Directional Well Planning: Choosing the Right Profile for Optimal Reservoir Access

 

Introduction

"What determines whether a well will follow a J-curve or an S-curve?" In directional drilling, the trajectory of the well is crucial to accessing the target reservoir efficiently. The path that the well follows is determined during the directional well planning phase. This article covers the stages of well planning and explores the different well profiles, such as build and hold, S-curve, horizontal wells, and multilateral wells, which are key to determining the well’s trajectory.

Overview of Well Planning Stages

1. Initial Well Design

The first step in directional well planning involves determining the overall objective of the well, such as its target depth, the type of reservoir being drilled into, and the most efficient path to reach the reservoir. This initial phase requires comprehensive geological, geophysical, and reservoir data to define the target zone and the desired well profile.

• Data Considerations: Reservoir maps, seismic data, and well history are used to assess the best possible drilling trajectory.
• Objective: To design a well that efficiently accesses the reservoir while minimizing drilling risks and maximizing recovery.

2. Wellbore Trajectory Design

Once the objectives are clear, engineers design the wellbore trajectory using the selected well profile. This stage includes determining the target depth, incidence angle, and azimuth. Different well profiles are considered based on the geological conditions, the need to avoid obstacles, and the desired production outcomes.

• Optimization Factors: Inclination, direction, and distance from the surface are calculated for optimal placement and reservoir contact.
• Outcome: The result is a detailed directional plan that can be followed by the drilling team.

3. Drilling and Monitoring

With the plan in place, the drilling phase begins. Monitoring of well parameters (such as azimuth, inclination, and deviation) occurs in real time to ensure that the well follows the planned trajectory. This is where tools like Measurement While Drilling (MWD) and Logging While Drilling (LWD) become crucial in providing feedback to the engineers.

• Real-time Data: These tools help adjust the drilling parameters to keep the well on course.
• Safety Measures: Early detection of deviations allows for timely corrections to avoid potential drilling problems.

Common Well Profiles in Directional Drilling

1. Build and Hold Profile

The build and hold profile is one of the most basic wellbore trajectories used in directional drilling. In this profile, the well begins vertically, then deviates at a steady angle until it reaches the desired target.

• Trajectory Characteristics: The wellbore builds to a specific angle and then holds that angle as it continues towards the target.
• Application: This profile is commonly used in conventional reservoir setups, where a straightforward, stable trajectory is enough to reach the target zone.

2. S-Curve Profile

The S-curve profile is designed to give a smoother, more gradual change in direction as the well transitions from vertical to horizontal. The curve is shaped like an "S" and allows the well to gradually change inclination without abrupt turns.

• Trajectory Characteristics: The well starts with a vertical section, then gradually curves to the desired angle, holding that angle to the target depth.
• Advantages: This profile reduces stress on the casing and minimizes the risk of wellbore damage. It is often used in long-reach wells, as it offers a balance between efficient drilling and trajectory flexibility.
• Application: Ideal for wells that need to access reservoirs located horizontally, especially when dealing with irregular geological formations.

3. Horizontal Wells

In horizontal wells, the wellbore is drilled to a high inclination (nearly 90 degrees) and then remains horizontal for the majority of its length.

• Trajectory Characteristics: The well reaches a point where it transitions from a vertical or inclined section to a horizontal trajectory.
• Advantages: Horizontal wells increase contact with the reservoir, enhancing production rates. They are especially effective in reservoirs with low permeability, where the well needs to cover a wide area to maximize resource extraction.
• Application: Common in shale plays, tight gas reservoirs, and offshore platforms.

4. Multilateral Wells

Multilateral wells are more complex and involve drilling multiple branches off a main wellbore, often to access multiple reservoir zones. These wells require advanced planning and coordination to ensure that all branches are drilled efficiently and safely.

• Trajectory Characteristics: A multilateral well starts with a main bore, then branches off into additional horizontal or vertical wells.
• Advantages: Multilateral wells allow for the exploration and production of multiple zones with a single surface location. They are especially useful in reservoirs with multiple productive layers or in offshore fields where space for additional wells is limited.
• Application: Widely used in deepwater and offshore environments to increase resource recovery while minimizing surface footprint.

What Determines the Well Profile: J-Curve vs. S-Curve?

The decision between a J-curve and an S-curve profile depends on several factors, including:

1. Reservoir Location and Depth

The location and depth of the reservoir often dictate the type of well profile chosen. For reservoirs that are located deep beneath surface obstructions, a J-curve may be required to reach the target efficiently. However, if the well needs to be drilled through multiple layers or formations, an S-curve is typically chosen for its smoother trajectory and reduced risk of wellbore instability.

2. Geological Formation

The type of geological formations encountered can also influence the choice of well profile. In highly deviated or challenging formations, an S-curve may be preferred for its ability to avoid abrupt changes in direction. In contrast, a J-curve might be used when the well needs to traverse a single, relatively straightforward geological layer.

3. Drilling Objectives

If the objective is to reach a target horizontally over a significant distance, a horizontal well or S-curve profile would be more beneficial. However, for simpler, more conventional wells, a build and hold trajectory might suffice.

4. Wellbore Integrity

Maintaining wellbore integrity is critical, and the selected profile must ensure minimal risk of casing failure or wellbore instability. Profiles like the S-curve tend to put less stress on the casing, making it an attractive choice for extended-reach and high-deviation wells.

Conclusion

Directional well planning is an essential step in ensuring efficient and cost-effective drilling operations. The well profile selected whether it’s build and hold, S-curve, horizontal, or multilateral depends on factors like reservoir location, geological conditions, and production goals. By understanding the benefits and challenges of each profile, drilling engineers can make informed decisions that optimize well performance and enhance reservoir access.