American Oil and Gas Reporter - September 2016 - 40

SpecialReport: Reservoir Optimization & Modeling
Finally, as opposed to a traditional
RTU, PLCs have native remote I/O functionality. This can save considerable installation cost. Also, when equipment skids are
added to a site, they can come with premounted and prewired I/O and instruments. Startup for these skids is as simple
as plugging an Ethernet cable into a
switch and configuring the I/O in the controller.
Life Cycle Management

The equipment components required on pad sites include separators, compressors,
vapor recovery units, lease automatic custody transfer units, and water transfer and
chemical injection systems. The size and scope of multiwell pads, and the amount of
equipment needed on location, require new solutions to manage and control production operations.

found a viable alternative in the form of
modular and scalable programmable logic controllers (PLCs). PLC technology has
been fine-tuned in industrial process control environments that are just as rugged
as upstream oil and gas production.
PLCs offer a number of benefits compared with RTUs. For one, they offer modularity and open-source architectures.
While some RTUs support modular hardware, balancing the module requirements
with application and control requirements
in an RTU is much more difficult than in
a PLC, and third-party modules for RTUs
typically are not available.
A wide variety of modules are available
for PLCs that enable monitoring and
control of a full range of field instrumentation. Communication support for many
network types also is supported through
modular configuration.
A second major benefit is scalability.
This is especially important when considering how a multiwell pad is put together. A single well often will be drilled and
completed at a pad site, and it may then
be months or even years before additional wells are drilled and more equipment is
installed and brought on line.
In these instances, equipment builders
want to be able to build equipment skids
off site and then simply plug them in and
configure them at the well pad. They want
to reduce the need for trenching, wire
pulling, wire termination, and panel and
40 THE AMERICAN OIL & GAS REPORTER

instrument installation in the field to help
cut costs. Scalability in both programming
and hardware is required, and that is exactly what PLC technology offers.
Historically, PLCs were viewed as a
"blank canvas" for automation and control.
Only engineering firms or experienced programmers knew how to start from scratch
and develop the control required in a PLC.
However, today's modern PLCs have libraries of predeveloped and documented
code that can be added quickly in an almost drag-and-drop fashion. Some PLC
vendors also have predeveloped upstream
oil and gas libraries that can be configured
on site. Oil and gas producers only need
to enable and configure the required data
from the human/machine interface to
start a PLC, or to add skid hardware into
an existing system.
This ease of programming can eliminate the need for a well technician with
specialized expertise to know the programming environment and write new code
when adding hardware. Moreover, program and configuration changes can be
made online in a PLC without shutting
down the process.
RTUs traditionally have not been able
to accept such changes without being taken off line and downloaded. This is a critical differentiator in a modern multiwell
pad environment because shutting down
the control system results in lost production.

Well pad life cycle management can be
a major headache for operations teams.
Wells can go through many flow states, including natural flowing, electronic submersible pumping, or progressive cavity
pumping and sucker rod pumping. The
flow type and life cycle can vary by well,
but wells in resource plays can require two
or more artificial lift methods in the first
year of production alone.
Multiple RTUs often are used simply
to control a well's various life cycles.
Switching a well from an ESP to gas lift
or a rod pump necessitates changing the
RTU, which usually means changing
communication drivers to the SCADA system because the physical RTU hardware
at the well likely will be from a different
vendor. Configuring the RTU to control the
application and setting up that hardware
at the system can be a painstaking process.
A PLC-based well pad can provide significant relief in this area, delivering efficiency and optimization benefits that operators never may have thought possible.
By using the modular and scalable I/O
hardware architectures that PLCs provide,
operators can install I/O modules where
instrumentation is located and send the
data to the PLC.
Regardless what I/O design approach
an oil or gas producer uses, the hardware
in the control system will remain constant,
communications to the SCADA system
will use the same drivers, and the system
will remain online and in control of all applications as updates are made. All the
while, there remains only one PLC program to maintain.
There also is the matter of vendor application support. Unlike the RTU-based
design approach, where vendors typical-

American Oil and Gas Reporter - September 2016

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