Advances
in Automation Improve Gas Lift Oil Production
By
Larry Hester
Emerson Performance Solutions
Abstract
Today,
efficient gas lift oil production is more than mechanics. Field applications
have demonstrated improvements in production of between 5 and 10% can be
achieved by applying continuous automatic control along with optimization
techniques. A primary aim of optimization is to ensure that lift gas is
applied to each individual well at a precise rate required to achieve the
maximum production from the field, commensurate with the minimum consumption
of compressed gas.
Some
of the benefits of gas lift management systems are; increased oil
production, reduced compressed gas consumption and maintenance, and
personnel efficiency and safety. Applications of gas lift optimization
systems over the past 20 years are highlighted as progress continues toward
production increases and reduced gas consumption .
A
key factor to the success of these projects are the selection of a suitable
Remote Telemetry Unit (RTU) to manage the wellhead activities. The
capabilities of the field equipment are the focus of this article.
Introduction
Today,
efficient gas lift oil production is more than mechanics. Field applications
have demonstrated improvements in production of between 5 and 10% can be
achieved by applying continuous automatic lift gas flow control along with
optimization techniques. A primary aim of gas lift optimization is to ensure
that lift gas is applied to each individual well at a precise rate required
to achieve the maximum production from the field, commensurate with the
minimum consumption of compressed gas.
A
key factor in optimization success is the selection of a suitable Remote
Telemetry Unit (RTU) to manage these wellhead activities. The capabilities
of RTU's in enabling optimized gas lift production are the focus of this
article.
Basics
The
gas lift method of lifting crude oil is used in approximately 10% of the
world's oil wells. Indeed, in fields where significant quantities of
associated gas are present and produced solids are involved, it is the
preferred method of augmenting the natural reservoir pressure and thus
increasing production.
Because
the technique it involves comparatively compact equipment at the wellhead,
it is especially attractive in areas where space is at a premium, such as
offshore, and where access for maintenance is restricted.
Gas
lift itself typically involves separating the associated gas from crude oil
as it is produced and then compressing the gas to a pressure higher than the
reservoir pressure, typically 800 to 1200 psi. The compressed gas is
injected down the annulus between the outside well-bore casing and the inner
production tubing string and introduced into the base of the liquid column
in the tubing string via specialized down hole gas lift valves. The effect
is to 'aerate' the crude oil, reducing its density and causing the resultant
gas/oil mixture to flow up the tubing.
At
the surface the gas is once again separated from the crude oil for
re-compression and re-injection. Surplus gas over and above that required
for continued production is typically sent onwards for sale, although it may
be vented or flared during production upsets.
Historically,
a surface manual injection gas choke has been used to regulate the flow of
injected gas. A manual production choke may also be used to regulate the
production pressure at the wellhead. These basic controls are set by
operators who use their experience to make the appropriate adjustments on
visits to the wellhead at intervals which may be as long as several weeks.
In the interim, the settings remain fixed, irrespective of variations in the
condition and rate of production of the well and the availability of lift
gas.
Lift
gas flow control then is the first level of control. It is typically
implemented at the wellhead. The next level of automation is optimization,
wherein the rate of lift gas injected is determined with respect to all the
wells in the production system, ensuring the lift gas is optimally
distributed.
History
Initially,
fixed orifice flow beans or chokes were used to set the lift gas rate
according to the gas lift design. The design was based on predicted flowing
conditions or well tests. A pressure regulator was used to maintain the line
pressure. The change in injection flow rate due to low line pressure, choke
wear, or well conditions were seldom accommodated.
Optimization
techniques were arduous manual calculations. New injection flow rates
calculated were set by operators traveling to the wellheads to adjust the
gas control choke. One can imagine the setting change had to be significant,
before it was an operator's priority.
With
the advent of dumb RTU's operating over telephone and radio and the use of
DCS systems for gas lift control, operators at a central site were able to
remotely position injection valves by telephone, radio, or wire.
As
spreadsheet programs became available, the optimization calculations became
easier and the number of wells that could be optimized larger. Those
operators fortunate to have substantial computing resources, were
calculating injections schedules of flow rates by computer which were then
manually set from their central site.
Automatic
well testing, afforded by PLC's, RTU's, and DCS systems, improved the well
performance data, showing the production rate yielded by the gas injection
rate, upon which the optimization calculations were based.
Advancing
technology, such as RTU's capable of remote gas rate measurement and
control, provided autonomous wellhead gas rate set point control without the
need valve for adjustment by a central system. Recently intelligence has
been added to remote equipment to enable a number of gas lift wellhead
functions, further relieving the central control computer of remote duties.
Host
advances continue with PC-based programs now available to calculate the
optimum lift gas rates at various operating conditions and enhanced
flexibility afforded by continued movement toward open and compatible
systems. This makes optimization available to a larger number of producers.
Automation
and Optimization
It
has been demonstrated that significant improvements in production rates and
reductions in gas usage could be achieved by replacing manual control with
fully automatic control of gas lift operations. Field results prove that if
gas lift operations are optimized not only on a well-by-well basis, but also
across groups of wells or even entire fields, improvements in performance
are realized. A primary aim of gas lift optimization then is to ensure that
gas lift is applied to each individual well at a precise rate required to
achieve the maximum production from the field, commensurate with the minimum
consumption of compressed gas. Field applications have demonstrated
improvements in production of between 5 and 10% are achievable by applying
continuous automatic control and higher with continuous optimization
techniques.
