Page 254 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Pressure Transient Testing of Oil and Gas Wells 423

+
skin factor (drawdown), s = 1.151 log( 2)
3.231
(5-1 37)
WSf
pressure drop due to skin, (Ap), = 0.869 ms (5- 138)

average drainage area pressure, j5 = pwf + (5-139)
kh

average pressure (steady - state), = pwf + (5-140)
kh

(5-132)

For these equations, the following nomenclature is applicable:
rate
time
formation volume factor
viscosity
thickness
porosity
total compressibility
external drainage radius
wellbore radius
semilogarithmic MTR slope
initiaI reservoir pressure
flowing bottomhole pressure
shut-in bottomhole pressure
pressure drop across skin region
MTR semilog intercept
In the equation for skin factor during the pressure buildup, P, is measured just
before shutting in the well, and plhr is obtained from the straight-line portion
(extrapolated if necessary) of the buildup curve one hour after shut-in (see
Figure 5-138a). Similarly, the straight-line portion of the drawdown data must
be extrapolated to one hour if the data do not fall on the semilog straight-line
(see Figure 5-138b).

vpe-Curves
While at least one author [197] questions the uniqueness of pressure-transient
type-curves and others [66,180] state that type-curves should only be used when
conventional techniques cannot be used, curve-matching techniques have recently
received more widespread use. In some cases where conventional analyses fail