Page 250 - Introduction to Petroleum Engineering

GAS WELL PRESSURE TRANSIENT TESTING 237
12.2.3 Radius of Investigation of a Liquid Well

The change in pressure induced by a change in flow rate at the well travels through
the reservoir at a rate that is a function of the properties of reservoir rock and fluid.
The distance traveled by the pressure transient away from the wellbore depends on
the elapsed time following the change in flow rate. This distance is known as radius
of investigation. The Horner analysis of a PBU test gives the following equation
for radius of investigation:

kt
.
r inv 0 029 (12.17)
c T

where radius of investigation r is in feet, Δt is the shut‐in time (hr), k is the perme-
inv
ability (md), ϕ is the porosity (fraction), μ is the viscosity (cp) of the mobile liquid
phase, and c is the total compressibility (1/psia).
T
Radius of investigation is an approximation of the distance traveled by the pressure
transient. This can be useful in reservoir characterization because it can be used to
estimate the distance to a feature in the reservoir that causes a change in the slope
of the pressure measured at the well. The shut‐in time corresponding to the change
in slope can be used to estimate the distance to the feature that caused the change in
slope. Features that can change the slope of the pressure response include no‐flow
barriers such as sealing faults or permeability pinch‐outs. The radius of investigation
gives information about the distance to the feature. The interpretation of the feature
should be consistent with all available data.

Example 12.4 Oil Well Radius of Investigation
Calculate radius of investigation of a pressure buildup (PBU) test in an oil well
when the shut‐in time (∆t) is 24 hr. Use the following data: permeability
k = 150 md, porosity ϕ = 0.14, initial liquid viscosity μ = 0.9 cp, and initial total
compressibility c = 8 × 10 psia .
−1
−6
T
Answer
kt 150 md 24 hr
r 0 029 0 029 1733ft
.
.
inv 16
.
.
.
a
c T 0140 90 cp 80 10 /psia
12.3 GAS WELL PRESSURE TRANSIENT TESTING
PTT of a gas well relies on the same general principles as PTT of an oil well.
Gas flow rate is changed, and the resulting change in pressure at the well is recorded
as a function of time. Results of gas well PTT can be used for two purposes:
to determine reservoir characteristics and to determine gas well deliverability.