What Does Hydraulic Fracture Mean?
Hydraulic fracture or hydraulic fracturing, also known as fracking, is the process of creating fractures in sub-surface rocks using high-pressure fluid. The rate at which fluid is injected into the formation is high enough to fracture it. Usually, water, sand, and chemicals under high pressure are injected into the bedrock formation through the well.
This method can be used to increase oil and gas flow to the well from formations and is commonly used in low-permeability rocks such as tight sandstone, shale, and some coal beds. It is also used to determine the in-situ stress of rocks and for applications such as tunneling, geothermal energy, groundwater remediation, and water well development.
Trenchlesspedia Explains Hydraulic Fracture
Fracking is carried out by injecting high-pressure fluid containing about 95% water, 0.5% additives and 4.5% proppant. When the fracture fluid is injected, the generated fractures expand as the proppant fills them and keeps them open.
Hydraulic fractures propagate perpendicular to the least principal stress, which in some formations is the overburden stress, resulting in a hydraulic fracture in the horizontal direction. In very deep reservoirs, the least principle stress will mostly be horizontal, creating a vertical hydraulic fracture. Fractures always form perpendicular to the minimum in-situ stress and in almost all cases, the vertical stress equals the weight of the overburden per unit area.
In some cases, higher sub-surface vertical stresses are created by upward forces greater than the overburden weight. At shallow depth, the minimum stress is the vertical stress, causing fractures in the horizontal direction. Proppants such as sand or gritty material that suspend in water-based or other types of drilling fluid are added to keep the fractures open.