Essentials of Planning a Directional Well
Directional drilling has changed the oil and gas industry by making it possible to reach oil and gas reserves that aren’t located directly under the drilling rig. Instead of drilling straight down, this method allows wells to be drilled at angles, helping to reach precise underground targets. This approach improves production efficiency and reduces the environmental footprint of drilling operations. The directional well planning and design process requires meticulous work and advanced technology solutions to ensure safety, efficiency, and success. The essential aspects of planning a directional well are discussed below, tailored for both young and experienced drilling professionals.
Planning a directional well is a complex process integrating geological, geophysical, and engineering expertise to achieve the desired objectives. The following sections outline the key considerations involved in directional well planning.
1. Objectives and Well Requirements Your Comments
The planning process begins with defining the well’s purpose, which could include:
Production: Extracting oil or gas from a known reservoir. A directional well helps optimize the reservoir contact, production rates, and drainage. This involves positioning the well to maximize drainage. They require precise coordinates regarding measured depth (MD), true vertical depth (TVD), and horizontal displacement. Production goals, such as expected flow rates (e.g., barrels per day for oil or cubic feet per day for gas), influence the well design and equipment selection.
Exploration/Appraisal: Exploration wells are drilled in areas with promising geological and geophysical characteristics to discover new oil or gas fields. After a discovery is confirmed, appraisal wells are drilled to assess the reservoir's dimensions, quality, and potential for profitable production. These wells are highly uncertain and need a flexible trajectory design with contingency options.
Injection: Injector wells are drilled with the objective of secondary or enhanced recovery through water or gas injection. The directional wells are planned to strategically place injectors relative to producers to maintain reservoir pressure and drive hydrocarbons toward production wells.
2. Geological and Geophysical Evaluation Your Comments
A thorough understanding of the subsurface is critical for successful directional drilling. This involves several considerations:
Depth: The vertical depth to the reservoir and the displacement of the target from the surface location guide the kick-off point, well trajectory, and the well's measured depth (MD). Depth also influences drilling time, cost, and rig capability selection. Deeper formations require longer and more complex well paths, often necessitating extended-reach drilling (ERD) or high-angle trajectories.
Dip: Formation dip is the angle at which the formation layer inclines relative to the horizontal plane. The wellbore must be angled in dipping formations to remain in the most productive zone and achieve optimal reservoir contact.
Thickness and Continuity: Refers to the vertical extent and lateral spread of the reservoir rock. A thick and laterally continuous reservoir is ideal for horizontal or multilateral well placement. In thin or discontinuous reservoirs, geosteering is required to keep the well within the productive zone to maximize reservoir exposure, recovery, and drainage.
Seismic Data: Seismic imaging is essential for visualizing what lies beneath the surface and spotting potential hazards before drilling starts. We need precise target coordinates to reach the reservoir at just the right spot and angle to create an effective drilling plan. Seismic surveys offer clear images of the subsurface, whether 2D, 3D, or 4D. They help us pinpoint the exact locations of reservoir structures and identify hazards like faults, salt domes, gas pockets, and areas with high pressure.
Formation Properties: Formation properties are key to the wellbore's stability, the drilling's efficiency, and the production performance. Important factors like porosity and permeability determine how well a reservoir can hold and move fluids. Areas with higher porosity and permeability are likely to deliver better production results. These characteristics also influence our approach to designing completions.
Formation pressure is crucial when choosing the right mud weight, helping us prevent kicks or blowouts. On the other hand, formation temperature affects equipment ratings and the types of drilling fluids we use. In high-pressure and high-temperature (HPHT) environments, we must rely on specialized materials and robust well control systems. Careful planning of drilling fluid density so that it is just right for pressure control without compromising the integrity of the formation is vital.
