Assignment
1. As a petrophysicist for your company, explain to your multi-disciplinary colleagues the subtle differences between the anisotropy and the heterogeneity of geological formations.
Solution
Anisotropy is the variation in a vectorial value with direction, such as the interval velocity of a formation increasing in a northerly direction, at a single point. Heterogeneity is the variation in vectorial or scalar values between two or more points within a given direction.
Anisotropy primarily describes the physical properties, such as the hardness or the average grain size, of a geological formation. Heterogeneity describes the point-to-point variations in compositions, geometries, or physical properties.
2. Summarize, for the benefit of your engineering colleagues, the main types of anisotropy found in carbonate formations. Comment on the resultant effects on the carbonate reservoir’s properties.
Solution
Carbonate formation anisotropy is primarily the result of either fracturing or diagenesis, both of which are post-depositional effects. To a lesser extent, anisotropy in carbonates can also be caused by changes in the carbonate mineralogy that are contemporaneous with deposition. These variations in mineralogy may be caused by changes in the carbonate content of the atmosphere and/or the formation waters.
Changes in carbonate mineralogy can result in differences in the reservoir rock’s texture, porosity, and permeability.
3. As a petrophysicist, comment on the main types of anisotropy found in clastic formations.
Solution
Anisotropy in clastic reservoirs is mainly the result of:
- Layering, usually attributed to changes in the sediment type and resulting in beds of varying materials or grain sizes.
- Ordering of the grains comprising the sandstone reservoir formation, which is caused by the directionality of the transporting medium, such as wind or water. Upon settling, the grains tend to align in the direction of least resistance to the movement of the water or wind.
- Post-depositional tectonic deformation causing shortening in one direction and resulting in the azimuthal distribution of grain assemblages.
4. As a petrophysicist, summarize for your multi-disciplinary work colleagues, the main causes of electrical anisotropy.
Solution
The main causes of electrical anisotropy are:
- Formation bedding geometry, such as thin-bedded or laminated sandstone and shale sequences
- Varying grain size distribution and sorting in sandstone beds
- Apparent dip—the angle that a wellbore makes with the horizontal, measured in any randomly oriented section rather than perpendicular to the strike. As the electrical anisotropy depends on the direction in which electrical current flows in the rock, both the angle at which the borehole penetrates the formation and the dip of that formation are factors that can cause electrical anisotropy.
- Differences between the hydrocarbon and the water legs of reservoirs, such as water-wet formations with large variability in their grain size being highly anisotropic in the oil leg while being isotropic in the water leg
Assessment
1. In a reservoir engineering numerical simulation model of a reservoir, what are often the most significant anisotropic elements? (Select all that apply.)
A .Vertical permeability
B .Horizontal permeability
C .Horizontal porosity
D .Vertical porosity
2. Which of the following directional differences can often have the most impact on geophysical interpretations? (Select all that apply.)
A .Time
B .Velocities of shear waves
C .Velocities of compressional waves
D .Fluid velocities
3. Which of the following are needed for the measurement of electrical anisotropy? (Select all that apply.)
A .Acquiring micro-spherically focused log data
B .Acquiring horizontal resistivity log data
C .Acquiring micro-resistivity log data
D .Acquiring vertical resistivity log data
4. Which of the following are post-depositional, diagenetic causes of formation anisotropy? (Select all that apply.)
A .Burrowing
B .Pressure solution
C .Depositional environment changes
D .Compaction
5. What are the main causes of anisotropy in carbonates in the subsurface? (Select all that apply.)
A .Diagenesis
B .Formation age
C .Compaction
D .Fractures
6. Which of the following can cause electrical anisotropy? (Select all that apply.)
A .Formation depth
B .Grain size in homogeneous sandstones
C .Formation bedding geometry
D .Intrusion of salt diapirs
7. What is the most important petrophysical interpretation implication of obtaining both vertical and horizontal resistivity measurements in the interpretation of low resistivity and low contrast pays?
A .Better permeability estimates
B .Faster petrophysical interpretation
C .Improved formation hydrocarbon saturation determination
D .Improved porosity interpretation
8. How do petrophysical interpreters best understand the relationship between the resistivities and the laminar shale volume?
A .Model the resistivity data
B .Merge the resistivity data
C .Deconvolve the resistivity data
D .Invert the resistivity data
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