Abnormal pore pressure

Pressure is one of the main concepts for a petroleum engineer because it affects every decision he takes in every stage of well life. Hydrostatic, overburden, pore pressure are the main types of pressure but we’ll concentrate on pore pressure especially abnormal pressure. This type of pressure is the reason why surface control equipment are extremely important.Geological, geochemical, geophysical, mechanical factors are classified as follows to indicate how abnormal case occurs in wells: depositional effects, diagenetic processes, tectonic effects, structural causes, thermodynamic effects.



Definitions:
Hydrostatic pressure:
It is the pressure exerted by column of fluids in the well pore.
Pore pressure:
It is the pressure acting on fluids in pore spaces of rocks.

It has three types:
  1. Normal pore pressure: When the pore pressure is equal to hydrostatic pressure. Pf = Ph

  2. Subnormal pore pressure: When the pore pressure is less than hydrostatic pressure. Pf < Ph

  3. Abnormal pore pressure: When the pore pressure is more than hydrostatic pressure. Pf > Ph



Causes of abnormal pore pressure:

After 3D seismic data is acquired, it needs to be processed to be ready for interpretation as the image is blurred. Data processing includes filtering, stacking and migrating and other forms of computer analysis. As computers have become more powerful and processing techniques more developed, it has become common to re-process seismic data acquired in earlier years, creating new opportunities for exploration that could not originally be derived from the 3D data. Processing of one 3D seismic survey may take months and cost a lot of money.



Depositional effects:
Depositional effects

Undercompaction:

First, one must know how normal compaction occurs; as sediments are deposited, other sediments form layers above them causing their volume to decrease then pressure is further increasing and as a result sediments reorganize their grains so porosity decreases depending on amount of fluids n pore spaces. Normal compaction leaves the pore fluids in normal pore pressure.

So, how does undercompaction occurs?
When sediments are deposited but in a fast rate, fluids will not have a chance to leave pore spaces.so fluids are trapped and cannot respond to the increase in pressure by flowing out causing the pressure to increase (abnormal pressure). To make it further clear, the increase in the rate of sedimentation or reduction of fluid expulsion by decreased permeability or blocking passages for fluids to flow by evaporites as example; totally impermeable for fluids or blocked by salt; no porosity, no permeability therefore can be a perfect seal, are the reasons why abnormal pressure may exist because fluids begin to support the increasing weight of overlying layers of sediments.Clay or shale undercompaction is common in Gulf of Mexico, Papua New Guinea and North Sea. Southern North Sea gas reservoirs owe their existence to Zechstein salt.



Diagenetic processes:

After deposition, both pressure and temperature may change in a way that will cause recrystallization or formation of new minerals or water generation.so, the new minerals will have different properties especially sealing ones and water will not have a way to escape.
Claydiagenesis(conversion of Smectite to Illite) as example:
Marine clays consists mainly of Smectite clays of which montomorillonite is by far the most common. Montomorillonite consists mainly of water in pore spaces or interlayer water between clay platelets. As compaction occurs due to burial, water in pore spaces is expelled and interlayer water takes its place causing the structure of clays to change.high temperature, high pressure, existance of potassium help conversion of Smectite to Illite to occur.In the North Sea, this process occur at 90-100 deg C and 6500-9750 ft.



Tectonic effects:

Folding, faulting, uplift can cause abnormal pressures. But how? Let’s see.
Folding:
Simply, horizontal stresses form folds but if these stresses increase, the fluids will be expelled. By the same way described above, if a seal prevents fluids from escaping, fluids will be over pressurized and abnormal conditions will show. Southern Iran (AgharJari fields),Tapungato field in Argentina are examples.
Faulting:
As layers move on the fault plane, impermeable layer may face another one which is permeable. If compaction affects the permeable layer, no fluids will be able to escape.
Uplift:
If a normally pressured formation is uplifted to a shallower depth then the formation will appear to have an abnormal pressure due to the fact that the formation pressure has more hydrostatic pressure than a corresponding normally pressured zone at the same depth.



Structural causes:

Different densities of water, oil and gas in reservoir structure may cause abnormal pressure especially in lenticular, dipping, anticlinal reservoirs. In dipping reservoirs, formation pressure behind water zone will be transmitted to updip part of the reservoir.



Thermodynamic processes:

They are helping factors to above mentioned factors.as example, thermal cracking of oil into gas may result in abnormal pressures.at high temperatures, complex hydrocarbons like kerogen will break down into simpler components.Because of thermal cracking, volume of hydrocarbons increases especially in a sealed environment. Fields of such type exist in central North Sea.



Abnormal pore pressure evaluation:

Abnormal pore pressure evaluation

Being important and one do not want to see a picture like this one in reality, abnormal pressures are carefully evaluated.
Basic techniques used to detect pressure are:

  1. Mud logging method.

  2. Measurement while drilling.

  3. Wireline logging methods.

  4. DST, Production tests and RFT.

We have discussed abnormal pore pressure; causes and methods of evaluation.it was a serious problem for the oil industry in its early days but up-to-date techniques help prevent large problems because of abnormal pressures. Safety is important in oil industry so engineers give due care to abnormal pressures.

References:

  • Well engineering and construction by HussianRabia.