3. Well Trajectory Design Your Comments
The well trajectory is the planned path from the surface to the target, designed to balance efficiency, safety, and equipment capabilities. Key considerations include:
Well Profile Selection: In directional drilling, well profile selection refers to choosing the path a wellbore will follow from the surface to the target reservoir. The well profile is not just a geometric path—it is a strategic decision that affects the entire drilling and production lifecycle. It directly impacts the drilling project's technical, economic, and operational success. It not only influences the Reservoir Access, drainage Efficiency, and well cost but also greatly influences Drilling Feasibility, dogleg severity, torque and drag, hydraulics, and borehole stability.
Build-and-Hold: It starts vertically, deviates to a target angle, and holds that angle to the target. It is suitable for shallow wells.
S-Curve: This type of curve includes a build section, a hold section, and a drop section to return to vertical or a lower angle.
Horizontal Well: This type of well extends horizontally or at a high inclination within the reservoir to maximize exposure. It is common in unconventional reservoirs like shale.
Multilateral Well: Multilateral well configurations involve drilling multiple horizontal or deviated branches, known as laterals, from a single primary wellbore. Each of these laterals is strategically designed to access and produce from different zones or sections of a hydrocarbon reservoir.
Extended Reach Drilling (ERD): ERD involves drilling wells with significant horizontal displacements relative to their true vertical depth (TVD). In ERD wells, the horizontal reach from the surface location to the well's endpoint often exceeds twice the vertical depth of the well.
Complex 3D wells: Complex 3D wells are designed with multiple changes in directions, both inclination and azimuth, to carefully steer around geological barriers or reach distant, scattered reservoir targets.
Key Points: When planning the path (or trajectory) of a directional well, several important factors must be carefully considered to ensure the well reaches the target safely, efficiently, and within design limits.
Kick-Off Point (KOP): The Kick-Off Point is the depth at which the well deviates from vertical. Choosing the right KOP depends on the formation and well profile. Selecting it too early or late can lead to challenges in controlling the well path or missing the target zone.
Build-Up Rate (BUR): The Build-Up Rate is measured in degrees per 100 feet and refers to how quickly the well’s angle changes. A typical rate is between 1° and 3° per 100 feet. A smooth, gradual curve is preferable to reduce the risk of equipment damage or borehole instability.
Tangent Section: Once the desired angle is achieved, the well typically enters a tangent section, where it continues in a straight line at a constant inclination.
Target Coordinates: Coordinates are based on those specified in MD, TVD, and horizontal displacement to ensure the well hits the target window.
Target Window: Even with precise planning, there's always a small margin of error. The target window defines the allowable area where the well must land to hit the productive reservoir. This is often visualized as a circle or rectangle around the target point.
Dogleg Severity Limits: Dogleg severity measures how sharply the well path bends. It is measured in degrees per 100 feet and is affected by both the rate of change of well inclination and azimuth. Staying within acceptable limits is important to prevent problems with equipment, especially when running casing or using tools like logging instruments.
Anti-Collision Planning: In fields where multiple wells are drilled from a single location (like multi-well pads/offshore platforms) or nearby locations, it’s crucial to avoid intersecting nearby wells. Anti-collision planning uses specialized software to model well paths in 3D and calculate safe distances between wells. This helps reduce the risk of collisions, which can lead to serious safety and environmental incidents.
4. Drilling Engineering and BHA Design Your Comments
The drilling engineering phase selects and configures equipment to execute the planned trajectory.
Coordinate Transformation: Accurate planning requires transforming coordinates between geodetic (latitude/longitude), grid (easting/northing), and local systems. Transforming coordinates between these systems ensures that the planned well path correctly matches the real-world position of the target underground. This helps drill the well in the right direction and hit the reservoir exactly where intended.
