Well Testing and Pressure Transient Analysis Flashcards

In gas well testing, why is it necessary to use 'pseudo-pressure' (m(p) or real-gas potential) instead of pressure in the flow equations?

Answer: Gas viscosity and z-factor are both pressure-dependent, so simple pressure analysis introduces significant error

Both gas viscosity (μg) and z-factor vary significantly with pressure; pseudo-pressure integrates μg and z through the pressure range, linearizing the gas flow equation for accurate analysis.

The concept of 'radius of investigation' (ri) in pressure transient testing describes which physical phenomenon?

Answer: The maximum distance from the well that a pressure disturbance has reached at a given time

The radius of investigation estimates how far from the wellbore a detectable pressure disturbance has propagated at time t, given by ri ≈ 0.029√(kt/φμct).

In pressure transient analysis, what does a 'unit slope' (slope = 1) on a log-log plot of Δp versus Δt during early time indicate?

Answer: The test is dominated by wellbore storage (afterflow)

A unit slope (1:1) on the log-log plot of Δp and its derivative at early time is the diagnostic signature of wellbore storage domination, where all flow comes from wellbore fluid expansion.

Which of the following correctly describes the difference between 'infinite conductivity' and 'finite conductivity' vertical fractures in pressure transient analysis?

Answer: Infinite conductivity fractures show half-slope on the derivative; finite conductivity fractures show quarter-slope

Infinite conductivity fractures (negligible fracture pressure drop) exhibit a half-unit slope (linear flow), while finite conductivity fractures show a quarter-unit slope (bilinear flow) because fluid flows in both the fracture and the formation simultaneously.

A well is tested and the extrapolated Horner straight line gives a false pressure P* instead of true average reservoir pressure (P-bar). Under what condition does P* equal P-bar?

Answer: When the well has been producing for an infinite time (the reservoir is effectively infinite-acting at shut-in)

P* equals P-bar only in an infinite-acting reservoir where no boundary effects occurred during production; in bounded reservoirs, corrections (e.g., Matthews-Brons-Hazebroek) are needed to convert P* to true average pressure.

During a multi-rate transient test using superposition theory, why must the principle of superposition be applied instead of simple single-rate analysis?

Answer: Superposition allows each rate change to be treated as an independent transient whose effects sum linearly

Superposition (based on the linearity of the diffusivity equation) allows multiple rate changes to be treated as superimposed independent transients, enabling correct analysis of complex rate histories.

In an interference test between two wells, what is the primary information obtained that a single-well buildup or drawdown test cannot provide directly?

Answer: Interwell permeability and porosity-compressibility product between the two wells

An interference test measures the pressure response at an observation well due to production at an active well, yielding interwell (large-scale) permeability and the φct product between the two wells.