SPE Well Testing and Pressure Transient Analysis 1 ā Questions and Answers
Question 1: During a buildup test, the Horner plot is constructed by plotting shut-in bottomhole pressure versus the Horner time ratio. What does a slope of 'm' on this plot represent?
- Formation permeability-thickness product (kh) (Correct answer)
- Skin factor indicating near-wellbore damage
- Reservoir drainage radius
- Reservoir pressure gradient
Correct answer: Formation permeability-thickness product (kh)
The slope 'm' on a Horner plot is directly related to the permeability-thickness product (kh) via the equation m = 162.6 qBĪ¼ / kh, allowing calculation of formation flow capacity.
Question 2: In pressure transient analysis, what does a positive skin factor (S > 0) indicate about the wellbore condition?
- Enhanced permeability near the wellbore due to stimulation
- Near-wellbore damage reducing effective permeability (Correct answer)
- A naturally fractured reservoir
- Gas slippage effects in a tight formation
Correct answer: Near-wellbore damage reducing effective permeability
A positive skin factor indicates near-wellbore damage (e.g., from drilling mud invasion or scale), which creates an additional pressure drop and reduces well productivity.
Question 3: Which flow regime appears as a half-slope straight line (slope = 0.5) on a log-log plot of pressure derivative during a pressure transient test?
- Radial flow
- Linear flow (Correct answer)
- Bilinear flow
- Spherical flow
Correct answer: Linear flow
Linear flow, which occurs in hydraulically fractured wells or elongated reservoirs, produces a half-unit slope (0.5) on the log-log pressure derivative plot.
Question 4: What is the primary purpose of performing a drawdown test rather than a buildup test in well testing?
- To measure static reservoir pressure more accurately
- To evaluate productivity without sacrificing production time by shutting in (Correct answer)
- To determine original oil in place using material balance
- To measure wellbore storage effects only
Correct answer: To evaluate productivity without sacrificing production time by shutting in
A drawdown test acquires pressure transient data while the well flows, avoiding the production loss associated with a buildup test shut-in period.
Question 5: In well testing, 'wellbore storage' (afterflow) distorts early-time pressure data. What causes wellbore storage in a producing oil well?
- Reservoir compressibility changes near the wellbore
- Fluid expansion or compression within the wellbore volume after shut-in or opening (Correct answer)
- Skin damage reducing flow into the wellbore
- Fracture closure pressure exceeding reservoir pressure
Correct answer: Fluid expansion or compression within the wellbore volume after shut-in or opening
Wellbore storage occurs because the wellbore volume contains compressible fluid; after shut-in, continued influx from this volume (afterflow) or fluid compression masks the true reservoir response.
Question 6: The dimensionless wellbore storage coefficient (CD) accounts for afterflow effects in pressure transient analysis. Which well characteristic most directly increases CD?
- Higher formation permeability
- Larger wellbore volume (e.g., larger casing ID or deeper well) (Correct answer)
- Greater reservoir thickness
- Higher bottomhole temperature
Correct answer: Larger wellbore volume (e.g., larger casing ID or deeper well)
CD is proportional to the wellbore volume (C = Vwb Ć ct); a larger casing inner diameter or longer wellbore increases the storage volume, raising CD.
Question 7: On a pressure derivative plot (Bourdet derivative), radial flow is identified by what signature?
- A unit slope (45Ā°) straight line
- A half-slope (0.5) straight line
- A flat (horizontal) straight line (Correct answer)
- A negative quarter-slope line
Correct answer: A flat (horizontal) straight line
Radial flow produces a constant (flat, horizontal) pressure derivative on a log-log plot, making it the hallmark diagnostic for the infinite-acting radial flow regime.
SPE Well Testing and Pressure Transient Analysis 1
During a buildup test, the Horner plot is constructed by plotting shut-in bottomhole pressure versus the Horner time ratio.
What does a slope of 'm' on this plot represent?