Most acceptable methods of gas well characterization require a series of drawdown-buildup sequences. The method described here requires only one drawdown followed by a buildup, thus reducing testing time and associated costs.
Many authors recognize that Darcy's law may not hold for flow of gas through porous media. The non-Darcy flow effect has been accounted for by a flow-rate-dependent skin. Thus, to adequately characterize and predict deliverability of a gas well, it is essential to resolve the skin calculated from a buildup or a drawdown into at least its two components: the physical or rate-independent component, and the nondarcy, or rate-dependent component. The total skin calculated from flow tests has been represented by the following equation.
st = s + Dq,................................(1)
where
st = total skin calculated from flow tests
s = skin caused by physical damage or improvement, plus pseudoskin effects caused by restricted entry to flow at the wellbore
D = nondarcy flow constant in 1/q
q = gas flow rate in any appropriate units.
Basically, there are two methods for estimating the non-Darcy flow constant, D. The first method calculates D by the following equation.
-15 2.715 × 10 M pbk D =, ...............(2) hr w T b
where 1 D = Mscf
M = molecular weight of gas, lb
P = base pressure, psi
k = permeability, md
mu = viscosity, cp
h = thickness, ft
r = wellbore radius, ft
T = base temperature, deg. R
beta = turbulence factor, 1/ft.
beta may be estimated theoretically by -5/4 -3/4 beta = (5.5 × 10) k (phi S),
where
phi = porosity, fraction
Sg = gas saturation, fraction.
The second method uses flow tests to calculate D. The literature reports that comparing values of D estimated by Eq. 2 with those obtained from flow-test data indicates that the theoretically determined values may be in error by as much as 100 percent. Because of this uncertainty, D should be obtained from flow tests when possible.
JPT
P. 1500
