Fracture (geology)

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces.

[4] Tensile fracturing may also be induced by applied compressive loads, σn, along an axis such as in a Brazilian disk test.

A load is applied on the top edge, the sides of the envelope open outward, even though nothing was pulling on them.

To fully understand the effects of applied tensile stress around a crack in a brittle material such a rock, fracture mechanics can be used.

A. Griffith during World War I. Griffith looked at the energy required to create new surfaces by breaking material bonds versus the elastic strain energy of the stretched bonds released.

By analyzing a rod under uniform tension Griffith determined an expression for the critical stress at which a favorably orientated crack will grow.

[4] where γ = surface energy associated with broken bonds, E = Young's modulus, and a = half crack length.

This means that for Modes II and III crack growth, LEFM and energy balances represent local stress fractures rather than global criteria.

It is also important to note that once the crack grows, the microcracks in the brittle process zone are left behind leaving a weakened section of rock.

Note that this description of formation and propagation considers temperatures and pressures near the Earth's surface.

This causes them to behave in the semi-brittle and plastic regimes which result in significantly different fracture mechanisms.

In other words, the fault typically attempts to orient itself perpendicular to the plane of least principal stress.

Shear-failure criteria is an expression that attempts to describe the stress at which a shear rupture creates a crack and separation.

σn is the normal stress across the fracture at the instant of failure, σf represents the pore fluid pressure.

The cumulative impact of asperities is a reduction of the real area of contact', which is important when establishing frictional forces.

Since the OH bond is much lower than that with O, it effectively reduces the necessary tensile stress required to extend the fracture.

[7] In geotechnical engineering a fracture forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.)

One aspect of the upstream energy sector is the production from naturally fractured reservoirs.

There are a good number of naturally fractured reservoirs in the United States, and over the past century, they have provided a substantial boost to the nation's net hydrocarbon production.

The key concept is while low porosity, brittle rocks may have very little natural storage or flow capability, the rock is subjected to stresses that generate fractures, and these fractures can actually store a very large volume of hydrocarbons, capable of being recovered at very high rates.

One of the most famous examples of a prolific naturally fractured reservoir was the Austin Chalk formation in South Texas.

However, tectonic stresses over time created one of the most extensive fractured reservoirs in the world.

Many people credit this field for the birth of true horizontal drilling in a developmental context.

Furthermore, the recent uprise in prevalence of unconventional reservoirs is actually, in part, a product of natural fractures.

However, while natural fractures can often be beneficial, they can also act as potential hazards while drilling wells.

If a higher pressured natural fracture system is encountered, the rapid rate at which formation fluid can flow into the wellbore can cause the situation to rapidly escalate into a blowout, either at surface or in a higher subsurface formation.

Conversely, if a lower pressured fracture network is encountered, fluid from the wellbore can flow very rapidly into the fractures, causing a loss of hydrostatic pressure and creating the potential for a blowout from a formation further up the hole.

Since the mid-1980s, 2D and 3D computer modeling of fault and fracture networks has become common practice in Earth Sciences.

[10] The technology consists of defining the statistical variation of various parameters such as size, shape, and orientation and modeling the fracture network in space in a semi-probabilistic way in two or three dimensions.

Computer algorithms and speed of calculation have become sufficiently capable of capturing and simulating the complexities and geological variabilities in three dimensions, manifested in what became known as the "DMX Protocol".