Horizontal drilling is a technique that starts with a vertical well, then gradually curves underground until the drill bit is moving sideways through a target rock layer. This allows a single well to travel thousands of feet through an oil- or gas-bearing formation that might only be 50 or 100 feet thick, exposing far more reservoir rock than a straight vertical hole ever could. The technique has become the backbone of modern oil and gas production in shale formations and is also widely used to install pipelines and utility lines beneath rivers, roads, and wetlands.
How the Drilling Process Works
Every horizontal well begins as a vertical one. The drill bores straight down, sometimes thousands of feet, until it reaches a predetermined depth known as the kickoff point. This is where directional drilling begins and the wellbore starts curving away from vertical. The kickoff point marks the start of what drillers call the “build curve,” a gradual arc that transitions the well from straight down to fully horizontal.
During the build section, the drill bit is steered at a controlled rate of angle change, measured in degrees per hundred feet. A typical well might gain two to six degrees of angle for every hundred feet drilled, though steeper or gentler curves are possible depending on the geology and well design. Once the wellbore reaches roughly 90 degrees from vertical, it enters the lateral section, running horizontally through the target formation. In the Permian Basin, one of the most active drilling regions in the U.S., average lateral lengths grew from less than 4,000 feet in 2010 to over 10,000 feet by 2022.
Steering the Drill Bit
The key piece of equipment that makes horizontal drilling possible is the steerable motor, a device powered by drilling fluid (mud) pumped down through the drill pipe. The motor contains a bent housing near the bottom that points the drill bit slightly off-center. When the entire drill string is held stationary and only the motor turns the bit, the well curves in the direction the bend is aimed. When the operator rotates the whole string from the surface, the bend averages out and the well drills roughly straight ahead. Switching between these two modes gives drillers precise control over direction and curvature.
More advanced systems called rotary steerable tools can change direction continuously while the drill string keeps spinning, which speeds up the process and produces a smoother wellbore. These tools can capture data about the surrounding rock roughly one meter from the bit itself, including natural radiation readings, well angle, compass direction, rotation speed, and temperature.
Real-Time Navigation Underground
Drilling a well two miles underground and then turning it sideways through a thin layer of rock requires constant feedback. Sensors embedded near the drill bit transmit measurements to the surface while drilling is underway, a capability known as measurement while drilling (MWD). The data stream includes the well’s depth, the rate and torque of rotation, downward pressure on the bit, and how fast the drill is advancing.
Once the well enters the target formation, a more specialized technique called geosteering takes over. Geosteering combines real-time sensor data with geological models to keep the bit positioned in the most productive zone, sometimes called the “sweet spot.” Sensors measure natural gamma radiation from the rock above and below the bit. If the reading from above differs from the reading below, it tells the drilling team whether the wellbore is angling upward or downward relative to the rock layers. This matters because formations are rarely perfectly flat. They tilt, fold, and shift across faults, so the drill path needs constant adjustment to stay within the target.
By comparing these radiation patterns with pre-drill seismic predictions, geologists on the surface can identify when the formation is steeper or shallower than expected and adjust the trajectory before the bit wanders out of the productive zone.
Why It Replaced Vertical Drilling in Shale
Shale oil and gas formations are typically thin horizontal layers of rock with very low permeability, meaning the oil and gas trapped inside don’t flow easily. A vertical well passing through such a layer might only contact 100 feet of reservoir rock. A horizontal well drilled through that same layer for 10,000 feet contacts 100 times more formation. After drilling, the lateral section is fractured (a process that creates tiny cracks in the rock to release trapped hydrocarbons), and the enormous contact area makes production economically viable where vertical wells could not.
Horizontal wells also reduce the surface impact of drilling operations. Because each well drains a much larger volume of rock underground, fewer wells are needed to develop a given area. Multiple horizontal wells can be drilled from a single surface pad, fanning out in different directions beneath the ground. This shrinks the total surface footprint of roads, well pads, and equipment compared to dotting the landscape with individual vertical wells.
A Brief History
The concept is older than most people expect. The first horizontal well was drilled in 1941 in Azerbaijan by Soviet driller Alexander Grigoryan. But the technology remained impractical for decades because there was no reliable way to steer a drill bit or know its position underground in real time. The commercial breakthrough came in 1982, when the first economically viable offshore horizontal well was drilled at Rospo Mare in Italy’s Adriatic Sea. Through the 1980s and 1990s, improvements in downhole motors, sensors, and computing power turned horizontal drilling from an expensive novelty into standard practice. Today, the vast majority of new oil and gas wells drilled in U.S. shale plays are horizontal.
Utility Installation and Other Uses
Horizontal directional drilling (HDD) is a closely related technique used outside the oil and gas industry to install pipelines, water mains, sewer lines, fiber optic cables, and electrical conduits beneath obstacles that would be expensive or environmentally damaging to dig through. Utilities have been built by HDD under rivers, highways, railroads, airport runways, and densely built urban areas.
The technique is especially valuable for crossing waterways and wetlands, where environmental agencies often prohibit open-cut trenching. A 42-inch redundant water line was installed under Mill Cove in Massachusetts’s Weymouth Fore River, spanning over 2,000 feet between two congested urban areas. A 30-inch steel sewer pipe was drilled 3,500 feet under the York River in Virginia to connect an area previously served only by septic tanks. An 18-inch natural gas pipeline crossed the Cape Cod Canal, completed in just five days at a length of 1,454 feet, installed 20 feet below the canal’s dredge line.
HDD is also used for environmental monitoring, allowing sampling of contaminated soil and groundwater beneath roads, rivers, and buildings without disturbing the surface. For any project involving a waterway or wetland crossing, HDD is now typically the first construction method considered.