A
reduction in compression costs and flaring are equally important benefits to
many producers. In addition, other benefits are realized with a gas lift
automation system:
-
safety
is improved, through reducing the need for personnel to visit wellheads
-
operations
manpower is more efficiently used
-
engineering
resources are better utilization
-
travel
to maintain or operate remote wells is reduced
-
operators
can quickly respond to upsets or problems
-
gas
lift valve life is increased
-
data
for historical trending and problem diagnosis can be archived
Wellhead
Control Required
A
key factor to the success of remote gas lift automation projects are the
selection of a suitable RTU to manage the wellhead activities. These devices
can be in the desert, off-shore on single well jackets, or in the jungle at
distances from the control room limited only by the communication system in
use, such as radio link, and have to be able to withstand the extremes of
ambient temperature, humidity, wind, and rain.
One
major operating company recently conducted a series of trials with different
vendors' RTU offerings to access their functionality and reliability in a
harsh desert environment. Fisher Controls' ROC312 (Remote Operations
Controller) was selected for the project, along with Rosemount transmitters,
Solarex solar powered battery packs, and Microwave Data Systems radio
modems.
The
unit selected is routinely used for gas lift automation. The ROC312 is an
advanced measurement and control device comprising a master control unit
with 6 built-in I/O points and expansion sockets for up to 6 I/O modules.
Larger ROC units can handle up to 64 I/O modules. I/O modules are available
for interfacing to analog, discrete, and HART devices of various types and
current ranges. Each contains its own signal conditioning and conversion
circuits. Modules can therefore be used in any combination and location in
the module racks.
Two
levels of programming are readily available making it easy to add gas lift
functions as requirements change. Function Sequence Tables (FST'S) a used
for most specific RTU programming needs. In addition, a range of standard
software applications can be supplemented by User-C Language tool kit which
allows for the creation of custom or proprietary programs and protocols.
This is typically a significant factor in selection of the ROC in wellhead
applications.
The
system is particularly suitable for operation in remote locations since its
power consumption of typically as little as one watt places minimal demands
on the solar powered battery pack.
At
wellheads, the ROC and power supply is normally, housed in a pole-mounted
enclosure. Data gathered and acted upon includes production and gas line
injection flow rates. Gas lift hosts use this data to monitor the gas lift
operation and, based on its simulation of the individual wells and of the
entire field, to compute updated control set points for the gas injection
rates yielding optimum production.
Other
Application Examples
Other
examples of ROC's used in gas lift include the following.
Advances
in Remote Equipment
As
well as managing injection and production related parameters, the ROC has
the capacity to gather and forward additional data on the condition of the
well and the wellhead equipment which could be used for control and
diagnostic purposes and to forecast the need for maintenance. They further
enhance the overall contribution of automation projects to raise project
value. Pre-Engineered systems further contribute to project value. Major
hardware components of pre-engineered systems are:
Remote
control gas lift automation equipment is available in several installation
configurations with orifice meter runs included and for existing meters.
Typical single well units, such as shown below, can be augmented to control
additional wells independently with a remote multivariable sensor and
control valve for each.
Typical
Packaged Gas Lift Remote Control Equipment With Integral
Meter
The
continued move by producers toward focusing on core business, while
out-sourcing related business activities means the availability of complete
engineered assemblies from an original equipment manufacturer is a welcome
solution. Field installation costs are reduced by having each wellhead
control unit sized, assembled, and tested before shipment to the field
locations for installation.
Intelligent
Control
A
number of functions specific to continuous gas lift are available due to the
ability to add application specific programs to intelligent remote
controllers at the factory, during integration, and in the field. Typical
gas lift controls and functions available are briefly described next.
-
Control
Type. The operator can choose between injection lift gas flow
rate control or casing pressure control. This selection is dependent on
the type of downhole valves being used.
-
Unloading.
Unloading provides an automatic means to unload gas lift wells
in a recommended method by ramping the measured casing pressure over
time in configurable stages. Improper unloading may result
in damage to the unloading valves that can prevent injection at the
desired depth, reducing production potential.
-
Allocation.
A local allocation function helps maintain the gas lift
injection rates for independently controlled wells as close to the
optimum economic operation solution as possible. It autonomously changes
the local control set point according to a locally stored allocation
curve or according to pre-defined alternate set points as supply
pressure varies. This function is used when individual controllers
are not part of a communications system and do not receive routine
optimum gas injection set points remotely, yet are supplied from a
common lift gas supply that reflects distribution system conditions.
The
controller calculates the incremental break-even production ratio of
produced oil versus injected lift gas based on operator entries for lift gas
injection compressor fuel costs, lift gas injection compressor maintenance
costs, other miscellaneous injection costs, water disposal cost, oil value,
other miscellaneous benefits, and production water cut.
Lower
Overall Costs
Benefits
of quality RTU's and remote supervisory and control systems used in gas lift
management systems include; increased oil production, reduced compressed gas
consumption and maintenance costs, and more efficient use of personnel.
Additional benefits are increased gas valve life, faster response to
changing well conditions or problems, reduced operator travel time, and
better record keeping for trending and diagnostics.
Developments
in gas lift optimization software and its deployment demonstrate the
viability of using real-time techniques in what once was a manual operating
environment. At the same time, Fisher Controls' ROC's for remote wellheads
prove electronic measurement and control equipment can be deployed in the
harshest of operating conditions.
The
industry has been pushing the envelope in application of gas lift
optimization systems for twenty years. Progress continues toward production
increases and reduced lift gas expenses.
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