Petroleum resources assignment GLY 826 REAL

UNIVERSITY OF PORT HARCOURT, CHOBA, PORT HARCOURT , RIVERS STATE

AN ASSIGNMENT
ON ORIGIN AND PREDICTION OF ABNORMAL FORMATION PRESSURES

WRITTEN BY

NAME: WILLIAM JAJA SAMPSON
REG NO: G2015/MSC/GEOL/FT/867
SUBJECT: PETROLEUM RESOURCES
COURSE CODE (GLY 826)

SUBMITTED TO THE LECTURER:
DR. R.U.Ideozu(Phd)

IN PARTIAL FULFILMENT FOR THE REQUIREMENTS FOR THE AWARD OF MASTER OF SCIENCE DEGREE(MSC) IN THE DEPARTMENT OF GEOLOGY. UNIVERSITY OF PORT HARCOURT(2015/2016 SESSION)

The various origins of abnormally-high formation pressures in various zones of the subsurface result from under-compaction, tectonics and other internal factors etc. These can be predicted using various techniques in clastic, carbonate and salt-bearing formations (Serebikayov, 2002).
Pressure is defines as the force per unit area operating within the mass of a body. There are tiny spaces or voids that exist in these formations known as the pore spaces. These spaces can be filled with either water or air or both existing together in proportions. Air or water in pore spaces are called pore fluid. Also, fractures and cracks can also be termed as structural pores in rocks and can contain the same fluids as mentioned above (Serebikayov, 2002).
Pore fluids are not in a state of equilibrium in soils or rocks, as they tend to exert pressure within the pores which confine them. These pressures build up and increase when there the soil is carrying load or the introduction of stress system to a point where there is maximum pressure built inside the poor spaces of the formation.
The influx of water from a nearby aquifer of water body can increase the rate of influx of water into the surrounding formations. As a result, the pore spaces of the formations will have more water inside them. This will increase their pressures.
According to Mohammed (2000), the various origins of abnormal-
ly-high formation pressure zones, result from the following:
1 Tectonic movements, 2. Aquifers in mountainous regions, 3. Charged Shallow reservoirs, 4. Natural build up of water in structural voids, 5. Differential compaction, 6. Hydrostatic Pressure gradient.

These can explained below under the various subheadings:
1. TECTONIC MOVEMENTS: Movement of plates of the lithospheric and oceanic crust result in uplift, deformation and faulting of rock formations. As a result of tectonic activities, pore fluids are pressured as crustal plates are crushed when they collide. Plate tectonics can create cracks which can also contain fluid, and older voids can be squeezed or eliminated during tectonic activity as the fluid contained in them will be abnormally pressured. The prevalence of stresses in tectonically active zone can directly or indirectly lead to abnormal formation pressure.
2. AQUIFERS IN MOUNTAINOUS REGIONS: Aquifers are rocks which confine subsurface water. Activities of underground water can be well-explained, located and analysed if the aquifer is known. Some aquifers are potential reserves of water since they primarily contain water. Aquifers containing a huge water reserve can exhibit overpressures. In most cases, the pores in the nearby country rock formations will be over-pressured due to recharge from the nearby aquifers from where they are receiving water. Abnormally high formation pressures are built-up within confined microscopic pores having a low degree of freedom. The pressures increase when the aquifer is in constant supply of water due to recharge.
3.CHARGED SHALLOW RESERVOIRS: Just like the aquifers, the reservoir formations also experience abnormally high pressure. A reservoir contains fluids such as oil, gas and water. These can be recharged and the formations will eventually be over pressured.
4. NATURAL BUILD-UP OF FLUIDS IN STRUCTURAL VOIDS: Structural voids include spaces found in cracks, joints and crevices which have the potential to store underground water. Structural voids increase the porosity of rock with formations very tiny voids (Pores). Fluids in the formation can build-up within microscopic pores having a low degree of freedom. The internal build-up of fluid within pore spaces can cause abnormally high-pressure. Formation (interstitial pore fluid) pressure is a pressure that deviates from the hydrostatic pressure Ph at very great depths D and termed as abnormal formation pressure. Some examples are given by Watts (1948) in the Ventura Field as a result of thrusting.
The causes and origin of abnormal pressure within formations depend on the lithology, mineralogy, tectonic action and rate of sedimentation.
5.DIFFERENTIAL COMPACTION: Interstitial (inter granular or formation pressures above or below the hydrostatic pressure occur around the world under a wide range of geological conditions. Fluids exert pressure between the matrix of the soil/rock particles. Compaction takes place when more sediments are deposited on top of the existing sediments, which causes the soil to be under load. The load becomes the pressure of the overburden. Hydrostatic pressure increases with more overburden load, as this super pressures increase the matrix-pore fluid pressure (Chilingar et al, 2002).





