Guidelines & Recommended Practices
Selection of Artificial Lift Systems
for Deliquifying Gas Wells
Prepared by Artificial Lift R&D Council
1.6 Know Your Reservoirs
This describes the knowledge you must have about your reservoirs; their recoverable reserves, the primary drive mechanism(s), the static bottom-hole
pressure, etc.
-
Reserves
If you are considering a significant expenditure on artificial lift equipment, it is essential to know the “size of the prize.” In other words, how
much gas is in place in the reservoir, how much of this is recoverable,
and how much is recoverable by the artificial lift method(s) that are being considered? Clearly, the recoverable gas must be sufficient to justify the expenditure that is contemplated. Normally, this is not a question that can be answered by the Artificial Lift Staff, but will require input from Reservoir Engineering.

Gas in place
To determine the gas reserves, certain tools are required and
certain data is needed to make these tools work properly.
Some of the important variables are:
-
Area. This is the area of the reservoir, or more particularly
the drainage area that can be drained by the well of interest.
Is it 640 acres, 40 acres, or some different size? The drainage area available to the well of interest is a function of the
number of wells drilled in the reservoir, where they are located in the reservoir (e.g. up dip, down dip, etc.). Another factor is interference of offset wells. If offset wells are produced
at high rates, this may limit the amount of gas that can be
produced from the well of interest.
-
Height. This is the height or thickness of the reservoir. The
area times the height gives the volume of the reservoir that
can be drained by the well of interest. Clearly, the height is
not necessarily constant across the reservoir. The value
needed here is the average height.
-
Porosity. This is the portion of the overall reservoir volume
that is not filled with solid rock. In other words, this is the
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Page 2
pore space where gas may be accumulated. Again, the actual porosity may vary across the reservoir, so the average
value is needed here. The volume of the area times the
height times the porosity gives the volume of pore space in
the reservoir.
-
Water saturation. Some of the pore space may (will probably) be filled by water. This limits the amount of pore space
that is actually filled with gas.
-
Bottom-hole pressure. Gas is compressible. The actual
amount of gas is a function if the volume and the pressure.
At lower reservoir pressure (static bottom-hole pressure or
SBHP), the gas is expanded and less is available in the reservoir to be produced.
-
Bottom-hole temperature. Temperature also has a significant impact on the volume of gas. The higher the temperature, the greater the pressure of gas in a specific physical
volume.
-
Pressure build-up. A pressure build-up test is conducted by
measuring the reservoir pressure over time after a flowing
well is shut-in. The shape of the build-up curve, and the
amount of pressure build-up, can provide significant information about important reservoir properties. Pressure buildup surveys are not free. The well must be shut-in which
means that production is stopped. Also, there is always
some risk when pressure measurement instruments are run
in a well.
-
Production analysis – type curves – Topaz, RTA. There are
various tools available to evaluate pressure build-up data.
The Reservoir Engineer should have access to these tools
and know how to use them.
-
Darcy’s Law. Darcy’s Law is a well known method for calculating the flow of fluids from a reservoir.
-
P/Z calculations. A method for calculating remaining producible reserves is use of P/Z calculations. This process will be
described in an annex to this Recommended Practice.
-
Reservoir simulators – Eclipse. Modern, computerized reservoir simulators can be used to estimate remaining produc-
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Page 3
ible gas reserves. The Reservoir Engineer should have access to Eclipse or other reservoir simulators.
Selection of Artificial Lift Systems for Deliquifying Gas Wells
-
Page 4
Abandonment pressure
If the reservoir produces under depletion drive, it is important
to now the abandonment pressure. This is the minimum reservoir pressure where enough gas can still flow from the
reservoir to the wellbore to sustain an economical production
rate. Knowing the abandonment pressure may have a significant impact on the type of artificial lift system chosen to
produce the final remaining reserves in a reservoir.

