The depth of a natural gas well is not a fixed number but represents a vast range, reflecting the complex geology of the Earth’s subsurface. Wells are drilled to access pockets of gas trapped in rock formations, and the distance to these formations varies dramatically across different regions. A gas well can be drilled to a relatively shallow depth of a few hundred feet or extend for miles into the crust. The target depth is determined by the specific geological conditions of a basin and the type of rock holding the gas, along with the limits of modern drilling technology and economic viability.
Understanding True Vertical Depth and Measured Depth
When discussing the depth of a gas well, two distinct measurements describe its trajectory beneath the surface. True Vertical Depth (TVD) represents the direct vertical distance from the drilling surface down to the hydrocarbon-bearing formation. This metric is a straight-line measurement, regardless of any curves or deviations the wellbore may take, and is important for geological calculations like estimating formation temperature.
The second measurement is Measured Depth (MD), which is the actual total length of the wellbore drilled along its entire path. For a perfectly straight vertical well, MD and TVD are identical. However, most modern gas wells are intentionally deviated or include long horizontal sections to reach distant targets. In these cases, the Measured Depth is always significantly greater than the True Vertical Depth.
Wells are classified based on their True Vertical Depth. Shallow wells might only penetrate a few thousand feet, while wells reaching formations deeper than 15,000 feet are classified as deep. Ultra-deep wells exceed 25,000 feet TVD, often targeting high-pressure, high-temperature reservoirs that are challenging to access.
Geological and Economic Factors Influencing Well Depth
The primary driver of a well’s depth is the underlying geology, specifically the location of the source and reservoir rock. Natural gas is generated from organic matter buried beneath sediment layers, requiring intense heat and pressure found at significant depth. Major gas deposits are consequently found in deep sedimentary basins. The exact depth depends on when the gas-rich layers were deposited and how much subsequent rock has accumulated.
The type of rock holding the gas also determines drilling depth. Conventional gas reservoirs consist of porous, permeable rock like sandstone, where gas has migrated and collected. Unconventional reservoirs, such as shale and tight gas, trap the gas within the low-permeability source rock itself. To unlock this gas, the well must be drilled directly into the source rock, which often occurs at depths greater than 8,000 feet to ensure the necessary pressure and temperature.
The economic viability of a project provides the ultimate constraint on drilling depth. The cost of drilling increases exponentially as depth increases, due to the need for specialized equipment and complex well control measures to manage high pressures and temperatures. Drillers must balance this escalating cost against the anticipated volume of gas and the market price. Deeper wells are more expensive but can access vast, highly pressurized reservoirs that yield substantially higher production rates.
Depth Profiles of Conventional Versus Unconventional Wells
The difference between conventional and unconventional gas production is reflected in their depth profiles and wellbore trajectories. Conventional wells are typically drilled vertically or with slight deviation to access the discrete, porous rock formation where gas has accumulated. The True Vertical Depth (TVD) of these wells often ranges from a few thousand feet up to 15,000 feet. Since the wellbore is mostly straight, the Measured Depth (MD) is nearly the same as the TVD.
Unconventional wells target low-permeability formations like shale and are characterized by a deep vertical section followed by a long horizontal segment. The vertical section must reach the deep shale layer, often requiring a TVD between 8,000 and 12,000 feet. Once the target formation is reached, the wellbore drills horizontally within the gas-bearing layer for thousands of feet. This horizontal extension causes the Measured Depth to far exceed the True Vertical Depth; for example, a well with a TVD of 10,000 feet might have an MD of 20,000 feet or more.
The deepest achievements are often seen in projects targeting high-pressure conventional gas. These ultra-deep wells penetrate beyond 25,000 feet TVD, seeking highly productive, pressurized reservoirs.