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Directional drilling technology is one of the most advanced drilling technologies in the global oil exploration and development field today. It relies on special downhole tools, measurement instruments, and process technologies to effectively control the wellbore trajectory, guiding the drill bit to reach the predetermined underground target along a specific direction. This technology breaks the limitation of vertical wells, which "can only develop resources directly below the wellhead". By adopting directional drilling technology, oil and gas resources restricted by surface or underground conditions can be developed economically and effectively, significantly increasing oil and gas production and reducing drilling costs. In essence, a directional well is a drilling method that guides the wellbore to reach the target formation along a pre-designed deviation angle and azimuth. There are three main types of its well profiles: (1) Two-section type: Vertical section + build-up section; (2) Three-section type: Vertical section + build-up section + tangent section; (3) Five-section type: Upper vertical section + build-up section + tangent section + drop-off section + lower vertical section A horizontal well is a type of directional well. Conventional oil wells penetrate the oil reservoir vertically or at a shallow angle, resulting in a short wellbore section passing through the reservoir. In contrast, after drilling vertically or at an angle to reach the oil reservoir, the wellbore of a horizontal well is turned to a near-horizontal direction to remain parallel to the oil reservoir, allowing long-distance drilling within the reservoir until completion. Equipped with high-strength heavy-weight drill pipes (HWDP) for horizontal sections and wear-resistant PDC (Polycrystalline Diamond Compact) bits, the length of the reservoir-penetrating section can range from hundreds of meters to over 2,000 meters. This not only reduces the flow resistance of fluids entering the well but also increases production capacity several times compared to conventional vertical or deviated wells, facilitating enhanced oil recovery. Ⅰ. Application Scenarios 1. Overcoming Surface/Underground Obstacles Surface obstacles: When there are buildings, railways, lakes, or ecological protection zones above the reservoir, directional wells can be drilled outside these obstacles to reach the reservoir at an angle (e.g., development of oil and gas reservoirs around cities). Underground obstacles: When bypassing hazardous geological features such as underground caves, salt domes, and faults, shock-resistant and collapse-proof drill collars and high-pressure blowout preventers (BOP) are used in coordination to avoid drilling accidents like pipe sticking and blowouts. 2. Enhancing Production Capacity of Unconventional Oil and Gas Reservoirs Unconventional reservoirs such as shale gas and tight oil have "extremely low permeability". Vertical wells can only access a small area of the reservoir, leading to limited production capacity. However, horizontal wells traverse the reservoir horizontally over a distance of several hundred meters, increasing the contact area with the reservoir by dozens of times. The daily gas production of a single horizontal well can be 5 to 10 times that of a vertical well, making it a core technology for unconventional oil and gas development. 3. Reducing Development Costs Offshore oil and gas fields: Drilling a cluster of wells from a single offshore platform is far less costly than building a separate platform for each target, resulting in a 30% to 50% reduction in development costs. Mature oil fields: Through "sidETracking" of directional wells (drilling branches from the wellbore of an old well to develop remaining oil reservoirs in the surrounding area), there is no need to drill new vertical wells, significantly reducing investment. Ⅱ. Advantages and Disadvantages Compared with Vertical Wells Advantages 1.Strong resource coverage capability: It can develop offset reservoirs and scattered reservoirs that are inaccessible to vertical wells, improving the production efficiency of oil and gas reservoirs. 2.High single-well production capacity: Horizontal wells, in particular, greatly increase the contact area between the wellbore and the reservoir, offering significant advantages in the development of unconventional oil and gas reservoirs. 3.Superior cost-effectiveness: Cluster wells and multi-lateral wells, supported by integrated drilling rigs and matched drilling equipment (such as top drives and mud pumps), reduce surface occupation and platform construction costs, making them suitable for offshore and intensive development scenarios. Disadvantages 1.High technical complexity: It requires professional directional drillers, rotary steerable systems (RSS), and MWD (Measurement While Drilling) equipment, resulting in a much higher technical threshold than vertical wells. 2.High costs: The investment in a single directional well is usually 20% to 50% higher than that of a vertical well of the same depth (due to increased costs of tools, equipment, and labor). 3.High risks: The complex trajectory leads to high circulating resistance of drilling fluid and increased difficulty in wellbore stability, resulting in a higher incidence of accidents such as pipe sticking and wellbore collapse compared to vertical wells. 4.Long construction cycle: Frequent trajectory adjustments and data measurements are required, leading to a 30% to 60% longer construction cycle than vertical wells of the same depth. Ⅲ. Conclusion In summary, directional drilling represents a milestone in the evolution of oil drilling from simple vertical development to complex and precise development. Currently, in global oil and gas resource development, the application proportion of directional wells has exceeded that of vertical wells, making it one of the core technologies for ensuring oil and gas supply.

