These are formations whose pore pressure is greater than that corresponding to the normal gradient of 0.465 psi/ft. As shown in Figure 11 these pressures can be plotted between the hydrostatic gradient and the overburden gradient (1 psi/ft). The following examples of overpressures have been reported:

overpressure

overpressure european wells

From the above list it can be seen that overpressures occur worldwide. Some results from European fields are given in Figure 11. There are numerous mechanisms which cause such pressures to develop. Some, such as potentiometric surface and formation foreshortening have already been mentioned under subnormal pressures since both effects can occur as a result of these mechanisms. The other major mechanisms are summarised below:

(a) Incomplete Sediment Compaction
Incomplete sediment compaction or undercompaction is the most common mechanism causing overpressures. In the rapid burial of low permeability clays or shales there is little time for fluids to escape. Under normal conditions the initial high porosity (+/- 50%) is decreased as the water is expelled through permeable sand structures or by slow percolation through the clay/shale itself. If however the burial is rapid and the sand is enclosed by impermeable barriers (Figure 12) , there is no time for this process to take place, and the trapped fluid will help to support the overburden.

barriers

(b) Faulting
Faults may redistribute sediments, and place permeable zones opposite impermeable zones, thus creating barriers to fluid movement. This may prevent water being expelled from a shale, which will cause high porosity and pressure within that shale under compaction.

(c) Phase Changes during Compaction
Minerals may change phase under increasing pressure, e.g. gypsum converts to anhydrite plus free water. It has been estimated that a phase change in gypsum will result in the release of water. The volume of water released is approximately 40% of the volume of the gypsum. If the water cannot escape then overpressures will be generated. Conversely, when anhydrite is hydrated at depth it will yield gypsum and result in a 40% increase in rock volume. The transformation of montmorillonite to illite also releases large amounts of water.

(d) Massive Rock Salt Deposition
Deposition of salt can occur over wide areas. Since salt is impermeable to fluids the underlying formations become overpressured. Abnormal pressures are frequently found in zones directly below a salt layer.

(e) Salt Diaperism
This is the upwards movement of a low density salt dome due to buoyancy which disturbs the normal layering of sediments and produces pressure anomalies. The salt may also act as an impermeable seal to lateral dewatering of clays.

(f) Tectonic Compression
The lateral compression of sediments may result either in uplifting weathered sediments or fracturing/faulting of stronger sediments. Thus formations normally compacted at depth can be raised to a higher level. If the original pressure is maintained the uplifted formation is now overpressured.

(g) Repressuring from Deeper Levels
This is caused by the migration of fluid from a high to a low presssure zone at shallower depth. This may be due to faulting or from a poor casing/cement job.The unexpectedly high pressure could cause a kick, since no lithology change would be apparent. High pressures can occur in shallow sands if they are charged by gas from lower formations.

(h) Generation of Hydrocarbons
Shales which are deposited with a large content of organic material will produce gas as the organic material degrades under compaction. If it is not allowed to escape the gas will cause overpressures to develop. The organic by-products will also form salts which will be precipitated in the pore space, thus helping to reduce porosity and create a seal.

Institute of Petroleum Engineering, Heriot-Watt University

The major mechanisms by which subnormal (less than hydrostatic) pressures occur may be summarised as follows:

(a) Thermal Expansion
As sediments and pore fluids are buried the temperature rises. If the fluid is allowed to expand the density will decrease, and the pressure will reduce.

(b) Formation Foreshortening
During a compression process there is some bending of strata (Figure 8). The upper beds can bend upwards, while the lower beds can bend downwards. The intermediate beds must expand to fill the void and so create a subnormally pressured zone. This is thought to apply to some subnormal zones in Indonesia and the US.Notice that this may also cause overpressures in the top and bottom beds.

pressure problems

(c) Depletion
When hydrocarbons or water are produced from a competent formation in which no subsidence occurs a subnormally pressured zone may result. This will be important when drilling development wells through a reservoir which has already been producing for some time. Some pressure gradients in Texas aquifers have been as low as 0.36 psi/ft.

(d) Precipitation
In arid areas (e.g. Middle East) the water table may be located hundreds of feet below surface, thereby reducing the hydrostatic pressures.

(e) Potentiometric Surface
This mechanism refers to the structural relief of a formation and can result in both subnormal and overpressured zones. The potentiometric surface is defined by the height to which confined water will rise in wells drilled into the same aquifer. The potentiometric surface can therefore be thousands of feet above or below ground level (Figure 9).

potensiometric

(f) Epeirogenic Movements
A change in elevation can cause abnormal pressures in formations open to the surface laterally, but otherwise sealed. If the outcrop is raised this will cause overpressures, if lowered it will cause subnormal pressures (Figure 10).

sedimentary basin

Pressure changes are seldom caused by changes in elevation alone since associated erosion and deposition are also significant. Loss or gain of water saturated sediments is also important.

