A horizontal well is a type of multi-directional drilling technique that drills with an inclination of at least 80 degrees to enhance reservoir performance. The horizontal technique is used as an alternative method for drilling oil and gas in situations where vertical wells are impossible or the shape of the reservoir is difficult to access.
Usually, drilling at non-vertical angles can stimulate oil and natural gas reservoirs in ways that a vertical well cannot manage. When combined with hydraulic fracturing, horizontal wells can be used to drill hydrocarbons in previously unproductive zones.
Traditionally, horizontal drilling was relatively unpopular in the oil and gas industry. The technique became more prominent in the 1990s when Mitchell Energy began drilling horizontal wells in the Barnett Shale in and around Fort Worth, Texas. With improvements in technology, horizontal drilling is now considered an effective reservoir-development technique, helping increase the production of oil and natural gas extraction in many places around the world.
A horizontal well is a drilling technique that drills with an inclination of greater than 80 degrees to a vertical wellbore.
Horizontal wells are used in situations where the reservoir is abnormally shaped, or excavation is impossible.
Horizontal drilling is preferred over vertical drilling due to its ability to access subsurface reservoirs that are not accessible from directly above.
How Horizontal Drilling Works
The process of drilling a horizontal well starts by drilling a vertical well. Once the drill bits hit the target rock underneath, the attached drill pipe is withdrawn and replaced with a mud motor, which is attached to the drill bit. The mud pump is a pumping mechanism that drills into the earth using a drilling fluid known as mud. The mud powers the motor as it drills to the target area, automatically rotating the drill bit. The process allows the drill bit to follow non-vertical directions and is different from the direction of the drill pipe.
With horizontal wells, the drill bit drills a path that changes from a vertical to a horizontal direction, and once the proper angle is attained, drilling continues in a fully horizontal direction. Modern drilling methods allow the use of drill bits that can bend to achieve the required angle. The drill bit bending can be achieved using either hydraulic jets or by a computer using Global Positioning Systems (GPS) to determine the direction that the drill bit will follow in relation to the target oil or natural gas reservoir.
Why Horizontal Wells are Used
Although horizontal wells are expensive compared to vertical wells, they are preferred due to higher oil and gas production. When combined with hydraulic fracturing, it is estimated that horizontal drilling can cost many times more per foot than vertical drilling. Horizontal drilling is used to reach targets lying below adjacent lands and can minimize surface-level disruption.
Listed below are several reasons for using horizontal wells when drilling oil and natural gas:
1. Reach difficult targets
Some of the target reservoirs may be located beneath a park, residential areas, or city where drilling may be impossible or even forbidden. Here, a vertical well is drilled down first and then curved to a horizontal angle to reach underneath the unreachable areas. In such a way, horizontal drilling helps reach the target locations without causing disruptions to a park or residential area.
2. Drain a wide area
Horizontal wells are also used to drill a broad area using a single drilling pad to reduce the footprint of oil and gas exploration. For example, when drilling for oil in 1,000 acres of land, an oil company can drill a single main well and effectively branch off multiple times to the target reservoirs. The horizontal technique significantly reduces the footprint of oil and gas exploration on the land.
3. Improve well productivity in a fractured reservoir
Some reservoirs include pore spaces that are structured in the form of fractures. The exploration process must penetrate the fractures to allow the free flow of oil and gas into the well. Horizontal wells achieve the free flow by drilling into the earth in a direction that intersects the maximum number of fractures. It requires the drilling direction to be at the appropriate angles in relation to the dominant fracture direction.
4. Install utility service where excavation is expensive or impossible
When installing utility services, such as electric lines or gas, utility companies may be forced to drill horizontal wells in areas where excavation is impossible or too expensive. For example, where electric lines are required to cross a busy highway, the power company may decide to dig a horizontal well beneath the highway to pass the electric lines to the other side of the highway.
Horizontal Drilling vs. Hydraulic Fracturing
Horizontal drilling is used as part of the hydraulic fracturing process in the development of low-permeability rocks. The rocks contain a significant amount of oil and gas and are located beneath large tracts of land. However, it is difficult to tap the reservoir in these rocks using the traditional vertical wells due to the tiny pore spaces in the low-permeability rocks.
To stimulate the productivity of the rocks, exploration companies drill horizontally through the low-permeability rock unit. Hydraulic fracturing is then used to produce an artificial permeability. Fracking fluid is composed of water, chemicals and sand, and is forcefully injected into the hydrocarbon-containing rock (shale rock). The force of the injections props the shale open, creating cracks and fissures that allow large volumes of hydrocarbons to be extracted.
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