Page 254 - Introduction to Petroleum Engineering

GAS WELL PRESSURE TRANSIENT TESTING 241
The pressure drawdown test can be used to estimate the limits of a gas reservoir.
The reservoir limit test is a pressure drawdown test that continues until pseudosteady‐
state (PSS) gas flow is achieved. The beginning of PSS flow begins at time

cA
t s 380 T (12.30)
k

where t is the stabilization time in hr, ϕ is the porosity (fraction), μ is the gas viscosity
s
−1
(cp), c is the total compressibility (psia ), k is the permeability (md), and A is the
T
drainage area of the well in ft . The drainage area for a radial system may be approx-
2
2
imated as a circular area A = πr where r is the drainage radius. Stabilization time t s
e
e
is an approximation because drainage radius r is not well known.
e
Example 12.6 Gas Well Stabilization Time
A. Stabilization time is inversely proportional to permeability. Assume stabi-
lization time in a conventional 10 md gas reservoir is 1 hr. How long is
stabilization time in a tight gas reservoir with 1 microdarcy = 0.001 md
permeability?
b. How long is stabilization time in a shale gas reservoir with 1 nanodarcy
= 1 × 10 md permeability?
−6
Answer
A. If we assume all other factors are equal, we have the relationship

tk tk 10 md hr
s conventional s tight gas
Stabilization time in a tight gas reservoir is 10 000 hr.
b. If we assume all other factors are equal, we have the relationship

tk tk 10 md hr
s conventional s shalegas

Stabilization time in the shale gas reservoir is 10 million hr.
These results show that stabilization time is most practical in conven-
tional gas reservoirs.

12.3.5 Rate Transient Analysis

Rate transient analysis (RTA) uses flow rate and flowing wellbore pressure to fore-
cast production rate for very low‐permeability reservoirs. RTA is a mechanistic
method that combines Darcy’s law, an equation of state, and material balance to
derive solutions that depend on different boundary conditions. The boundary
conditions represent different well orientations and reservoir geometries. The solu-
tion that best matches production data is used to calculate reservoir characteristics