Directional drilling (DD), also known as directional boring, horizontal drilling, slant drilling, or deviated drilling, is a method of trenchless technology that involves drilling a non-vertical well or borehole. The oil well or borehole is drilled horizontally or laterally up to a 90-degree angle from the surface. Variations of directional drilling are used in oil drilling, utility installation drilling, and in-seam drilling of methane from a coalbed.
In the oil and gas industry, directional drilling involves laterally drilling multiple wells through a zone of oil- or gas-bearing rock at angles or slants from a vertical well hole. Most oil wells are drilled vertically straight down from the surface, penetrating only a few feet down into an oil or gas zone. Alternatively, the horizontal drilling of a well creates the ability to reach a greater amount of oil or gas within a reserve.
Directional drilling is also used in the installation of utility pipelines and conduits. A pilot borehole is drilled along a pre-determined bore path from the surface with minimum disturbance. It is used for making many types of crossings such as rivers and roads and for installing pipes and conduits to transport gas and oil, chemicals, cables, sewage, water, and other products. Pipe installation lengths of 21,326 feet (6,500 m) and diameters of 48 inches (122 cm) have been achieved with directional drilling.
Directional drilling is seen as a marriage of conventional road boring and the directional drilling of oil wells. It was first practiced in the 1920s in the drilling of oil wells as a means of increasing oil production. Early applications of directional drilling were, however, limited by low build rates and very short extension possibilities. For example, in the method of slant drilling, it took up to 2,000 feet (610 m) for the well to complete a transitional curve from vertical to horizontal. Modern technology has enabled directional drilling to be completed at a 90-degree turn in less than 100 feet (31 m). The most common method to extract oil and gas was to drill a vertical hole down from the surface directly into the oil or gas reserve. A steel casing was then inserted inside the hole to prevent it from caving in. A perforated gun was then inserted into the hole to puncture holes in the steel casing at depths where there was oil and gas. One of the drawbacks of this method was that only a small amount of oil- or gas-bearing rock was penetrated. The quantity of gas or oil that could flow out of the holes in the casing was therefore very limited. Today directional drilling increases oil and gas production by as much as 20 times that of a vertical drilled well. Directional drilling technology was then later adapted in the early 1990s for use in utility installations.
The process of directional drilling begins with a tracked or trailer-mounted directional boring machine that pushes a bore head connected to a hollow pipe into the ground at an angle. When each joint of the drill gets pushed into the ground, another one is placed behind it. The pilot hole is drilled horizontally across an obstacle along a predetermined path. An electronic beacon in the drill string behind the cutting head monitors the drill. A driller on the surface relies on the information relayed by the beacon to steer the bore along a specified path.
Many directional boring machines also use a drilling fluid or slurry called mud. Other machines use foam or air, or both. The drilling fluid or mud is composed of bentonite clay and water with additives for enhancing performance. The higher-pressure jet of fluid is used in soft ground to cut through the soil. The cuttings then get suspended in the fluid. As more fluid is pumped down into the hole, the cuttings are brought back up to surface where they settle in a pit or are removed mechanically.
An angled bit can also be used to bore a hole or well horizontally in softer soils. The angled bit and the pipe are rotated to achieve a straight bore. To steer the bit in a specific direction rotation is stopped. The angle of the bit is then aligned and forward thrust is applied. The jet of drilling fluid and the forward thrust cuts a new path through the soil.
In hard rock, a mud motor is often used. Mud motors are designed to convert the hydraulic pressure of the drilling fluid in mechanical rotation. This mechanical rotation is used to rotate the drill bit while the drip pipe remains stationary. Steering the pipe is achieved by aligning the angle of the mud motor in the chosen direction.
Sometimes when the ground is really soft or unstable, a washover pipe or steel casing is pushed down into the borehole to prevent it from caving in. When the pipe has reached the exit portal the bit is detached and the end of the drill pipe is attached to a reamer to enlarge the hole. The reamer is then pulled back while rotating the drill bit to enlarge the borehole. Sometimes the process involves multiple passes. The drill pipe is added in behind the reamer so that there is always a drill pipe in the borehole.
Sometimes when the hole is much larger than the pipe to be installed, the end of the drill pipe is joined to a reamer that is attached to a swivel and connected to the product pipe. Drilling fluid is then pumped down into the hole to provide lubrication and the pipe is pulled down into the hole while rotating the drill pipe and reamer. The swivel prevents the product pipe from rotating inside the hole.
New drilling sensors and global positioning technology have also brought vast improvements to directional drilling in recent years. Drill bit angles can be controlled with greater accuracy through the use of real-time technology. Some of the other tools used in directional drilling include whipstocks, bottomhole assembly (BHA) configurations, three-dimensional measuring devices, mud motors, and specialized drill bits.
There are also many advantages to directional drilling. The most obvious advantage is directional drilling minimizes the environmental impact of an oil well. It is also more cost-efficient. A deeper and longer installation of pipeline or conduit is also possible. Directional drilling also has greater directional capabilities and requires no access pit or large portal to be excavated at the surface, thereby lessening the need for surface restoration. It also requires a shorter amount of construction time than an open-cut excavation.
- ↑ Horizontal Drill Techniques and Directional Drilling Methods. EnergyindustryPhotos.com. 07-05-2009.
- ↑ Directional Boring. Directional Boring Central. 07-05-2009.
- ↑ Directional Drilling. EnerMax. 07-05-2009.
- ↑ How Does Directional Drilling Work? RigZone. 07-05-2009.
- ↑ Technology Overview for Directional Drilling. Directional Technologies Inc. 07-05-2009.