Two sandstone reservoir rocks R1 and R2 have 25 % porosity each but the permeability of R1 is higher than that of R2. Which of the following can explain this difference in permeability?
Two sandstone reservoir rocks R1 and R2 have 25 % porosity each but the permeability of R1 is higher than that of R2. Which of the following can explain this difference in permeability?
Show Hint
- Clay content in R1 is more than that in R2
- Microfracture density in R1 is higher than that in R2
- Sorting in R1 is better than that in R2
- R1 has more angular grains than R2
The Correct Option is C
Solution and Explanation
Permeability is the ability of a rock to transmit fluids through its pore spaces. It depends on factors like porosity, the shape of the pore spaces, the size of the pores, and the sorting of the grains. Although both rocks R1 and R2 have the same porosity, permeability can still differ due to variations in these factors.
Step 2: Analyzing each option.
- Option (A): Clay content in R1 is more than that in R2: Clay content reduces permeability because clay particles tend to block pore spaces and reduce fluid flow. Therefore, if R1 has more clay content than R2, its permeability should be lower, not higher. This option is incorrect.
- Option (B): Microfracture density in R1 is higher than that in R2: Microfractures can increase permeability by creating additional flow paths. However, this option does not explain the difference based purely on the given porosity, as we are not told anything about microfractures in either rock. This option is unlikely to be the cause.
- Option (C): Sorting in R1 is better than that in R2: Sorting refers to the uniformity of grain sizes in the rock. Better sorting (more uniform grain sizes) leads to larger, more connected pores and, hence, better permeability.
If R1 has better sorting than R2, it would result in higher permeability. This is the most plausible explanation and the correct answer.
- Option (D): R1 has more angular grains than R2: Angular grains tend to interlock more tightly, reducing the size of pore spaces and lowering permeability. This would not explain why R1 has higher permeability than R2. This option is incorrect.
Step 3: Conclusion.
The correct explanation for the difference in permeability between rocks R1 and R2 is that R1 has better sorting than R2, leading to larger, more connected pores and higher permeability. Therefore, the correct answer is (C).
\[ \boxed{\text{Sorting in R1 is better than that in R2}} \]
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