Other factors that may affect the amount of gas that can be
produced
-
Fractures vs. permeability. A major factor in the production
of reservoirs is access to the gas in the reservoir by the
wells. Permeability describes the ability of the reservoir to
support flow of gas and liquids through the reservoir. If the
permeability is high, flow into the wellbore can be easy. If
the reservoir is very tight and has very low (or no) permeability, it may be necessary to fracture (break) the reservoir rock
to allow flow to occur. Now access to the gas depends on
the size, type, and quality of the fractures.
-
Reservoir drive mechanism – water drive, depletion. Another factor is the reservoir drive mechanism. If a reservoir has
a strong water drive, water may enter the reservoir as gas is
produced. This may maintain the pressure in the reservoir at
or near its original value. If the reservoir has no or limited
natural water drive, it may be possible to supplement this by
water injection. If there is no water drive or injection, the
reservoir pressure will decrease (deplete) as the gas is produced. This will reduce the drive mechanism to move the
gas to the wells.
-
Trends in pressure over time. It is important to keep track
and plot the trend in static bottom-hole pressure over time.
This provides important information on the type of reservoir
drive mechanism – water drive, partial water drive, depletion
drive, etc. It is also important information for use in reservoir
performance models, P/Z calculations, etc.
-
Multiple zones open in a wellbore. A major factor can be the
numbers of zones (or actually the number of separate reservoirs) that are open (perforated) in a wellbore. If more than
one zone or reservoir is open to the wellbore, this may lead
to unequal production or depletion of the separate zones, the
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Page 5
potential for cross flow or loss of liquid into lower pressure
zones, and other potential problems.
-
Inflow from Reservoir to Wellbore
Once the amount of gas in the reservoir that can be produced is
known, the next important factor is the ability of this gas to be produced from the reservoir to the well. A number of factors are important
in this determination.

IPR
One of the most important factors is the inflow performance relationship or IPR. This relationship describes the gas inflow rate
as a function of the operating or flowing bottom-hole pressure.
Since different types of artificial lift can achieve and sustain different operating or flowing bottom-hole pressures, it is very important to know the IPR for each well. And, in most cases, the
IPR is different for each well in the same reservoir, for many
reasons. Therefore, it is important to determine the IPR for
each well on a periodic basis. And a well’s IPR can and will
change over time as reservoir conditions and inflow conditions
change.

IPR curve – Vogel
There are various ways to describe the inflow performance relationship. One of the best known and simplest to use is the Vogel method. This is described in detail in an annex to this Recommended Practice. The Vogel method was developed for solution (depletion) drive oil reservoirs but it works acceptable well
for gas reservoirs.

Backpressure tests – Fetkovich
Another method is the one developed by Fetkovich.
wishes, this may be used in lieu of the Vogel method.

If one
Data needed to determine IPR
The following information is needed to determine the Inflow Performance Relationship for a well.
-
Bubble point pressure. The IPR curve behaves differently
above and below the bubble point pressure. Therefore, the
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Page 6
bubble point pressure and its relationship to the Static Bottom-Hole pressure must be known.
-
SBHP. The static bottom-hole pressure (SBHP) must be
known. This can be determined from a Pressure Build-Up
Test (discussed above) or from a SBPH pressure measurement. If the reservoir does not have a strong water drive, the
SBHP will change with time; so it must be determined or accurately estimated on a period basis. A frequency of at least
once per year is recommended.
-
FBHP. The flowing bottom-hole pressure (FBHP) must be
determined in conjunction with a well test. The best way is
to measure the FBHP during a stable well test. If this is not
possible, it may be possible to infer the FBHP based on fluid
level measurements or vertical pressure calculations,.
-
Production rate. The well’s production rate must be measured at the same time as the flowing bottom-hole pressure.
Ideally the well test should be taken during a stable production period, when the flowing bottom-hole pressure in the
well is stable and being measured.
-
Impact of type of reservoir on ability to obtain SBHP. In
some cases, the type of reservoir may impact the ability to
obtain an accurate static bottom-hole reservoir. If the reservoir is very tight, it could take a long time (days or weeks) to
obtain a fully built-up static reservoir pressure. In a fractured
reservoir, the SBHP measurement may only “see” the pressure in the parts of the reservoir that are in communication
with the wellbore via the fracture system. In some cases, it
may be possible to overcome these problems with special
pressure build-up analysis or reservoir simulators.
-
Inflow constraints – skin, damage, water blockage. Many
factors can affect inflow performance other than “normal”
reservoir parameters such as area, porosity, permeability,
etc. These may include skin, near wellbore damage caused
by invasion of drilling materials, blockage caused by water
invasion into the reservoir from the wellbore, etc. Detection
and treating of these is a science all by itself. If inflow constraints are expected, it is best to consult with an expert Production Technologist who is familiar with this type of diagnosis and treatment.
Selection of Artificial Lift Systems for Deliquifying Gas Wells
-
-
Page 7
Productivity – relative permeability – water imbibition into
reservoir. Since the primary focus of these recommended
practices is gas well deliquification, a major issue is the impact of water accumulation in the wellbore on the reservoir
and its productivity. This is not a perfectly well understood
subject, but it is well known that a continuous level or head
of water in the wellbore can allow or cause some of the water to enter or be imbibed into the reservoir. And it is well
known that higher water saturation in the near wellbore region will have a negative impact on the productivity of gas.
And, it is well documented that it can sometimes take a long
time to remove water from the formation once it has been allowed to accumulate there. All of this argues for the importance of removing water from the wellbore and not allowing it to stand or accumulate over the formation interval.
Other Important Reservoir Factors
There are a number of other factors that must be considered and understood to optimize gas recovery from reservoirs.