The level of underpressuring is usually so slight it is not of any practical concern. By far the largest number of abnormal pressures reported have been overpressures, and not subnormal pressures.

Institute of Petroleum Engineering, Heriot-Watt University

Pore pressures which are found to lie above or below the “normal” pore pressure gradient line are called abnormal pore pressures (Figure 5 and 6). These formation pressures may be either Subnormal (i.e. less than 0.465 psi/ft) or Over pressured (i.e. greater than 0.465 psi/ft). The mechanisms which generate these abnormal pore pressures can be quite complex and vary from region to region. However, the most common mechanism for generating over pressures is called Under compaction and can be best described by the under compaction model.

subnormal pressure

The compaction process can be described by a simplified model (Figure7) consisting of a vessel containing a fluid (representing the pore fluid) and a spring (representing the rock matrix). The overburden stress can be simulated by a piston being forced down on the vessel. The overburden (S) is supported by the stress in the spring (σ) and the fluid pressure (p). Thus:

S = σ + p

If the overburden is increased (e.g. due to more sediments being laid down) the extra load must be borne by the matrix and the pore fluid. If the fluid is prevented from leaving the pore space (drainage path closed) the fluid pressure must increase above the hydrostatic value. Such a formation can be described as overpressured (i.e. part of the overburden stress is being supported by the fluid in the pore space and not the matrix). Since the water is effectively incompressible the overburden is almost totally supported by the pore fluid and the grain to grain contact stress is not increased. In a formation where the fluids are free to move (drainage path open), the increased load must be taken by the matrix, while the fluid pressure remains constant. Under such circumstances the pore pressure can be described as Normal, and is proportional to depth and fluid density.

over pressure

Institute of Petroleum Engineering, Heriot-Watt University

During a period of erosion and sedimentation, grains of sediment are continuously building up on top of each other, generally in a water filled environment. As the thickness of the layer of sediment increases, the grains of the sediment are packed closer together, and some of the water is expelled from the pore spaces. However, if the pore throats through the sediment are interconnecting all the way to surface the pressure of the fluid at any depth in the sediment will be same as that which would be found in a simple colom of fluid. The pressure in the fluid in the pores of the sediment will only be dependent on the density of the fluid in the pore space and the depth of the pressure measurement (equal to the height of the colom of liquid). it will be independent of the pore size or pore throat geometry. The pressure of the fluid in the pore space (the pore pressure) can be measured and plotted against depth as shown in Figure 1. This type of diagram is known as a P-Z diagram.

pore pressure

The pressure in the formations to be drilled is often expressed in terms of a pressure gradient. This gradient is derived from a line passing through a particular formation pore pressure and a datum point at surface and is known as the pore pressure gradient. The reasons for this will become apparent subsequently. The datum which is generally used during drilling operations is the drillfloor elevation but a more general datum level, used almost universally, is Mean Sea Level, MSL. When the pore throats through the sediment are interconnecting, the pressure of the fluid at any depth in the sediment will be same as that which would be found in a simple colom of fluid and therefore the pore pressure gradient is a straight line as shown in Figure 1. The gradient of the line is a representation of the density of the fluid. Hence the density of the fluid in the pore space is often expressed in units of psi/ft.

This is a very convenient unit of representation since the pore pressure for any given formation can easily be deduced from the pore pressure gradient if the vertical depth of the formation is known. Representing the pore pressures in the formations in terms of pore pressure gradients is also convenient when computing the density of the drilling fluid that will be required to drill through the formations in question. If the density of the drilling fluid in the wellbore is also expressed in units of psi/ft then the pressure at all points in the wellbore can be compared with the pore pressures to ensure that the pressure in the wellbore exceeds the pore pressure. The differential between the mud pressure and the pore pressure at any given depth is known as the overbalance pressure at that depth (Figure 2). If the mud pressure is less than the pore pressure then the differential is known as the underbalance pressure. It will be seen below that the fracture pressure gradient of the formations is also expressed in units of psi/ft.

mud density

Most of the fluids found in the pore space of sedimentary formations contain a proportion of salt and are known as brines. The dissolved salt content may vary from 0 to over 200,000 ppm. Correspondingly, the pore pressure gradient ranges from 0.433 psi/ft (pure water) to about 0.50 psi/ft. In most geographical areas the pore pressure gradient is approximately 0.465 psi/ft (assumes 80,000 ppm salt content) and this pressure gradient has been defined as the normal pressure gradient.Any formation pressure above or below the points defined by this gradient are called abnormal pressures (Figure 3). The mechanisms by which these abnormal pressures can be generated will be discussed below. When the pore fluids are normally pressured the formation pore pressure is also said to be hydrostatic.

formation pressure

 

Institute of Petroleum Engineering, Heriot-Watt University