Question

Which of the following statements regarding artificial gravel pack are correct?
A. It is required when the aquifer material is homogeneous with a uniformity coefficient (Cu) of less than 3 and an effective grain size (D10) of less than 0.25 mm
B. It stabilizes the aquifer tapped by the well
C. It provides a zone of high permeability surrounding the well screen
D. It decreases the well radius (known as ‘effective radius’ of the well) and well yield
E. The maximum grain size of a gravel pack should be less than 1 cm
Choose the correct answer from the options given below:

• A. A, B, C and E only
• B. B and C only
• C. B, C and E only
• D. B, C and D only

Hint:

An artificial gravel pack behaves like an extension of the well screen.
It increases the effective radius of the well, which lowers the intake velocity of the water, prevents sand pumping, and increases the overall well yield.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
An artificial gravel pack consists of clean, well-sorted, permeable gravel placed in the annular space between the well screen and the borehole wall.
It acts as a filter to keep sand out of the well and improve hydraulic efficiency.
Detailed Explanation:
Let us evaluate each statement:
- Statement A:
Standard well design criteria state that a gravel pack is required when the aquifer material is highly uniform (uniformity coefficient \(C_u < 3\)) and fine-grained (effective size \(D_{10} < 0.25 \text{ mm}\)).
Without a gravel pack, a simple well screen would easily become clogged with fine sand.
Thus, Statement A is correct.
- Statement B:
The gravel pack physically supports the borehole wall, preventing cave-ins and stabilizing the surrounding aquifer.
Thus, Statement B is correct.
- Statement C:
Because the gravel pack consists of clean, highly porous gravel, it provides a high-permeability zone around the well screen.
This reduces friction losses as water flows into the well.
Thus, Statement C is correct.
- Statement D:
The gravel pack increases the effective radius of the well to match the borehole diameter, which actually increases the well yield rather than decreasing it.
Thus, Statement D is incorrect.
- Statement E:
To act as an effective filter, the gravel pack material must be relatively fine and well-sorted (usually between 1 to 5 mm).
Using large gravel coarser than 1 cm would allow fine sand to pass through, causing clogging.
Thus, Statement E is correct.

Step 2: Final Answer:
The correct statements are A, B, C, and E only.