Natural gas trapped in deep shale formations, commonly referred to as shale gas, is methane locked within fine-grained, low-permeability sedimentary rock. Unlike conventional deposits, this gas cannot flow easily through the dense rock structure. Unlocking this resource requires combining two advanced engineering methods: directional drilling and high-volume hydraulic fracturing. These technologies allow operators to access and stimulate a large area of the deep, gas-bearing rock before the trapped gas can be released and brought to the surface.
Accessing the Shale Layer Through Directional Drilling
The extraction process begins with the creation of the wellbore, the deep hole that reaches the target shale layer, often between 1,500 and 4,000 meters deep. Traditional vertical drilling is inefficient for thin, horizontally extensive shale layers because it contacts only a small portion of the gas-containing rock. Directional drilling overcomes this limitation by allowing the wellbore to turn.
Drilling first proceeds vertically until the bit is a few hundred feet above the target formation. Downhole motors and measurement-while-drilling (MWD) tools steer the drill bit, gradually curving the wellbore from vertical to horizontal. This horizontal section, often called the lateral, can extend for a mile or more within the shale formation, maximizing exposure to the reservoir.
As the wellbore is created, steel pipe sections, known as casing, are run into the hole to line the walls and prevent collapse. Cement slurry is then pumped into the annulus, the space between the casing and the rock formation, to secure the pipe and create a permanent barrier. This isolates different subsurface zones, including fresh water aquifers far above the shale layer, and ensures the well maintains structural integrity under the high pressures used in later stages.
The Hydraulic Fracturing Process
Once the wellbore is secured, hydraulic fracturing, or “fracking,” is initiated to stimulate gas flow from the tight shale rock. A perforating gun is lowered down the horizontal section of the well to create small holes through the casing and cement into the surrounding shale rock. These perforations establish fluid communication between the wellbore and the gas-bearing formation.
A high-pressure fluid mixture is then pumped through these perforations into the rock at pressures that can exceed 10,000 pounds per square inch (PSI). This immense pressure is calculated to surpass the stress of the rock formation, causing the shale to crack and form tiny fissures. The fracturing fluid is primarily composed of water (over 90% of the total volume), along with proppants and a small percentage of chemical additives.
Proppants, typically fine-grained materials like silica sand or ceramic beads, are suspended within the fluid and carried into the newly created fractures. The function of the proppant is to “prop” the fissures open once the injection pressure is released. Without these solid particles, the natural closure stress of the deep rock would cause the fractures to close, trapping the gas again.
The chemical additives, which usually constitute less than 0.5% of the fluid volume, are included to modify the fluid’s properties. For instance, friction reducers minimize pumping resistance, biocides prevent microbial growth, and scale inhibitors prevent mineral precipitation.
Separating and Transporting the Gas
Following the completion of the fracturing process, the pressure is relieved, allowing the liberated natural gas to flow up the wellbore. This flow is accompanied by a portion of the injected fracturing fluid, which returns to the surface as “flowback” water. The volume of flowback water can vary significantly, ranging from 15% to sometimes 100% of the total fluid injected over the life of the well.
This flowback water, collected in steel tanks or lined pits, contains high concentrations of dissolved solids, salts, naturally occurring heavy metals, and residual chemical additives. Operators must manage this water through treatment for reuse in future fracturing operations or through disposal in authorized deep injection wells.
Meanwhile, the raw natural gas, which is mostly methane, flows to the surface. It is often mixed with other components, including water vapor, heavier hydrocarbons, and non-hydrocarbon gases. The raw gas must be routed to midstream processing facilities for separation and purification.
At the processing plant, separation equipment removes liquids, sediment, and impurities to meet pipeline quality specifications. This involves removing water vapor and separating out valuable condensable hydrocarbons, known as Natural Gas Liquids (NGLs). Once purified, the pipeline-quality natural gas is ready for transport. It is pushed into local gathering pipelines, which connect to the extensive network of major transmission pipelines that carry the gas to distribution centers and end-users.