For example, Geodetic Coordinates Latitude: 30.123456° N and Longitude: -97.654321° W tell you where the well is on the globe. However, they’re not ideal for calculating distances or directions over short ranges like wellbore paths. To simplify measurements, the geodetic coordinates are converted into grid coordinates, such as in the UTM (Universal Transverse Mercator) system; Easting: 620,000 meters, Northing: 3,330,000 meters, Zone: UTM Zone 14N, which are easier for planning. To simplify things further at the drilling site, a local coordinate system uses the reference point (0,0) at the rig location. The target reservoir might now be described as: X = 2000 m (east), Y = 0 m (north), and Depth = 2500 m (downward). Now, everything is measured relative to the rig, making it easier to track drilling progress on-site.
Bottom Hole Assembly (BHA): Selecting and designing the BHA is a key step in directional well planning. It plays an important role in precisely steering, stabilizing, and monitoring the well as it progresses toward the reservoir target, minimizing risks and maximizing drilling efficiency.
Tools: Include mud motors, rotary steerable systems (RSS), MWD, and logging while drilling (LWD) tools. Mud motors with bent subs allow steering by rotating only the bit, while RSS enables steering with continuous string rotation for better efficiency. MWD and LWD tools collect and transmit real-time data about the well’s direction, inclination, and formation properties, helping drillers make better decisions on the fly.
Directional Control: The BHA is designed to build, hold, or drop angles as needed. Different assembly configurations are used based on the trajectory. Fulcrum assemblies are used when building angles. ‘Stabilization’ assembly setups are designed to hold the current angle, whereas ‘Pendulum’ assemblies help drop the angle when needed. BHA Configurations for Directional Control are summarized in the table below.
Configuration Purpose Stabilizer Placement
Fulcrum Build angle Near bit, length determines build rate
Stabilization Hold angle At 0–30 ft, 30 ft, 60 ft from bit
Pendulum Drop angle First stabilizer 30–45 ft behind bit
Steering Capability: With tools like RSS and MWD, drillers can make precise, real-time adjustments to the well direction. This ability to steer as drilling progresses ensures that the well stays on course and accurately reaches the intended target zone.
String Stability: The BHA includes stabilizers and reamers to keep the drill string stable while drilling. These tools reduce unwanted vibrations and help prevent problems like key-seating (where the string gets stuck in a groove it creates). A stable string also improves drilling efficiency and tool life.
Torque and Drag Considerations: In deviated wells, friction between the drill string and borehole can cause excessive torque and drag, leading to pipe sticking or equipment wear. Software models these forces to optimize BHA design and drilling parameters. Based on the torque and drag analysis, multiple versions of the well profile may need to be developed before an optimum profile is selected.
Well-Planning Software: Well-planning software is used to design trajectories, torque and drag analysis, BHA behavior analysis, hydraulics, visualize the subsurface, and avoid surface and underground constraints, such as existing wells or geological hazards.
Bit Selection: Drill bit selection in directional wells is based on formation type, desired well path, and compatibility with the BHA and steering system. The bit must be steerable enough for directional control, efficient in the specific rock types, and Stable to maintain hole quality. The right drill bit ensures the well reaches the target accurately, efficiently, and safely.
Hydraulic Analysis: Hydraulic analysis ensures safe and efficient drilling, especially in directional wells.
Hole Cleaning: In directional wells, gravity causes the rock cuttings to settle along the lower side of the wellbore instead of falling straight down as in vertical wells. This makes cleaning the hole more challenging. To transport cuttings effectively, it requires high flow rates and optimized drilling fluid rheology (e.g., high yield point to plastic viscosity ratio).
Cooling: Drilling generates a lot of heat, especially from the drill bit and bottom hole assembly (BHA). This heat can damage tools or reduce their performance in deep or high-temperature formations. Drilling fluid flows past these tools, absorbing and carrying away the heat, helping to keep equipment cool and functioning properly.
Pressure Control: Drilling fluids are also used to control formation pressures. If the pressure from the rock formation is too high, it can cause a blowout. If it’s too low, fluid can be lost into the formation (lost circulation). Fluid weight and properties are carefully selected to balance the pressure from the surrounding rock, and flow rates are adjusted to maintain annular velocity, ensuring efficient cuttings removal.