American Oil and Gas Reporter - February 2017 - 54

SpecialReport: Shale Resource Science
speed, it is believed that adding a liquid
recirculation system would allow the
pump to achieve higher efficiencies even
when the process fluid entering the pump
is at 100 percent GVF.
Virtually no field installation of a PCP
recirculates fluid from the pump discharge
to the pump suction. Trapping and cooling
a small amount of liquid at the pump discharge and routing it to the pump inlet
should improve the run life of the stator
and allow better dissipation of compression
heat generated when GVFs are continually
in the 90-100 percent range.
Another Texas A&M research project
tested high-GVF operation using a Leistritz
TSP, and successfully demonstrated the
ability of a surface twin-screw pump to
provide wet gas compression. With this
design, compression is generated by the
backflow of liquid from the pump discharge to the pump suction though chambers created by the intermeshing of the
two screws.
As shown in Figure 3, a small vessel
is utilized to trap a small amount of liquid
and re-circulate it to the pump discharge.
This allows the pump to run without
liquid flow for some time, if needed.
Cooling the recirculation stream with an
in-line passive air cooler (not shown) improves efficiencies for periods when the
GVF exceeds 90 percent.
Keys To Success
Multiphase pumps utilized for a padbased MPF need to tolerate some amount

of solids from the flow back of proppant
used in the fracturing treatments. PCPs
handle solids well, and are even utilized
as slurry pumps in other applications,
but the life of the replaceable rubber
stator is reduced. For TSPs, higher-strength
materials have been shown to extend the
life of the metal screws.
The keys to the success of a multiphase
pump as part of an MPF are correct
sizing, and cost-effective packaging and
auxiliaries. Most multiphase pumps for
wet-gas service have been oversized.
Properly selecting the motor is a key to
unlocking the benefits of the MPF and
reducing costs. Typically, pad applications
have high production rates early in well
life and require little boosting. In later
life, the boost requirement increases, but
rates are lower.
It is important that the maximum expected rate and the maximum expected
boost not be paired when calculating
power. This can result in a motor that is
one or even two frame sizes larger than
required.
Speed control is another area that has
driven up costs. The variable frequency
drive housing needs to be fit-for-purpose
and appropriate. For instance, in arid and
relatively mild temperature conditions
such as in California, housing can be
simple coverings rather than air-conditioned buildings. A hydraulic torque converter is an alternative method for speed
control that can be of benefit for applications where power exceeds 250 horse-

FIGURE 4
Multiport Selector Valve for Well Testing

54 THE AMERICAN OIL & GAS REPORTER

power. Torque converters have been used
successfully with TSPs in both laboratory
settings and in the field.
The multiphase meter is another key
component at the heart of the MPF approach. Multiphase metering has become
a best practice for deepwater developments, but these three-phase meters often
are considered too expensive for use on
shore. Methods utilizing compact separation are most affordable, and are very
effective under the high-GVF conditions
normally seen in shale pad production.
The compact separator-based metering
approach uses conventional gas, liquid
and water-cut meters to reduce cost, and
a compact gas-liquid cylindrical cyclone
separator concentrates the liquid so that
it can be measured more accurately.
Automated well testing is made possible by a multiport selector valve (Figure
4). This valve enables a compact testing
manifold, which greatly reduces
piping/valves. It uses a single actuator to
route one well at a time to the multiphase
meter for testing.
Flow Assurance
The improvements in efficiency and
reductions of cost derived from an MPF
depend on solving the unique flow assurance issues associated with multiphase
flow in surface flowlines. There has been
great progress in four, broad, flow assurance categories:
* Flowline leak detection;
* Flowline blockage detection and
prevention;
* Flow pattern management; and
* Corrosion/erosion.
A pad-based development will have a
gas flowline connecting the pad to a gas
gathering system or central processing
facility. Therefore, some leak detection
method is required, typically a mass-balance method or a safety pilot. Multiphase
flow complicates leak detection since
mass-balance methods are not responsive.
Several methods are possible for multiphase flow. Internal flow-based methods
use pressure/temperature sensors and flow
meters to quickly detect leaks with minimal
additional costs. External, diffusion-based
methods allow very small leaks to be detected that are not detectable with traditional methods. Periodic pigging also can
be used to inspect flowlines to mitigate
risks. The bottom line is that leak detection
is possible to enable using an MPF.
Hydrates, wax, scale and other block-

American Oil and Gas Reporter - February 2017

Table of Contents for the Digital Edition of American Oil and Gas Reporter - February 2017