Question

Which of the following improve(s) the productivity index of a vertical well producing from an oil reservoir?

• A. Hydraulic fracturing
• B. Increasing the perforation length
• C. Increasing the skin factor
• D. Reducing the oil viscosity by steam injection

Hint:

Anything that lowers the skin factor or lowers oil viscosity in the productivity index equation increases the productivity index, while a higher skin factor reduces it.

Answer 1

The Correct Option is A, B, D

Step 1: Recall the productivity index equation:
For steady state radial flow of oil to a vertical well, the productivity index, defined as \(J = q/(p_r - p_{wf})\), can be written as \(J = \dfrac{kh}{141.2\,B\mu\left[\ln(r_e/r_w) + S\right]}\), where \(k\) is permeability, \(h\) is net pay thickness, \(B\) is the formation volume factor, \(\mu\) is oil viscosity, and \(S\) is the skin factor. Anything that increases the numerator, or decreases the bracketed term in the denominator, increases \(J\), meaning the well can produce more oil for the same pressure drawdown.
Step 2: Evaluate option A, hydraulic fracturing:
Hydraulic fracturing creates a highly conductive fracture that extends well beyond the near wellbore region, dramatically improving the effective connectivity between the reservoir and the wellbore. This is equivalent to making the skin factor strongly negative, which shrinks the bracketed term in the denominator and therefore increases \(J\). Hydraulic fracturing is a standard well stimulation technique used precisely for this purpose, so option A is correct.
Step 3: Evaluate option B, increasing the perforation length:
A longer perforated interval, or perforation tunnel length, provides a larger effective flow area and reduces the flow convergence near each perforation, which lowers the mechanical or perforation related skin. A lower skin factor again shrinks the bracketed denominator term, so increasing the perforation length improves \(J\), and option B is correct.
Step 4: Evaluate option C, increasing the skin factor:
The skin factor \(S\) sits directly in the denominator of the productivity index equation. Increasing \(S\) makes the bracketed term \(\ln(r_e/r_w) + S\) larger, which makes the whole denominator larger and therefore makes \(J\) smaller. A higher skin factor represents more formation damage or restriction near the wellbore, which always hurts productivity, so option C is not correct.
Step 5: Evaluate option D, reducing oil viscosity by steam injection:
Oil viscosity \(\mu\) sits directly in the denominator of the productivity index equation. Steam injection heats the oil and lowers its viscosity substantially, especially for heavy oil. A smaller \(\mu\) in the denominator directly increases \(J\), so reducing viscosity through steam injection improves productivity, and option D is correct.
Final Answer:
\[ \boxed{\text{Hydraulic fracturing, increasing perforation length, and reducing viscosity by steam injection improve } J} \]