Based on the geological, geographical conditions and engineering requirements of oil and gas exploration and development, wells are divided into two main types: vertical wells and directional wells. These two are core well types in the oil and gas drilling field, with the latter further categorized into conventional directional wells, horizontal wells, cluster wells, etc. The core difference between them lies in whether the wellbore trajectory is perpendicular to the ground, and they also differ significantly in design purposes, technical characteristics, application scenarios and construction difficulty. Next, we will discuss vertical wells. Ⅰ. Vertical Wells In drilling engineering terminology, a vertical well refers to a well type whose designed trajectory follows a vertical line, with the wellhead and bottomhole having the same geographical coordinates. Its total angle change rate is generally no more than 3°/30m. The wellbore verticality is ensured by drill string assemblies such as packed hole assemblies and pendulum assemblies, and it is suitable for scenarios such as coalbed methane development where geological units need to be divided. Vertical Well Drill String：The conventional drill string assembly consists of a rotary table rig + drill pipe + roller cone bit, which relies on the rigidity of the drill string itself to maintain verticality. Currently, the deviation prevention and straight drilling technology for vertical wells is mainly realized by improving the structural combination of the drill string: Deviation prevention: Rigid packed hole assemblies, tower-type assemblies, and square drill collar anti-deviation assemblies are mainly used. Deviation correction: Pendulum assemblies, flexible pendulum assemblies, eccentric weight drill collar assemblies, and downhole motor assemblies are mainly used. Ⅱ. Application Scenarios The application scenarios of vertical wells and directional wells are fully centered around three core needs: "resource distribution, surface conditions, and development efficiency". There is no absolute advantage or disadvantage between them, only differences in adaptability. Vertical wells are a cost-effective choice for simple scenarios. Their core advantages are "low cost and high efficiency", so they are suitable for scenarios with simple surface and underground conditions and concentrated resources. No complex design is required—only the target formation depth needs to be determined, and the drilling can be planned along a vertical path. The drilling process is carried out at a constant speed throughout, with only regular well deviation angle measurements required and no frequent adjustments. 1. Conventional Oil and Gas Reservoir Development When the oil and gas reservoir is directly below the wellhead, with a large reservoir thickness (>10 meters) and concentrated distribution, vertical wells can be drilled vertically to the target formation. The single-well productivity meets the demand, and there is no need for additional investment in directional costs. 2. Shallow Resource Exploration and Development For shallow oil and gas reservoirs, groundwater, and geothermal resources with a burial depth of <1000 meters, vertical wells do not require complex trajectories and can quickly complete drilling and production. 3. "Preliminary Exploration" with Exploration Wells In the early stage of oil and gas exploration, to obtain basic data such as underground formation lithology, porosity, and oil-gas bearing property, vertical wells (called parameter wells) are usually drilled. Due to their simple trajectory, vertical wells can more truly reflect the vertical formation sequence and provide a basis for the subsequent design of directional wells. Ⅲ. Advantages and Disadvantages Advantages 1.Low cost: The costs of equipment, construction, and maintenance are all lower than those of directional wells, and the single-well investment can be reduced by 30%~50%. 2.Simple technology: It has low technical requirements for the construction team, no need for professional directional engineers, and is easy to operate. 3.Short cycle: No frequent trajectory adjustments are required, and the construction cycle for the same depth is 30%~40% shorter than that of directional wells. 4.Low risk: The incidence of accidents such as wellbore collapse and pipe sticking is lower than that of directional wells (due to the simple trajectory, the drilling fluid circulation is more stable). Disadvantages 1.Trajectory limitation: It can only develop resources directly below the wellhead and cannot bypass obstacles or cover scattered reservoirs. 2.Low development efficiency: For unconventional oil and gas reservoirs, the contact area between the wellbore and the reservoir is small, resulting in low single-well productivity (e.g., the daily output of vertical shale gas wells is only 10,000~20,000 cubic meters). 3.Large land occupation: To develop multiple scattered reservoirs, multiple vertical wells need to be drilled, which consumes a large amount of land platform resources. Ⅳ.Conclusion With the advancement of oil and gas development towards "unconventional, deep-layer, and offshore" areas, the application proportion of directional wells continues to increase. However, vertical wells are still irreplaceable. In scenarios such as conventional oil and gas reservoirs, shallow resources, and exploration wells, vertical wells will exist for a long time due to their advantages of "low cost and high efficiency". In small and medium-sized oilfields in some regions, vertical wells remain the main development well type. The choice between the two is essentially a trade-off between "development needs and cost-effectiveness"—on the premise of ensuring the development target, vertical wells are selected for simple scenarios, and directional wells for complex scenarios, jointly supporting the efficient exploitation of global oil and gas resources.