Overpressure
Encyclopedia
The term Overpressure is applied to a pressure
difference, relative to a "normal" or "ambient" pressure, in various circumstances:
An article on dvice.com (including an interview with a ballistics engineer) describes how an EOD suit worn by bomb-disposal experts protects against the deadly effects of BOP.
where:
to escape as the surrounding mineral matrix compacts under the lithostatic pressure caused by overlying layers. Fluid escape may be impeded by sealing of the compacting rock by surrounding impermeable layers (such as evaporite
s, chalk
and cemented sandstone
s). Alternatively, the rate of burial of the stratigraphic layer may be so great that the efflux of fluid is not sufficiently rapid to maintain hydrostatic
pressure.
Common situations where overpressure may occur: in a buried river channel filled with coarse sand that is sealed on all sides by impermeable shales, or when there is an explosion within a confined space.
at the well-head with possibly disastrous consequences.
Because overpressured sediments tend to exhibit better porosity
than would be predicted from their depth, they often make attractive hydrocarbon
reservoirs and are therefore of important economic interest.
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
difference, relative to a "normal" or "ambient" pressure, in various circumstances:
- In engineeringEngineeringEngineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
: the pressure difference over the wall thickness of a pressure vesselPressure vesselA pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...
. It is the overpressure, not the absolute pressure, which causes mechanical yieldYield (engineering)The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed...
and, if the overpressure exceeds the vessel's capacity, ultimately the failure of the vessel. - In geologyGeologyGeology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
: the pressure regime in a stratigraphic unitStratigraphyStratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
that exhibits higher-than-hydrostatic pressure in its porePorosityPorosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
structure. This phenomenon is the primary cause of "oil gushers". This is described below. - In militaryMilitaryA military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
terminology, the pressure caused by a shock waveShock waveA shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
over and above normal atmospheric pressureAtmospheric pressureAtmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...
. The shock wave may be caused by sonic boomSonic boomA sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion...
or by explosionExplosionAn explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
, and the resulting overpressure receives particular attention when measuring the effects of nuclear weaponNuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s or thermobaric bombs.
Blast overpressure
According to an article in the journal Toxicological Sciences,"Blast overpressure (BOP), also known as high energy impulse noise, is a damaging outcome of explosive detonations and firing of weapons. Exposure to BOP shock waves alone results in injury predominantly to the hollow organ systems such as auditory, respiratory, and gastrointestinal systems."
An article on dvice.com (including an interview with a ballistics engineer) describes how an EOD suit worn by bomb-disposal experts protects against the deadly effects of BOP.
Overpressure calculation (explosive)
Overpressure is determined using "Wiebull's formula":where:
- 2410 is a constant based on 1 bar (100 kPa; 14.5 psi)
= net explosive mass calculated using all explosive materials and their relative effectiveness
= volume of given area (primarily used to determine volume within an enclosed space)
| Pressure | Effects |
|---|---|
| 10 pound per square inches (68,947.6 Pa) |
|
| 4 pound per square inches (27,579 Pa) |
|
| 2 pound per square inches (13,789.5 Pa) |
|
Geologic overpressure
Geological overpressure in stratigraphic layers is caused by the inability of connate pore fluidsConnate fluids
The term connate fluids in the context of geology, and of sedimentology in particular, refers to the liquids that were trapped in the pores of sedimentary rocks as they were deposited. These liquids are largely composed of water, but also contain many mineral components as ions in solution.As...
to escape as the surrounding mineral matrix compacts under the lithostatic pressure caused by overlying layers. Fluid escape may be impeded by sealing of the compacting rock by surrounding impermeable layers (such as evaporite
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
s, chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
and cemented sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s). Alternatively, the rate of burial of the stratigraphic layer may be so great that the efflux of fluid is not sufficiently rapid to maintain hydrostatic
Fluid statics
Fluid statics is the science of fluids at rest, and is a sub-field within fluid mechanics. The term usually refers to the mathematical treatment of the subject. It embraces the study of the conditions under which fluids are at rest in stable equilibrium...
pressure.
Common situations where overpressure may occur: in a buried river channel filled with coarse sand that is sealed on all sides by impermeable shales, or when there is an explosion within a confined space.
Implications
It is extremely important to be able to diagnose overpressured units when drilling through them, as the drilling mud weight (density) must be adjusted to compensate. If it is not, there is a risk that the pressure difference down-well will cause a dramatic decompression of the overpressured layer and result in a blowoutBlowout (well drilling)
A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....
at the well-head with possibly disastrous consequences.
Because overpressured sediments tend to exhibit better porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
than would be predicted from their depth, they often make attractive hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
reservoirs and are therefore of important economic interest.