A further cause of kicks from drilling accidentally into abnormally pressured permeable zones. This is because we had ignored the warning signals that occur, these help us detect abnormal pressures. Some of these warning signals are: an increased penetration rate, an increase in background gas or gas cutting of the mud, a decrease in shale density, an increase in cutting size, or an increase in flow-line temperature, etc.
In some areas, there were adequate sands that were continuous and open into the sea or to the surface. In these areas the water squeezed from the shale formations, travelled through the permeable sands and was released to the sea or to a surface outcrop. This de-watering allowed the formations to continue to compact and thereby increase their density.
Figure 2.7
In other areas, or at other times, the sands did not develop or were sealed by deposition of salt or other impervious formations, or by faulting such as we have indicated here. Although the shale water was squeezed, it could not escape. Since water is nearly incompressible, the shales could not compress past the point where the water in the shale started to bear the weight of the rock above. This section caused a condition in which the weight of the formation - that is, the overburden - was borne not by the shale alone, but assisted by the fluids in the shale. In this situation the shale will have more porosity, and a lower density, than they would have had if the now pressured water had been allowed to escape. These
formations, both sand and shale, are then overpressured. If a hole is drilled into an overpressured formation, weighted mud will be required to hold back the fluids contained in the pore space.
Figure 2.8
Abnormally high formation pressure is defined as any formation pressure that is greater than the hydrostatic pressure of the water occupying the formation pore spaces. Abnormally high formation pressures are also termed surpressures, overpressures and sometimes geopressures. More often, they are simply called abnormal pressures. PERMEABLE ZONE NORMAL PRESSURE
SEA
FAULT ABNORMAL PRESSURESEA
Abnormally high formation pressures are found worldwide in formations ranging in age from the Pleistocene age (approximately 1 million years) to the Cambrian age (500 to 600 million years). They may occur at depths as shallow as only a few hundred feet or exceeding 20,000 ft and may be present in shale/sand sequences and/or massive evaporite-carbonate sequences.
The causes of abnormally high formation pressures are related to a combination of geological, physical, geochemical and mechanical processes.
As defined, the magnitude of abnormally high formation pressures must be greater than the normal hydrostatic pressure for the location, and may be as high as the overburden pressure. Abnormally high pressure gradients will thus be between the normal hydrostatic gradient (0.433 to 0.465 psi/ft) and the overburden gradient (generally 1.0 psi/ft).
However, locally confined pore pressure gradients exceeding the overburden gradient by up to 40% are known in areas such as Pakistan, Iran, Papua New Guinea, and the CIS. These super pressures can only exist because the internal strength of the rock overlying the super pressured zone assists the overburden load in containing the pressure. The overlying rock can be considered to be in tension.
In the Himalayan foothills of Pakistan, formation pressure gradients of 1.3 psi/ft have been encountered. In Iran, gradients of 1.0 psi/ft are common and in Papua New Guinea, a gradient of 1.04 psi/ft has been reported. In one area of Russia, local formation pressure in the range of 5870 to 7350 psi at 5250 feet were reported. This equates to a formation pressure gradient of 1.12 to 1.4 psi/ft.
In the North Sea abnormal pressures occur with widely varying magnitudes in many geological formations.
The Tertiary sediments are mainly clays and may be overpressured for much of their thickness. Pressure gradients of 0.52 psi/ft are common with locally
occurring gradients of 0.8 psi/ft being encountered. An expandible clay (gumbo) also occurs which is of volcanic origin and is still undergoing compaction. The consequent decrease in clay density would normally indicate an abnormal
pressure zone but this is not the case. However, in some areas, mud weights of the order of 0.62 psi/ft or higher are required to keep the wellbore open because of the swelling nature of these clays. This is almost equal to the low overburden
gradients in these areas.
In the Mesozoic clays of the North Sea Central Graben, overpressures of 0.9 psi/ft have been recorded. One reported case indicated a formation pressure gradient of 0.91 psi/ft in the Jurassic section. In the Jurassic of the Viking Graben, abnormal formation pressure gradients of up to 0.69 psi/ft have been recorded.
In Triassic sediments, abnormally high formation pressures have been found in gas bearing zones of the Bunter Sandstone in the southern North Sea. Also in the southern North Sea, overpressures are often found in Permian carbonates,
evaporates and sandstones sandwiched between massive Zechsteins salts.
2.3.6 SHALLOW GAS SANDS
Kicks from shallow sands (gas and water) whilst drilling in the top hole section with short casing strings can be very hazardous, as documented by many case histories. Some of the kicks from shallow sands are caused by charged formations: poor cement jobs, casing leaks, injection operations, improper abandonments, and previous underground blowouts can produce charged formations.