Reservoir treatments
It is important to know what reservoir treatments have been
conducted and if any are anticipated. By reservoir treatments,
such things as secondary recovery projects (e.g. water flood),
acid stimulations, fracture jobs, etc. are included. Each of these
can have a major impact on the productivity of the reservoir.

Gas properties
Clearly it is important to know the composition and properties of
the gas. This includes knowledge of such things as:
-
Gas/condensate ratio.
Gas/water ratio.
PVT properties.
Gas composition (hydrocarbon make-up).
CO2 content, if any.
H2S content, if any.
Paraffin content, if any.
Tendency to form scales
Solids content, if any.
Selection of Artificial Lift Systems for Deliquifying Gas Wells

Page 8
Hydrocarbon make-up of condensate
It is important to know the hydrocarbon make-up of any condensate that is associated with the gas. This can have a significant
impact on the choice of surfactants or other chemicals that may
be used as part of the artificial lift system.

Source of water – formation, condensation, fracture water
It is important to know the source of water that needs to be removed from the well. The reason is that the strategy to remove
this water may be very different depending on its source.

-
Is the water being produced from the formation itself? If yes,
this may mean that the reservoir is a water drive or partial
water drive reservoir and this may have a significant impact
on the choice of artificial lift system.
-
Is the water being condensed from the gas as the gas cools
during production? If yes, the primary strategy may be to
prevent the condensation in the wellbore.
-
Is the liquid water that was injected during a fracture job or
an acid treatment? If yes, one form of artificial lift may be
required to remove this water and another form of lift, or perhaps no lift at all, may be required once the treatment water
has been removed.
Water chemistry – chlorides, scale, specific gravity
What is the chemistry of the water? What is its chloride content? How heavy is it? Does it exhibit scaling tendencies?

Solids – sand, coal fines, scale, salt, clay fines
What types of solids are or may be produced with the gas? Is
there a tendency to produce sand, coal fines, scale, salt, shale,
or clay fines? .The presence of solids can have a significant
impact on the type of artificial lift system in that some systems
can handle solids much better than others. Also, if a well produces solids, it may be necessary to consider some method to
control the solids such as sand control, periodic treatments to
remove salt deposits, periodic treatments to remove or prevent
scale deposition, etc.
Selection of Artificial Lift Systems for Deliquifying Gas Wells

Page 9
Calculate potential gain if switching to another form of artificial lift
A question that always arises is how much production gain or
uplift will occur if a change is made in the artificial lift system.
This primarily depends on the reduction in flowing bottom-hole
pressure that can be obtained and sustained by the new artificial lift system. However, this is also affected by the impact that
the lower FBHP has on the productivity of the well and the inflow from the reservoir. In some cases, reducing the FBHP can
make a substantial gain in both current production rate and ultimate recovery. In other cases there may be almost no gain at
all. Clearly it is essential to understand which is the case for the
well under consideration.

Extra production – acceleration or increase in reserves
Another question that must be answered is whether an increase
in production rate will result in more gas production (an increase
in ultimate recovery from the reservoir) or will it result in recovering the same amount of gas faster (acceleration). This can be
important in the determination of the economics of the project to
improve the production rate of the well.

How and when to collect “good” reservoir data
There is often a debate about how and when to collect reservoir
data. Reservoir engineers want to know the reservoir pressure
and characteristics. Production and Artificial Lift Engineers want
to know well inflow performance. Operators want to maximize
production, minimize downtime, and minimize risk. These objectives can be in conflict with one another. A recommended
practice is for management, with input from all relevant parties,
to establish a reservoir management policy that states which information is needed, how it is to be obtained, and how frequently

Well test data – how to allocate production to wells
The flow rate of many gas wells is measured on a continuous
basis. If it is, and if the daily measurements are accurate and
are used, this information can be used to determine the actual
production from each well. However, in many cases, daily production is not measured on each well, or the daily measure-
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Page 10
ments are not carefully read and used. In such cases, it may be
necessary to allocate production to individual wells based on the
total amount of gas produced to a production station or other
bulk facility. The allocation may be made based on estimates of
production from each well from periodic measurements or well
tests, and some indication of “on production” time. If a production automation system is used to continuously monitor the
wells, there will be accurate information available on both the
daily production rates/volumes and any down (off production)
time. This data can then be used to accurately allocate production back to each well and its associated part of the reservoir.