Types of Wells: Vertical, Directional, and Multilateral

Petroleum wells are the physical link between hydrocarbon reservoirs and the surface, and their design depends on geology, project objectives, and environmental constraints. This chapter explores the main types of wells—vertical, directional, and multilateral—detailing their advantages and applications in oil and gas exploration and production. Understanding these categories is essential for linking geological fundamentals, well definitions, and drilling equipment, setting the stage for more advanced topics like rig organization and safety.

Types of Wells and Their Characteristics

Petroleum wells are classified based on their trajectory and purpose, which determine their complexity, cost, and application. The three main types are vertical, directional, and multilateral wells, each designed to suit the reservoir’s characteristics and operational objectives. Below, we describe their characteristics, advantages, and applications, with practical examples.

Vertical Wells

Vertical wells are the simplest and most traditional, drilled straight down to reach a reservoir rock. Their trajectory follows a direct line from the surface to the target, making them ideal for conventional reservoirs with clear vertical distribution, such as anticlines.

Advantages:

1. Simplicity: Require less technology and planning than directional wells, reducing initial costs.
2. Shorter drilling time: The direct trajectory minimizes non-productive time, especially in stable formations.
3. Low operational complexity: Use standard equipment, like conventional rigs, and are less prone to issues like drill string sticking.

Applications:

• Initial exploration: Vertical wells are common in exploratory (wildcat) wells, such as those drilled in the Neuquén Basin to assess Vaca Muerta’s potential.
• Conventional reservoirs: In fields like Ghawar in Saudi Arabia, vertical wells access high-porosity carbonate reservoir rocks.
• Shallow wells: In regions with reservoirs close to the surface, where directional drilling is unnecessary.

Challenges: Vertical wells are limited in extensive reservoirs or under inaccessible areas, such as urban zones or bodies of water. Additionally, their contact with the reservoir rock is limited, which can reduce production in unconventional reservoirs.

Directional Wells

Directional wells deviate from the vertical to reach targets not directly beneath the drilling site. Their trajectory can be J-shaped (inclined), S-shaped (with multiple direction changes), or include horizontal sections. These wells rely on technologies like mud motors and measurement-while-drilling (MWD) systems (Chapter 2) to guide the bit.

Advantages:

1. Access to remote targets: Enable drilling under restricted areas, such as cities, lakes, or limited offshore platforms.
2. Greater reservoir contact: In horizontal wells, the extended section through the reservoir rock increases production, especially in low-permeability formations.
3. Resource optimization: A single directional well can replace multiple vertical wells, reducing costs and environmental impact.

Applications:

• Unconventional reservoirs: In the Permian Basin, horizontal wells traverse shale layers to maximize oil and gas production.
• Offshore drilling: From fixed platforms or jack-ups, directional wells reach multiple targets from a single location.
• Avoiding geological obstacles: Allow bypassing salt domes or faults, as in the Gulf of Mexico.

Challenges: Directional drilling is more costly and complex, requiring advanced tools like rotary steerable systems (RSS) and constant monitoring to maintain the trajectory. It also increases risks like pipe sticking in unstable formations.

Multilateral Wells

Multilateral wells consist of a main wellbore with multiple branches extending into different zones of a reservoir. Each branch can be vertical, inclined, or horizontal, depending on the design. These wells are an evolution of directional wells, using advanced technologies to maximize reservoir access from a single surface location.

Advantages:

1. Greater reservoir exposure: Multiple branches increase contact with the reservoir rock, boosting production.
2. Cost reduction: Enable exploitation of multiple zones without drilling multiple surface wells, saving on infrastructure.
3. Efficiency in complex fields: Ideal for reservoirs with multiple productive layers or irregular geometries.

Applications:

• Mature fields: In Venezuela’s Maracaibo field, multilateral wells access separate sandstone layers to optimize recovery.
• Offshore reservoirs: In the North Sea, multilateral wells reduce the need for multiple platforms, lowering costs in deep waters.
• Compartmentalized reservoirs: In basins with faults or stratigraphic traps, branches reach isolated zones.

Challenges: Multilateral wells are technically complex, requiring advanced control and cementing systems to isolate branches. They also pose higher risks of collapse or interference between branches, demanding precise geological planning.

The following table compares the types of wells:

Summary

Vertical, directional, and multilateral wells address the specific needs of each reservoir, from the simplicity of vertical wells in conventional fields to the high exposure of multilateral wells in complex reservoirs. Each type offers unique advantages, such as low costs or greater reservoir contact, but also faces challenges that require advanced technologies and precise geological planning. These concepts connect with petroleum geology, well purposes, and drilling equipment, laying the groundwork for understanding rig organization and advanced operations.

Practical Exercise

1. Reflection question: How do you think choosing a directional well over a vertical one affects the profitability of a project in an unconventional reservoir?
2. Research task: Investigate an oilfield (e.g., Eagle Ford in the U.S.) and write a paragraph describing which type of well (vertical, directional, or multilateral) is predominantly used and why.
3. Technical question: Explain how multilateral wells can improve recovery in a reservoir with multiple productive layers.

Bibliography

• Books used:

  • Hyne, N.J. (2012). Nontechnical Guide to Petroleum Geology, Exploration, Drilling & Production. PennWell Books.
    Explains well types and their applications in an accessible manner.
  • Bourgoyne, A.T., Millheim, K.K., Chenevert, M.E., & Young, F.S. (1986). Applied Drilling Engineering. SPE Textbook Series.
    Details the technical characteristics and challenges of directional and multilateral wells.

• Recommended books:

  • Mitchell, R.F., & Miska, S.Z. (2011). Fundamentals of Drilling Engineering. SPE Textbook Series.
    A technical resource on well design and operation. Available at: https://store.spe.org/Fundamentals-of-Drilling-Engineering-P113.aspx.
  • Azar, J.J., & Samuel, G.R. (2007). Drilling Engineering. PennWell Books.
    Ideal for deepening knowledge on directional and multilateral wells. Available at: https://www.pennwellbooks.com/drilling-engineering/.

• Direct links:

  • SPE (Society of Petroleum Engineers): Resources on well types and drilling. https://www.spe.org/en/.
  • AAPG (American Association of Petroleum Geologists): Information on geological applications of wells. https://www.aapg.org/.
  • PetroSkills: Courses on well design and directional drilling. https://www.petroskills.com/en/training/courses/directional-horizontal-and-multilateral-drilling---dhm.