PBIOS - October 2021 - 121

FIGURE 2
2,400
2,200
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
100
200
300
400
500
Gas Lift Rate (QG)
nature from the per-foot temperature profile
can confirm whether valves are open
or closed at their depth.
· Downhole flowing pressure and liquid
production measurement at a particular
moment pinpoint the characteristics of
the inflow performance curve (type curve)
of the well.
· Wellhead data on pressure and temperature,
along with liquid production,
water cut, gas injection flow rate, formation
gas/liquid ratio and API oil gravity measurements,
can be used to analyze multiphase
flow correlations or models to find
what is best suited for current operating
conditions.
· Pressure surveys run at varying stabilized
injection gas flow rates to measure
the change in liquid production can estimate
the extent of formation damage,
the reservoir pressure and the well's productivity
index.
Converting the survey's pressure and
temperature data into these insights takes
skill and the right tools. For accurate results,
the surveys should be interpreted
by a professional who is familiar with
gas lift design and understands the ways
gas passes through the 16 ports in a gas
lift valve.
The Process
To conduct a gas lift survey, a trained
team runs wireline with temperature and
pressure quartz gauges from surface to
packer depth. The procedure involves
two trips down hole. Both trips can and
in fact should be performed while the
well is flowing, which is one of the
reasons gas lift surveys have a low cost.
The first trip cleans the tubing with a
circular metallic ring while calibrating
the line management system box. The
wireline team trips in with this ring to
packer depth, then pulls out to remove
any restrictions, such as paraffin, wax or
scale. The goal is to ensure the temperature
and pressure gauges that follow can run
clean, without encountering wellbore debris
as they enter and leave the hole.
After the cleaning run, two pressuretemperature
memory gauges are connected
to a weight bar, then connected to the
slickline. The gas injection rate must remain
constant at a set pressure before
and at the survey so gas lift valves can
work themselves down (compensating
for any frictional pressure loss from gas
flow across each valve). This also captures
the effective resolution of pressure and
temperature (late stages of the unloading
process) through the survey.
This job runs at a relatively constant
line speed of 75 feet a minute throughout
the entire trip.
Wolfcamp Well Insights
Figure 1 shows the data obtained
during the survey of the Wolfcamp well
mentioned earlier.
The trip-in-hole (TIH) pressure differential
(PRESSURE_G) in Figure 1 is
erratic at 1,500 feet and deeper, giving
the impression the gauges encountered
surging behavior going against the flow.
Figure 1 shows there is no hole in the
tubing. The well merely has slowed because
its flowing pressure has fallen to
FIGURE 3
2,000
1,800
1,600
1,400
1,200
1,000
800
600
400
200
100
200
300
400
500
Gas Lift Rate (QG)
OCTOBER PBIOS 2021 121
600
700
800
900
1,000
Simulation of Injection Rate's Effect After Survey
Tubing ID = 2.441 (s1)
IPR Base
600
700
800
900
1,000
Simulation of Injection Rate's Effect Before Survey
Tubing ID = 2.441 (s1)
1,600 psi. The new bottom-hole pressure
will be the foundation for simulations.
A survey should be run immediately
IPR Base
after any new valve installation to serve
as a baseline for future comparison and
to evaluate valve design efficiency.
Joules-Thompson cooling effect considerations
in survey readings are evident
where gas pressure changes reflect temperature
changes. If the total pressure
drop was achieved in one stage, that
could cause hydrates to form in a flowing
medium. In this survey, none of the
valves are open, so there is no indication
of this event.
A comprehensive checkup is required
before creating a simulation matrix to
ensure that root-cause analysis has been
performed on a well-specific basis. An
example of such a checkup follows.
Figure 2 shows that the Wolfcamp
well began gas lift with a bottom-hole
pressure of 2,100 psi. After starting gas
injection from the backside of tubing
through the gas lift valve, bottom-hole
pressure decreased (which is the purpose
of gas lift), creating a differential for
reservoir pressure.
Figure 2 also shows that, after injecting
at 800 Mcf/d, increasing injection would
not lower the bottom-hole pressure significantly.
This gas injection rate would
be the target when designing for 2,100
psi bottom-hole pressure.
With gas lift survey results, a much
lower bottom-hole pressure of 1,600 psi
was deduced. The simulation shown in
Figure 3 for 1,600 psi indicates there is
no decline in bottom-hole pressure after
injecting 500 Mcf/d, meaning that the
injection rate should be reduced from
800 Mcf/d to 500 Mcf/d to create a better
gas lift operating envelope.
As predicted by the simulation and
shown at the right end of the well's
production history (Figure 4), reducing
the injection rate improved production
Bottom Hole Pressure (psia)
Bottom Hole Pressure (psia)
PBIOS - October 2021

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