Page 251 - Hydrocarbon Exploration and Production Second Edition

238 Production Testing and Bottom Hole Pressure Testing

gas
other wells
Main Separator Train liquid
wireline
gas
FTHP
routinely measure
production vs FTHP
choke oil
water
Test Separator

non-routine
downhole pressure
measurement
tubing using downhole gauge
casing

downhole pressure gauge
perforations
Reservoir

Figure 10.9 Bottom hole pressure testing.

It is common practice to record the bottom hole pressure firstly during a
flowing period (pressure drawdown test), and then during a shut-in period (pressure
build-up test). During the flowing period, the FBHP is drawn down from the initial
pressure, and when the well is subsequently shut-in, the bottom hole pressure builds
up (Figure 10.10).
In the simplest case, for a pressure drawdown survey, the radial inflow equation
indicates that the bottom hole flowing pressure is proportional to the logarithm of
time. From the straight line plot of pressure against the log (time), the reservoir
permeability can be determined, and subsequently the total skin of the well. For a
build-up survey, a similar plot (the so-called Horner plot) may be used to determine
the same parameters, whose values act as an independent quality check on those
derived from the drawdown survey.
Drawdown and build-up surveys are typically performed once a production well
has been completed, to establish the reservoir property of permeability (k), the well
completion efficiency as denoted by its skin factor (S) and the well PI. Unless the
routine production tests indicate some unexpected change in the well’s productivity,
only SBHP surveys may be run, say once a year. A full drawdown and build-up
survey would be run to establish the cause of unexplained changes in the well’s
productivity. In addition to pressure and temperature gauges, a whole suite of further
data may be acquired at the same time in one production log. These production
logging techniques (PLTs) may include spinners to measure flowrates, density meters to
measure water, gas and oil contents and other more sophisticated measurements for