Figure 1:Illustrating normal and abnormal pressures of rocks under compaction.
Pressures above the hydrostatic pressures are often regarded to abnormally-high formation pressure (AHFP) or super pressure whereas pressures below the hydrostatic pressure may be referred to as an abnormally low formation pressure (ALFP) or sub pressure.
Abnormal pore pressure gradients are generated under compacted regions because the shale matrix cannot support the overburden stress (Chilingar and Robertson, 2002).
5.HYDROSTATIC PRESSURE GRADIENT IN THE OIL-GAS WATER INTERPHASE IN THE RESERVOIR: Within the reservoir, pressure exist as a result of oil-gas-water interphase. Abnormal formation pressure occurs at the upper end of the reservoir.Compaction may increase or reduce the pore fluid pressure.








Fig 2: Abnormal pressures resulting from confinement within a reservoir
6. FAULTING: In faulted regions, similar formations have been displaced relative to each other, thus large blocks of the same the same crust are up thrown through upward displacement to shallow depths.
On the other hand, the down thrown blocks of faulted rocks have been displaced through downward displacement to greater depths relative to the up-thrown blocks. Since density and pressure increases with depth, the formations in the down-thrown block will be under intense stress than the formations of the up-thrown blocks.
In summary, the origin and causes of abnormal formation pressures relating to geologic processes are explained using some specific geologic examples are:
1 -High pressures in formations are associated with salt domes along the Gulf coast of Texas and Louisianna and often attributed to faulting and diastrophism (Lewis et al, 1970).
2- Differential compaction of clay formation during diagenesis is another geologic process which causes abnormal formation pressures.
3- Under-compaction of clay formation during rapid sedimentation and burial of sediments containing large quantities of clay minerals. Such sediments exhibiting incomplete expulsion of water which result in abnormal pressures (Ruby and Hubbet, 1959).
4- Formation (interstitial pore fluid) are pressures Ph which deviate from hydrostatic pressure at any depth D and is termed as abnormal formation pressure. Example is given by Watts (1948) in the Ventura field, Carlifornia as a result of thrusting.
5- Gas migration is one mechanism responsible for the formation of abnormal pressures during the migration of hydrocarbons from upper to lower horizons along faults (Lariech and Tumireve, 2003).








REFERENCES

Larech V. I and Tumireve A. J (1987): Pre- Jurassic Complex of Mangyshlak in A.E Gureuzh et al(Eds). Foundation fluid pressure: Nedra; Leningrad: pp 64.
Lewis C.R, Rose S.C (1970): A Theory Relating To High- temperatures and Over-Pressures. Journal of Petroleum Technology. Vol(22) 11-12.
Muhammed E.S (2000): Detection and Prediction of Abnormal Formation Pressures. Journal Of Geology. Pp 5
Serebikayov V.A, Robertson J.O, Chilingarian G.V( 2000): The Origin And Prediction Of Abnormal Formation Pressures. Gulf Pub. Co Millenial Edition Page 20-23
Watts E.V(1948): Some Aspects of High Pressures in the Ventura Avenue .Institute of Mineralogy. England.P174




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