Assignment Logging Tools For Evaluating Low Resistivity And Low Contrast Pay
1. You are a petrophysicist for your company. Make some general comments to your subsurface colleagues about the vertical resolution of logging tools, and how this affects well log data in thinly laminated, shaly sandstone reservoirs. Summarize two solutions from logging service companies for this interpretation problem.
Solution
The measurements of all standard well logging tools are a compromise between their vertical resolution and their horizontal depth of investigation. This causes interpretation challenges in thinly laminated, shaly sandstones. In the ideal scenario, deep resistivity logging tools would read deep enough into the formation to acquire data beyond the drilling mud filtrate invaded zone to measure the true formation resistivity of the virgin reservoir and they would also resolve thin beds vertically.
Interpreters need the vertical resolution of the well log measurements to be no thicker than the thinnest bed, or lamination. This goal is not always realized with conventional resistivity tools, as some thin beds or laminated zones may be thinner than the vertical resolution of standard logging while drilling (LWD) and wireline logging tools.
Logging service companies have developed innovative tools to overcome the limitations of the standard tools in thinly laminated, shaly sandstone reservoirs. The following are two such examples:
- The Schlumberger Array Induction Imager Tool (AIT) uses eight induction coil arrays operating at multiple frequencies to generate five resistivity curves. The log curves have median depths of investigation of 10, 20, 30, 60 and 90 inches, and vertical resolution options of 1, 2 and 4 feet.
- Baker Hughes has developed a resistivity tool unique to the industry, which is designed to overcome the limitations of conventional induction tools in thin bedded, low resistivity, shaly sandstone formations. The Baker Hughes 3D Explorer Induction Logging Service (3DEX) provides both vertical and horizontal resistivity measurements independently of the borehole deviation and formation dip.
2. Summarize, for the benefit of your engineering colleagues, the operations and uses of resistivity imaging tools in thinly laminated, shaly sandstone reservoirs.
Solution
Resistivity imaging tools evolved from dipmeter logging technology. These tools have either four or six independent arms, each with articulating pads and multiple electrodes. The combination of multiple pads and electrodes results in significantly improved vertical resolution, down to less than one inch.
Resistivity imaging tools use a fixed contrast display for gross correlations, and a dynamic averaging display to enhance the local geological and stratigraphic features. The fixed contrast presentation enables the interpreter to correlate color values between different zones of interest within the well or between images from different wells. The dynamic averaging display is applied to local events to enable the interpreter to differentiate smaller scale geological features.
3. As a petrophysicist, summarize to your colleagues how a state-of-the-art 3D induction resistivity logging tool can overcome the limitations of the more conventional induction tools in thin-bedded, low resistivity, shaly sandstone formations.
Solution
The 3D induction resistivity logging tool generates both vertical and horizontal resistivity measurements, which are independent of the borehole deviation and the formation dip. The tool has three transmitter-receiver coil arrays, mounted orthogonally in the X, Y and Z planes relative to the tool’s axis. These coil arrays provide 3D coverage in their resistivity measurements. Two coils, XX and YY, measure the resistivity in transverse directions, parallel to the tool body. A third coil, ZZ, measures resistivity in the direction of conventional resistivity tools, perpendicular to the tool body. In addition, there are cross-component measurements, XY and XZ.
These arrays induce currents that flow mainly across the laminated sandstone and shale sequences and are far more sensitive to the hydrocarbon-bearing sandstone resistivity.
Assessment Logging Tools For Evaluating Low Resistivity And Low Contrast Pay
1. If the combined NMR and resistivity-based saturation petrophysical analysis indicates that the entire sequence is at irreducible water saturation, what fluids would be expected to be produced on testing?
A .It is not possible to predict
B .Hydrocarbons only
C .Water and oil
D .Water only
2. Approximately what percentage pad coverage of an 8-inch diameter borehole is achieved using Schlumberger’s Formation MicroImager (FMI) tool?
A .40%
B .60%
C .80%
D .20%
3. Clays and shales, unless extremely compacted after deep burial, are characterized by generally _______
acoustic velocities.
4. Of what petrophysical parameter is the spontaneous potential (SP) curve a reasonable indicator?
A .Grain size
B .Grain density
C .Water saturation
D .Porosity
5. To what do NMR measurements primarily respond?
A .The water salinity
B .Dry gas
C .Hydrogen protons in the formation pore spaces
D .The formation’s bulk density
6. What are some of the interpretation challenges particular to thinly laminated, shaly sandstones using traditional/conventional logging tools? (Select all that apply.)
A .Measuring the true formation resistivity of the undisturbed zone
B .Thin beds or laminated zones may be thinner than the vertical resolution of the logging tools
C .Deep resistivity logging tools “seeing” deep enough into the formation to get beyond the drilling mud invaded zone
D .Tool malfunctioning in the downhole environment
7. How many independent arms, each with articulating pads containing multiple electrodes, do many modern resistivity imaging tools have? (Select all that apply.)
A .12
B .6
C .2
D .4
8. In resistivity imaging tools, what porosity/permeability characteristics are indicated by dark colors? (Select all that apply.)
A .Low porosity
B .High permeability
C .Low permeability
D .High porosity
9. What factors can cause the formation’s resistivity to be lowered? (Select all that apply.)
A .Bound water content
B .Permeability
C .Depth
D .Surface charge of clay and shale
10. In most situations, shale and clay in the formation cause conventional resistivity logs to read too _________ , while causing porosity logs to read too ____________ .
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