateral wellbore(s), the following questions should be answered:

• What type of equipment will be required in the main and lateral bores at any given time between completion and abandonment?
• What equipment will remain in the wellbore prior to drilling the lateral well(s)?
• Is isolation between laterals required during the drilling process?
• Will remedial operations such as stimulation, water shutoff and (or) abandonment be required during the life of the wells?
• How will remedial and stimulation work be performed?
• Is selectivity of production required?

Full Bore Systems

In many cases it is desirable to leave the parent casing unobstructed after operations are completed. For these applications positive re-entry into the laterals is not required. The completion can take on a variety of configurations, from totally commingled to selectively produced. In re-entry applications, two systems currently exist to accomplish this, one using a Retrievable Anchor-Stock* and the other a Retrievable Pack-Stock.*

The Retrievable Anchor-Stock is a hydraulic-set anchor run on the bottom of the whipstock.² The design versatility of this system allows the whipstock to be set at any depth in the wellbore. Retrieval of the assembly leaves the casing unobstructed. This feature eliminates the need to run and retrieve any equipment across laterals, thus reducing potential problems.

The disadvantage of this system is that no reference point is left in the well to re-enter the lateral. However, this often is not a future wellbore requirement, thus suiting the application. When entry into the lateral will be required at a later date, a bent joint is used to facilitate this operation. Although old technology, new product enhancements have provided consistent operation time and again.

Operation of the retrievable anchor / whipstock is as follows, Fig. 1:

• Scrape wellbore.
• Pick up whipstock and milling assembly.
• Run in hole to setting depth.
• Orient whipstock to kickoff point (using MWD or SRG).
• Set anchor.
• Shear mill from whipstock.
• Displace fluid as required.
• Mill window and rathole.
• Pull mill out of hole.³
• Run drilling assembly.
• Drill lateral.
• Retrieve whipstock.

Fig. 1. Operation of the retrievable anchor / whipstock is as follows: A) run in hole; B) orient whipstock; C) shear mill; D) mill window; E) drill lateral.

This process can be repeated to add lateral wellbores uphole from the one just completed.

Field example. In the Middle East, this technology is being used to produce thin, tightly stacked oil reservoirs. Multilateral wells have enhanced production, improved recovery in the reservoir, delayed water breakthrough and helped manage production more efficiently. Utilization of the retrievable anchor / whipstock to access these reservoirs has lowered associated costs 36% and afforded the operator an array of options in drilling and completing these wells. These options include drilling various profiles that can access the hydrocarbon-bearing zones, or allow for the drilling of multiple open-hole laterals through a single window.^4,5

The retrievable packer / whipstock provides an alternative to the retrievable anchor / whipstock when isolation of the lower wellbore(s) is required during lateral drilling. This system allows the lower wellbore(s) to be drilled and completed prior to moving uphole, which saves time and money. An estimated cost savings of 20 – 30% will be recognized. This applies to each lateral leg drilled when the lower zone(s) is completed and the packer / whipstock is used to isolate the completed zones from the lateral being drilled. Operation of this system is identical to the retrievable anchor / whipstock.

Permanent Platform Systems

One of the oldest platforms used for casing sidetracking is a packer assembly. This system has evolved into a methodology from which multilateral wells can be constructed.⁶ Further enhancing this system, are the selective options of either commingled or isolated completions.

Operation of the system is as follows:

• Scrape wellbore.
• Run packer on wireline or pipe.
• Establish packer direction.
• Orient whipstock and latch based on packer direction to kick off point.
• Pick up whipstock and milling assembly.
• Run in hole to setting depth and latch into packer.
• Shear mill from whipstock.
• Displace fluid as required.
• Mill window and rathole.³
• Pull mill out of hole.
• Run drilling assembly.
• Drill lateral.
• Retrieve whipstock.

The shear value of the anchor is variable and can be changed on the surface prior to running. This provides the versatility for adjusting to changing wellbore conditions as they arise. Additional laterals can be drilled by spacing the whipstock further up the hole.

Field example. The packer and retrievable anchor has proven successful in many geographic areas. Two of the most prevalent areas are Aneth field in southern Utah and Prudhoe Bay field, Alaska.6,7 This system has proven to be both economical and functional. Multilateral wells have improved sweep efficiencies and the ultimate recovery from the reservoir. Infill drilling, as documented, would not have been economically successful. By running a packer in the hole, a permanent reference point in the wellbore is maintained for future depth and orientation correlation. Access to each lateral can be achieved by using the known depth and azimuth of the packer and placing a deflector at the optimum window opening.

If the main wellbore is to remain on production, the packer limits the size of equipment that can be run to that zone. This is an important post-well factor in determining which system should be run. If coiled tubing or slimhole workstrings are acceptable methods for performing remedial operations on the lower zone, this system can reduce time and minimize cost over the well’s life.

Conclusions

Over the last decade, technology has evolved to meet industry’s requirements for performing successful wellbore construction on Level 2 multilateral wells. In meeting industry requirements, different systems emerged to address various wellbore concerns. These systems have been applied successfully, both technically and economically. This practice provides a means to expose and produce hydrocarbons that otherwise would have remained in place.

Applications have ranged from accessing unswept hydrocarbons, improving sweep efficiencies, producing hydrocarbons from thinly bedded reservoirs and accessing pools trapped in compartments within the reservoir. With proper planning before wellbore intervention and construction, the wellbore geometry most conducive to that particular application can be installed.

Literature Cited

¹ Diggins, E., "A Proposed Multi-Lateral Well Classification Matrix," World Oil, Nov. 1997.

² Hebert, J., and Bailey, T., "Development and Field Testing of Retrievable Anchor-Stock," 1994.

³ Dewey, C., Miller, G., and Saylor, J., "Sidetracking in a Single Trip," SPE / IADC paper 37665, 1997 SPE / IADC Drilling Conference, Amsterdam, March 4 – 6, 1997.

⁴ Ismail, G., and El-Khatib, H., "Multi-Lateral Horizontal Drilling Problems & Solutions Experienced Offshore Abu-Dhabi," SPE paper 36252, 1997 SPE Middle East Oil Show, Bahrain, March 15 – 18.

⁵ Hassan, M., and Abu-Sharkh, AF., "Review of Horizontal Completion Practices Offshore Abu Dhabi," SPE paper 29830, 1995 SPE Middle East Oil Show, Bahrain, March 11 – 14.

⁶ Hall, S., "Multi-Lateral Horizontal Wells Optimizing a 5-Spot Waterflood," SPE paper 35210, Permian Basin Oil & Gas Recovery Conference, Midland, Texas, March 27 – 29, 1996.

⁷ Aubert, W., "Variations in Multilateral Well Design and Execution in the Prudhoe Bay Unit," SPE paper 39388, 1998 IADC / SPE Conference, Dallas, Texas, March 3 – 6.

The authors
	Rocky Seale is Lindsey Business Development Manager. He has 10 years of engineering and operational experience in the oil and gas industry, including total wellbore construction and engineered solutions for onshore and offshore applications for both deepwater and multilateral wells. He holds a BS in petroleum engineering from Montana Tech.
	Greg Nairn has been with Smith International for 24 years. Greg is a member of SPE and IADC, and holds three drilling tool patents and has co-authored 12 technical papers. His current position is Manager of U.S. Field Engineering.