Normal formation pore pressure is equal to the hydrostatic pressure of the most abundant, naturally-occurring water extending from the surface to the subsurface formation of interest. Having stated that, one must consider the geological environment. In some areas the local water table is close to the surface, while in other areas, it is not. Therefore, an averaging of the various water densities is done to arrive at some estimate of normal formation pressure.
And as long as water can escape from a permeable formation at the same rate that deposition is taking place, normal formation pressures are created. Additionally, in order for a formation to be considered normally pressured, it is usually undisturbed with regards to localized tectonics such as faulting, uplift, folding, etc.
An analogy is at left. Here is a block of foam rubber filled with water and we’ll imagine the water is not naturally leaking out. The gauge is measuring the hydrostatic pressure of the water.
A stone is placed on top compressing the foam rubber and causing water to escape. This in turn would cause the gauge to register a lower pressure due to the “shorter” water column.
As stated on the previous page, natural water densities can vary; thus, normal formation pressures of varying magnitudes are created. Below is a chart illustrating this point. And as seen from the chart, normal formation pressures can range from
8.3 ppg to 9.2 ppg.
Subnormal formation pore pressure is any formation pressure that is less than what is considered “normal” for a certain geographic area. Subnormal formation pressures can be naturally-occurring or can be due to production and depletion.
In general, a sub-normally pressured formation will not flow formation liquids to the surface; however, formation gas can be produced. This is due to the respective hydrostatic pressures of produced gas versus produced liquids. Subnormally- pressured oil reservoirs rely on various types of artificial lift to achieve production.
Let’s use the block of foam rubber again to describe this. The water-filled block of foam rubber has now been sealed with melted wax. The stone is once again placed on top. Because of the wax seal no water can “leak out” of the foam rubber.
The gauge registers a slightly high pressure. This is due to the water being forced to take on some of the weight of the stone. Additional weight would further increase the internal pressure of the foam rubber block.
Similarly, when deposition takes place which exceeds the rate at which water can escape, a seal is formed and the formation becomes abnormally pressured
Salt beds can also serve as sealing mechanisms. When salt is deposited and
buried, it forms an impermeable barrier against natural upward migration of formation fluids. But when salt is exposed to extreme pressure and temperature it becomes pseudo-plastic in nature and offers little support to the overlying
Salt domes originate from deeply buried salt beds. Under extreme pressure and temperature,
salt becomes semi-plastic in nature and slowly “intrudes/flows” through over-lying formations in its attempt to get to the surface,
Many domes around the world are
Abnormal Pressure
inactive but there are quite a few in and along the US Gulf
Coast that are active – slowly moving toward the surface. In this case, the daily deposition of the Mississippi River continually increases the overburden thus “driving” the domes upward.
Around the immediate vicinity of the domes, an area which can be quite extensive, formation pressures are affected by the intrusive action of the domes
Often when this happens, formations in the up-thrown fault are sealed retaining much of the original formation pressure. In the illustration the wells on the left and right have normally pressured formations.
The center well has identical geology lifted to a shallower depth. In many cases impermeable seals are formed along the flanks of the uplift trapping existing pressures. Higher-than-normal mud weights are required to balance these pressures.
The rig at left, although penetrating the same formation, finds no abnormal pressure. But the rig at right, at a lower elevation, encounters abnormal pressure due to the hydrostatic difference between the outcropping of the formation and the location and depth it is penetrated. This is more common in mountainous areas. Although an artesian system is usually associated with fresh water horizons, they are known to be the drive source for some shallow fields.