Question

Which of the following option(s) is/are CORRECT for well testing analysis of a reservoir?

• A. Permeability, skin and reservoir geometry are calculated using data from pseudo steady state.
• B. Permeability, skin and reservoir geometry are calculated using data from transient state.
• C. Reservoir geometry is calculated using data from pseudo steady state.
• D. Absolute open flow potential is calculated from back pressure test for a gas well.

Answer 1

The Correct Option is C, D

In well testing analysis for reservoirs, several parameters are evaluated to understand reservoir characteristics. Let's evaluate each option to determine the correct statements:

1. Permeability, skin, and reservoir geometry are calculated using data from pseudo steady state.

   Explanation: Pseudo steady state occurs when the pressure within the reservoir boundary stabilizes over time during production, and the rate of change of pressure becomes constant. While some parameters can be evaluated during pseudo steady state, typically transient analysis is more common for permeability and skin evaluations. Thus, this option is generally incorrect for initial calculations.

2. Permeability, skin, and reservoir geometry are calculated using data from transient state.

   Explanation: Transient state analyses are crucial in evaluating well and reservoir properties right after a change in flow rate or pressure. During this period, permeability and skin factor are indeed analyzed. However, reservoir geometry is often deduced in later stages like pseudo-steady state. The statement is partially correct but not entirely.

3. Reservoir geometry is calculated using data from pseudo steady state.

   Explanation: Reservoir geometry often requires stable conditions to evaluate, such as during pseudo steady state, when the pressure derivative is constant over time. Therefore, this statement is correct for well testing analysis.

4. Absolute open flow potential is calculated from back pressure test for a gas well.

   Explanation: An Absolute Open Flow (AOF) potential is a measurement of the maximum flow rate that a well could feasibly achieve in the absence of any back pressure. The back pressure test aids in deriving this potential for gas wells. This methodology is standard practice, making this statement correct.

Based on the evaluation, the correct options are:

• Reservoir geometry is calculated using data from pseudo steady state.
• Absolute open flow potential is calculated from back pressure test for a gas well.

Answer 2

Step 1: Identify what is obtained from transient (infinite-acting) data.
In well testing, the transient (infinite-acting radial flow) period is primarily used to estimate:
Permeability ($k$) from the semilog slope, and
Skin ($s$) from the semilog intercept (or equivalent transient relationships).
Hence, $k$ and $s$ are transient-state estimates, not pseudo-steady-state estimates.
Step 2: Identify what is obtained from pseudo-steady-state / boundary-dominated flow.
When the pressure response becomes boundary-dominated (pseudo-steady-state in closed/drainage systems),
the data reflect reservoir size/geometry effects (e.g., drainage area, boundaries).
Thus, reservoir geometry is commonly inferred from pseudo-steady-state / late-time behavior.
So, statement (C) is correct.
Step 3: Evaluate Absolute Open Flow Potential (AOFP) statement.
For a gas well, back pressure (deliverability) tests are used to determine the deliverability equation and estimate
the absolute open flow potential (AOFP) (rate at $p_{wf} \to 0$) by extrapolation.
So, statement (D) is correct.
Step 4: Check each option.
(A) Incorrect: $k$ and $s$ are not primarily calculated from pseudo-steady-state data.
(B) Incorrect: reservoir geometry is not generally obtained from early transient data unless boundary effects are observed; it is classically obtained from late-time/boundary-dominated behavior.
(C) Correct.
(D) Correct.
Step 5: Conclusion.
The correct options are: \[ \boxed{(C)\ \text{and}\ (D)} \]