Assignment Cation Exchange Capacity and Irreducible Water Saturation
1. You are a petrophysicist for your company. Summarize for your colleagues the concept of cation exchange capacity (CEC) in clays.
Solution
The cation exchange capacity (CEC) is the amount of a cation (positively charged ion) that can be exchanged by another cation on the surface of a clay mineral. This cation exchange capacity is related to the concentration of compensating cations on the clay surface that can be exchanged for other cations available in a solution.
Clay minerals consist of thin sheets made by lattices of alumina octahedra that are combined with silica tetrahedra. The clay sheets usually display negative electrical surface charges, which help to lower the resistivity readings seen by the downhole LWD and wireline well logging tools. The most common cause of this excess negative surface charge is an exchange of Al+++ in the clay lattice with ions of a lower positive valence. The most common exchange cations are Ca++, Mg++, H+, K+, NH+4, and Na+. After such substitution, the structure of the crystal remains the same.
2. You are a petrophysicist for your company. Define, for the benefit of your subsurface colleagues, the concept and implications of the irreducible water saturation.
Solution
The irreducible water saturation, which is sometimes termed the critical water saturation, is defined as the maximum water saturation that a formation with a given permeability and porosity can retain without producing water. This irreducible water, although present, is held in place by capillary forces and will not flow from a producing subsurface reservoir.
3. You are a geologist for your company. Comment on the broad relationship between sandstone depositional environments, the grain size of their sediments and the level of the resulting irreducible water saturation.
Solution
The level of the irreducible water saturation is strongly influenced by the grain size, which normally reflects the depositional environment. Fine-grained sediments, often the result of low energy depositional environments, usually have high irreducible water saturation levels with correspondingly lower initial hydrocarbon saturations than in coarser-grained clastic reservoirs. Coarse-grained sediments, often formed in high energy depositional environments, tend to have low irreducible water saturation levels and relatively initial high hydrocarbon saturations.
Assessment Cation Exchange Capacity and Irreducible Water Saturation
1. Which techniques can be used to measure the CEC in the laboratory? (Select all that apply.)
A .Sieving
B .Wet chemistry
C .Membrane potential
D .Multiple salinity
2. On what calculated petrophysical parameter do shale and clay have the most effect?
A .Water saturation
B .Porosity
C .Petrophysical parameter “m”
D .Apparent grain density
3. What property of a clay does the cation exchange capacity quantify?
A .The clay’s conductivity
B .The clay’s radioactivity
C .The temperature of the clay
D .The ability of a clay to release cations
4. Thin sheets made by lattices of which tetrahedra combine to form clays? (Select all that apply.)
A .Silica tetrahedra
B .Calcium tetrahedra
C .Carbon tetrahedra
D .Alumina tetrahedra
5. If an oil-bearing, permeable reservoir section with water saturations 10% above the irreducible water saturation is completed, what fluid(s) will likely be produced?
A .It is not possible to predict
B .Oil only
C .Water and oil
D .Water only
6. Which two of the following clay minerals have the highest cation exchange capacities? (Select all that apply.)
A .Kaolinite
B .Montmorillonite
C .Illite
D .Chlorite
7. What aspect of clay sheets helps to lower the resistivity readings seen by the downhole LWD and wireline well logging tools?
A .Lack of electrical surface charges
B .Positive electrical surface charges
C .Negative electrical surface charges
D .Variable electrical surface charges
8. Capillary pressure is the result of what forces in the subsurface? (Select all that apply.)
A .Extensional tectonic forces
B .Kinetic forces
C .Rock-fluid adhesion
D .Fluid-fluid interfacial tension
9. At the microscopic level, which of the following is the prime factor contributing to low resistivity pays?
A .Clay
B .Shale
C .Sandstone
D .Water
Which of the following are common exchange cations? (Select all that apply.)
A .O+
B .Na+
C .Mg++
D .Ca++
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