Types of Directional Wells in Oil and Gas Drilling
Directional drilling is crucial for oil and gas extraction today. It allows drillers to precisely guide wellbores to reach underground reservoirs not directly beneath the drilling rig. This technique improves resource recovery, reduces environmental effects, and lowers operational costs by enabling access to several targets from one surface location. Below, you'll find an easy-to-understand overview of the main types of directional wells, including their uses, advantages, challenges, and other important points, suitable for newcomers and seasoned drilling professionals.
1. Sidetracking Your Comments
Sidetracking involves drilling a new wellbore from an existing one to bypass obstructions or access new reservoir zones. It’s often used when equipment, like a stuck drill string, cannot be retrieved or when a different part of the reservoir needs to be targeted.
Sidetracking a well is a critical technique in the oil and gas industry. It requires placing a high-density cement plug or a Whipstock above the obstruction or at the desired kick-off point. An appropriately designed bottom hole assembly (BHA) is run in the well to apply lateral force to initiate the new path. Measurement-while-drilling (MWD) tools ensure the new trajectory is followed.
Usually, the time drilling technique is used to slowly deviate the well path by closely watching the returns on the shale shaker. Soft or improperly set cement plugs can fail, causing the bit to be pushed back into the original hole due to formation resistance, and instead drilling the cement plug itself. Once the new hole is established, a new bottom hole assembly could be run to continue drilling the new well path.
2. Build and Hold Angle ("J" Type) Your Comments
The build-and-hold profile, often called the "J" type, involves drilling vertically to a kick-off point, building to a specified inclination, and maintaining that angle to the target. The hold section is also called the "Tangent Section." The kick-off points and selection of the well profile depend on the formation characteristics, well depth, well objectives, and horizontal displacement.
The “J” profile wells are simpler to plan and execute than complex profiles. They effectively reach offset targets with minimal directional changes and could be a good choice for simpler wells. However, the tangent section requires continuous and precise control to maintain the well's inclination and intended direction, increasing the well cost due to the need to deploy directional drilling equipment and services until the well reaches its target.
3. Build, Hold, and Drop Angle ("S" Type) Your Comments
The S-type profile, commonly referred to as a "build-hold-drop" wellbore trajectory, is characterized by a distinct sequence of directional changes that, when viewed in profile, form a shape resembling the letter "S." This drilling pattern begins with a build section, where the wellbore gradually deviates from the vertical by increasing its inclination at a controlled rate. Once the desired angle is achieved, the trajectory enters a hold section, where the inclination is maintained consistently for a specified distance. This portion allows the drill to reach the target reservoir horizontally or at a desired angle while optimizing well placement. Finally, the trajectory transitions into a drop section, steadily reducing the inclination, bringing the wellbore back toward a more vertical orientation.
This approach is often used to navigate complex subsurface formations, optimize reservoir contact, create well separation, or reduce drilling and completion risks by avoiding problematic zones. Since complex directional drilling tools and services may not be needed once the well drops back to vertical, the well cost may be less than other complex well profiles. However, the shallow kick-off point results in high torque and drag, requiring careful planning and analysis.
4. Horizontal Wells Your Comments
Horizontal wells begin with a vertical hole down to a specified depth. After reaching a planned depth, the hole's angle is adjusted to create a nearly horizontal trajectory, typically at 80 degrees or more. The horizontal or near horizontal section runs parallel to the reservoir, maximizing contact with the pay zone and boosting production. This approach significantly improves the well’s interaction with the reservoir, resulting in a more efficient resource extraction.
Horizontal wells mitigate water or gas coning issues, enhance drainage, and boost production due to increased reservoir contact. When combined with hydraulic fracturing, horizontal wells are effective in low-permeability reservoirs such as shale plays. Horizontal wells have higher costs as they require advanced tools like rotary steerable systems for precise control and complex well completions. However, they reduce the number of wells needed to achieve higher production, lowering effective cost and the environmental impact.
5. Multilateral Wells Your Comments
Multilateral wells are advanced well configurations that 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.
This approach maximizes reservoir contact and recovery efficiency while minimizing surface footprint and infrastructure costs. By branching off from a single main well, operators can effectively drain complex or compartmentalized reservoirs and access heterogeneous or isolated zones that would otherwise require multiple vertical or horizontal wells. Multilateral wells reduce the need for multiple surface sites, resulting in higher returns on investment (ROI) than multiple single wells with minimum environmental impacts.
Depending on geological conditions, production goals, and technological capabilities, the design and complexity of multilateral wells can vary significantly, ranging from simple junctions to highly sophisticated systems with individually controlled laterals. Multilateral wells are categorized based on the “Technology Advancement of Multilaterals (TAML)” classification system, which ranks them from Level 1 to Level 6 as below.
Level 1: Open-Hole Laterals Without Mechanical Junctions are the simplest form of multilateral wells. The laterals branch off from the main wellbore without mechanical support at the junction. They have minimal control or isolation.
Level 2: Cased and Cemented Main Bore with Open-Hole Laterals. The main bore is cased and cemented, but the lateral is an open hole. It is slightly more stable than Level 1 but doesn’t have a mechanical junction.
Level 3: Cased Main Bore and Lateral with No Pressure Isolation at Junction. This configuration has both the main bore and lateral cased and cemented, but the junction does not provide pressure or flow isolation. The result is stronger structures with improved longevity and stability.
Level 4: Pressure-Isolated Junction With Liner. Level 4 includes mechanical equipment to isolate the junction between the main bore and the lateral. It allows selective production and pressure management between laterals.
Level 5: Junction With Full Pressure Integrity and Selective Flow Control. This level provides full pressure and flow control at the junction and enables re-entry into the main bore and laterals for intervention and stimulation.
Level 6: Intelligent Multilateral Wells. This is the most advanced multilateral system used for complex reservoir management. It is equipped with integrated sensors, valves, downhole control systems, real-time monitoring, and the ability to control flow remotely.
6. Extended Reach Drilling (ERD) Your Comments
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, typically expressed as a horizontal-to-vertical ratio (ERD ratio) greater than 2:1. Alternatively, a well may also qualify as an ERD well if its measured horizontal displacement surpasses 20,000 feet (approximately 6,100 meters), regardless of the TVD.
Drilling ERD wells enables operators to access distant subsurface targets from a single surface location, making it ideal for developing offshore fields from fixed platforms. It also minimizes environmental footprints in sensitive areas and enables reaching reservoirs under mountains or urban areas or bypassing any surface obstacles. However, to overcome the technical challenges associated with such long-reach trajectories, ERD operations require careful planning, advanced drilling technologies, enhanced torque, drag, Equivalent Circulation Density (ECD), and pressure management.
7. Complex 3D Wells Your Comments
Complex 3D wells are designed with multiple changes in direction, both inclination and azimuth, to carefully steer around geological barriers or reach distant, scattered reservoir targets. These wells follow intricate paths that allow operators to bypass faults, unstable rock formations, or other subsurface hazards. They also enable access to multiple reservoirs from a single wellbore and improve well placement in uneven or variable rock layers, which is especially valuable in complex fields.
Drilling such wells requires advanced 3D planning and visualization software to map the subsurface path accurately. Precision tools like rotary steerable systems ensure control and reduce the likelihood of drilling mistakes, equipment malfunctions, or unexpected deviations. While more technically demanding, complex 3D wells offer significant advantages: they can significantly boost hydrocarbon recovery in geologically challenging areas and reduce the environmental impact on the surface by minimizing the number of well pads needed.