Page 284 - Fundamentals of Reservoir Engineering

OILWELL TESTING 221

lead to an instantaneous perturbation in the wellbore pressure without any associated
transient effects. This pseudo skin can be determined as a function of two parameters,
the penetration ratio b and the ratio h/r w, where

the total interval open to flow
b =
the total thickness of the producing zone

and

h thickness of the producing zone
r w = wellbore radius

The latter definition is somewhat more complex than it appears since if the well is open
to flow over several sections of the total producing interval then h represents the
thickness of the symmetry element within the total zone. This point is made clear in
fig. 7.31 (a)-(c), which has been taken from the Brons and Marting paper and illustrates
three possible types of partial well completion. In all three cases the ratio
b = 30/150 = 0.2 while the ratio of h/r w is 150/.25 = 600 for case (a), 75/.25 = 300 for

case (b) and 15/.25 = 60 for case (c). Having thus determined the values of b and h/r w
the pseudo skin S b can be determined using the chart presented as fig. 7.32. For the
three geometric configurations shown in fig. 7.31, the pseudo skin factors are
approximately 17, 15 and 9, respectively. Once the pseudo skin has been calculated it
must be subtracted from the total skin measured in the well test to give the mechanical
skin factor.

7.10 SOME PRACTICAL ASPECTS OF WELL SURVEYING

This section deals with some of the practical aspects involved in the routine pressure
testing of wells in a producing oilfield.

a) Wireline Pressure Recording Instrument

Due to its accuracy and ruggedness the most popular wireline pressure gauge is the
Amerada (RPG), a rough schematic diagram of this tool is shown in fig. 7.33(a). The
continuous trace of pressure versus time is made by the contact of a stylus with a chart
which has been specially treated on one side to permit the stylus movement to be
permanently recorded. The chart is held in a cylindrical chart holder which in turn is
connected to a clock which drives the holder in the vertical direction. The stylus is
connected to a bourdon tube and is constrained to record pressures in the
perpendicular direction to the movement of the chart holder. The combined movement
is such that, on removing the chart from the holder after the survey, a continuous trace
of pressure versus time is obtained as shown in fig. 7.33(b), for a typical pressure
buildup survey. Both the clock and pressure element can be selected to match the
maximum time and pressure anticipated for the particular survey. With careful
handling, regular calibration and accurate reading of the pressure chart with a
magnifying instrument, an accuracy of about 0.2 can be achieved.