overpressure

The term Overpressure (Δp) is applied to a pressure difference, relative to a "normal" or "ambient" pressure, in two different circumstances:

• In geology: the pressure regime in a stratigraphic unit that exhibits higher-than-hydrostatic pressure in its pore structure. This phenomenon is the primary cause of "oil gushers". This is described below.
• In military terminology, the pressure caused by a shock wave over and above normal atmospheric pressure. The shock wave may be caused by sonic boom or by explosion, and the resulting overpressure receives particular attention when measuring the effects of nuclear weapons or thermobaric bombs.

Geologic overpressure

Geological overpressure in stratigraphic layers is fundamentally caused by the inability of connate pore fluids 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 evaporites, chalk and cemented sandstones). 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.

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 blowout 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.

Overpressure calculation (explosive)

Overpressure is determined using "Wiebull's formula" where: ∆р = 2410(w/v)^0.72

• 2410 is a constant based on 1 bar or 100 kPa = 14.51 psi
• Note: 0.72 is an exponent
• W = explosive weight
• v = volume of given area (primarily used to determine volume within an enclosed space)

Effects of overpressure

See also

• Lithification
• petroleum geology

References