SPE Well Testing and Pressure Transient Analysis 2 — Questions and Answers
Question 1: A pressure buildup test is shut in after a production time 'tp'. The Horner approximation works best when which condition is met?
- Shut-in time (Δt) greatly exceeds production time (tp)
- Production time (tp) greatly exceeds shut-in time (Δt) (Correct answer)
- Skin factor is zero
- Wellbore storage coefficient is greater than 100
Correct answer: Production time (tp) greatly exceeds shut-in time (Δt)
The Horner approximation (using superposition with a single producing time tp) is most accurate when tp >> Δt, ensuring that the transient is still within the early buildup response.
Question 2: Which reservoir boundary condition produces a doubling of the semi-log straight line slope in a buildup or drawdown pressure transient test?
- A constant pressure boundary (aquifer)
- A sealing fault (no-flow boundary) at some distance from the well (Correct answer)
- An infinite-acting reservoir
- A high-permeability natural fracture network
Correct answer: A sealing fault (no-flow boundary) at some distance from the well
When a sealing fault is encountered, flow transitions from radial to hemi-radial, effectively halving the drainage area and doubling the slope on the semi-log plot.
Question 3: In a drill stem test (DST), the 'Initial Shut-In Pressure' (ISIP) measured at the start of the test is most useful for estimating which parameter?
- Formation permeability
- Skin damage around the wellbore
- Static reservoir pressure or pore pressure (Correct answer)
- Wellbore storage coefficient
Correct answer: Static reservoir pressure or pore pressure
The ISIP in a DST approaches static reservoir pressure as equilibration occurs, making it the primary indicator of pore pressure or original reservoir pressure.
Question 4: What does the term 'pseudosteady-state flow' (PSS) describe in reservoir engineering and well testing?
- Transient flow where pressure disturbance has not yet reached a boundary
- Flow regime where reservoir pressure declines uniformly at a constant rate throughout the drainage area (Correct answer)
- Flow through a hydraulic fracture at constant bottomhole pressure
- Multiphase flow in the near-wellbore region
Correct answer: Flow regime where reservoir pressure declines uniformly at a constant rate throughout the drainage area
Pseudosteady-state (PSS) occurs after transients reach all boundaries; the reservoir then depletes uniformly, with pressure declining at the same constant rate everywhere in the drainage volume.
Question 5: In a pressure falloff test performed on an injection well, which parameter can be derived using the same analytical methods as a standard buildup test?
- Fracture gradient of the formation
- Formation permeability and skin factor (Correct answer)
- Capillary pressure curves
- Relative permeability endpoints
Correct answer: Formation permeability and skin factor
A pressure falloff test (shut-in after injection) is the injection-well analog to a buildup test; the same semi-log and derivative analysis yields formation permeability (k) and skin (S).
Question 6: Which deliverability test method for gas wells involves flowing the well at four different stabilized rates to establish a flow-after-flow curve?
- Isochronal test
- Modified isochronal test
- Backpressure (flow-after-flow) test (Correct answer)
- Pressure buildup (Horner) test
Correct answer: Backpressure (flow-after-flow) test
The backpressure (or flow-after-flow) test flows a gas well at four increasing rates, each to apparent stabilization, to construct the deliverability curve against backpressure.
Question 7: When analyzing a naturally fractured reservoir using pressure transient data, the Warren-Root model predicts what distinctive feature on the pressure derivative plot?
- A unit-slope line indicating wellbore storage dominated flow
- A dip (valley) in the derivative between two parallel semi-log straight lines (Correct answer)
- A doubling of slope indicating boundary effects
- A constant half-slope indicating infinite conductivity fracture
Correct answer: A dip (valley) in the derivative between two parallel semi-log straight lines
The Warren-Root dual-porosity model exhibits a characteristic valley (dip) in the pressure derivative as fluid transfers from matrix to fractures, appearing between the early fracture-dominated and late total-system radial flow regimes.
SPE Well Testing and Pressure Transient Analysis 2
A pressure buildup test is shut in after a production time 'tp'.
The Horner approximation works best when which condition is met?