Hydraulic fracturing, often called fracking, is a technique used to extract oil and natural gas trapped deep within low-permeability rock formations, such as shale. This process involves pumping a high-pressure mixture of water, sand, and chemical additives down a wellbore to create small cracks in the rock. These fractures release the hydrocarbons, allowing them to flow into the well and be brought to the surface. This method unlocks previously inaccessible energy reserves from the Earth’s subsurface.
Defining the Dimensions of a Fracking Well
A modern fracking well involves two primary measurements. The True Vertical Depth (TVD) measures the straight-down distance from the surface to the point where the well begins to turn horizontally, representing the actual depth of the reservoir rock layer.
The second dimension is the Lateral Length, the horizontal section of the wellbore that extends outward into the target formation. The total length of the well, measured along its entire trajectory, is known as the Measured Depth (MD). The MD is always significantly greater than the TVD because it includes the length of this horizontal arm.
For example, a well might have a TVD of 8,000 feet but an MD of 16,000 feet or more, accounting for a lateral section that runs for over a mile. This horizontal drilling allows the well to intersect a much larger volume of the resource-bearing rock layer, and the fracturing process is confined to this section.
Typical Depth Ranges for Target Formations
Hydraulic fracturing consistently takes place far below the surface, usually extending multiple miles beneath the ground. A comprehensive analysis of fracturing depths across the United States found the mean depth to be approximately 8,300 feet, or about 1.6 miles underground. A majority of wells are fractured at depths greater than one mile.
The range for the deepest wells can extend beyond two miles, with the 90th percentile of operations occurring at depths reaching 11,900 feet or more. For instance, the Marcellus Shale is typically targeted at True Vertical Depths between 5,000 and 9,000 feet. Other formations may be deeper, with some wells penetrating rock layers at depths exceeding 12,000 feet. These figures refer to the specific geological zone where the fracturing fluid is injected.
Factors Influencing Well Depth Variation
The depth of a fracking well is not uniform across all regions and is determined by specific geological conditions. The location of the target shale or tight rock formation dictates the necessary True Vertical Depth, as these resource-bearing layers exist at different levels in the Earth’s crust. Some formations are naturally shallower basins, while others are situated in much deeper subterranean structures.
Pressure and temperature gradients within the rock also influence the required depth and well engineering. Deeper formations often experience higher internal pressures, known as overpressure, which impacts the force required for fracturing and the overall well design. The composition of the rock layer, including its thermal maturity and structural characteristics, determines how effectively the formation can be fractured.
The economic viability of the project also plays a role, as drilling deeper wells requires more specialized equipment and incurs higher costs.
Protecting Groundwater: The Role of Depth and Casing
The deepest sources of fresh groundwater, known as freshwater aquifers, are typically located within the first few hundred to a thousand feet below the surface. This creates a vast vertical separation between drinking water sources and the zone where hydraulic fracturing occurs. Residential water wells generally draw water from depths averaging only about 200 feet.
This depth difference provides a natural barrier, as target formations are thousands of feet below the deepest protected groundwater. To ensure complete isolation, every well is constructed with multiple layers of steel casing and cement barriers. These protective layers are set and cemented in place through the shallow zones containing fresh water, effectively sealing the wellbore from the surrounding rock.
The cemented casing prevents any movement of fluids from the deep target zone or the wellbore into the overlying groundwater. While hydraulic fractures can propagate upward, studies show that in typical deep wells, the maximum possible upward movement of a fracture still leaves a considerable buffer of thousands of feet of solid rock between the fractured zone and the deepest freshwater aquifer.