Assignment
You are your company’s petrophysicist responsible for establishing the porosity of the reservoir formation objective in an exploration well. Explain to your team colleagues why you will be using the density-neutron crossplot as your primary method to determine the porosity within the reservoir formation.
Solution
A major advantage of the density-neutron crossplot method of well log porosity estimation is that the porosity generated is relatively unaffected by unknown or variable lithology through the reservoir interval being logged, as may well be the case in this exploration well.
Corrections for any gas effect using the density-neutron crossplot are readily achievable. The use of computer algorithms which iterate until there is stability on the calculated matrix density is the preferred technique for making light hydrocarbon corrections.
Corrections can also easily and quickly be made for the shale content of the reservoir formation in order to establish the shale-corrected porosities using the density-neutron crossplot.
FAQs (Frequently Asked Questions)
Q1: How does porosity affect oil and gas production? Porosity directly influences the storage and flow of oil and gas within a reservoir. Higher porosity allows for greater hydrocarbon accumulation and facilitates their movement through the rock formation, leading to increased production rates.
Q2: Can porosity be measured directly during well logging? Porosity is not measured directly but is inferred from well logging data using various techniques such as sonic, density, and neutron logs. These measurements provide information about the physical properties of the formation, which can be used to estimate porosity.
Q3: What are the typical porosity values in oil and gas reservoirs? Porosity values in oil and gas reservoirs can vary significantly depending on the geological characteristics. Generally, porosity ranges from a few percent to over 30%, with the optimal porosity for hydrocarbon storage and flow varying for different types of reservoirs.
Q4: How is porosity related to permeability? Porosity and permeability are related but distinct properties. Porosity refers to the volume of pore spaces, while permeability refers to the ability of fluids to flow through those pore spaces. Although high porosity generally correlates with high permeability, the relationship can be influenced by factors like pore size distribution and connectivity.
Q5: Can porosity change over time in a reservoir? Porosity can change over time due to various factors, including diagenesis, compaction, and fluid movement. Understanding the evolution of porosity is crucial for reservoir management and predicting changes in hydrocarbon production.
Assessment
1. A sonic logging tool normally triggers on the first arrival of what type of wave at the tool’s receivers?
A .Love wave
B .Rayleigh wave
C .Compressional wave
D .Shear wave
2. What is generally the most effective method of analyzing density and neutron logs together?
A .Shading the interval between them on digital playbacks
B .Overlaying them
C .Crossplotting them on an X-Y framework
D .Inversely overlaying them
3. What is the density of a pure limestone formation with no porosity and no shale content?
A .2.71 gm/cc
B .2.87 gm/cc
C .2.98 gm/cc
D .2.65 gm/cc
4. A neutron porosity log is recorded on a limestone matrix setting and gives a value of 15% neutron porosity (limestone matrix) in a clean, water-bearing interval. If the formation is sandstone, what is the true porosity?
A .19%
B .21%
C .12%
D .15%
5. In shale-free formations, what element of a formation dominates the neutron porosity logging tool’s response?
A .Calcite
B .Quartz
C .Dolomite
D .Liquid-filled porosity
6. A bulk density of 2.4 gm/cc is recorded by a density log run in a sandstone section with matrix density of 2.65 gm/cc when a salt water base drilling mud of density 1.1 gm/cc was used. What is the density porosity?
A .16.1%
B .11.2%
C .14.6%
D .19.7%
7. Which of the following well logs is least affected by shale and gas content in estimating porosity?
A .Neutron
B .Density
C .Sonic
8. What types of porosity does a sonic logging tool measure? (Select all that apply.)
A .Vugular porosity
B .Total porosity
C .Intergranular porosity
D .Primary porosity
9. Which of the following elements have an influence on the formation’s bulk density reading on a density well log? (Select all that apply.)
A .The volume of shale and its density
B .Formation porosity
C .Density of the logging tool
D .Density of the rock matrix
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