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THE PURPOSE
Two types of injection wells are used in oil and gas
operations. Each type is used for a particular purpose.
The first and most common is an injection well used
to enhance production or stimulate production from
other wells. An enhanced recovery injection well may
be a formerly productive well whose production has
decreased over time or a well specifically drilled as an
injection well.
This type of injection well is used to
inject fluid into the oil producing rock displacing oil
towards a second well where the oil is extracted.
When the fluid injected into the well is water, the
practice is commonly referred to as "water flooding"
or "water injection".
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The second type of injection well is for disposal of fluids produced along with oil or gas. These
fluids may be nearly freshwater or, more frequently, water high in chloride called "brine".
This
water, or brine is produced with oil and gas and is pumped back into the ground into rocks that can
contain these fluids. In this way there is no discharge into the land surface or into nearby surface
water way and environmental impact is reduced.
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THE PROCESS
A tailor made process design
Both fresh and produced water which have to be
injected need to be treated in order to prevent
wells plug.
The treatment may involve oil traces
removal, suspended solids removal as well as
chemical conditioning.
The designer must have
wide experience in water treatment problems and
should master a wide range of technologies in
order to select the most suitable ones and to
conceive a "tailor made" system.
Unidro, with its 360 degrees scope on water
treatment and specific experiences on well head
processes is one of the partners to consider when
water injection is involved and can provide the
best combination of technologies for each case.
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Produced water treatment for reinjection
The oil/water emulsion from producing wells in a field are piped from the well heads to a
production facility, gathered into a manifold and sent to two or three phase separator vessels where
the free water is removed from the produced fluid. The water may be then sent to large tanks for
settling, where skimmers are often used to further remove oil.
The water stream leaving oil skimmers is usually sent to Produced Water Treatment Units for
further purification prior to re-injection or waste.
Treatment may require further oil removal of residual oil by CPI and/or flotators (DAF or IAF),
and also suspended solids reduction by filtration (Media Filters, Continuous Sand Filters). Final
filtration on cartridge filters may be required for additional wells protections.
Treated water is then pumped via distribution manifolds to the injection wells.
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Fresh water injection
When fresh water is available for injection into
wells to enhance oil production, a treatment is
normally required to reduce turbidity to a level
which does not cause wells plugging.
Limiting factors are therefore total SS content and
max sizes of particles. The treatment consists on
deep bed filtration using carefully graded media
which may be preceded by pre-treatment in case of
poor raw water quality at inlet.
Similarly to produced water reinjection treatment,
oxygen scavengers and scale inhibitors dosage my
be required as well as final cartridge filters.
Treated water is then pumped via distributing
manifold to the injection wells.
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THE TECHNOLOGIES
Primary separation: Deoiling with CPI
Water deoiling using Corrugated Plate Interceptors is
commonly applied to Produced Water Treatment, thus
saving space and improving efficiency compared to
API separators.
Unidro provides engineered units, designed case by
case to fit all process and project requirements.
CPI may be installed in atmospheric tanks, (normally
blanketed with fuel gas or nitrogen to prevent oxygen
from air to dissolve into the water) or in pressure
vessels, when the downstream system requires feed
pressure.
Material selection for vessels and for corrugated plate
packs is carefully evaluated for each case in order to
prevent corrosion and guarantee equipment lifetime
even in the severe conditions occurring when
produced water is handled.
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Secondary separation: Deoiling with flotator
Dispersed oil may be efficiently removed from
produced water by means of flotation, i.e. by injecting
a suitable amount of gas (usually air is not preferred
as oxygen should not be added to the water to limit
corrosion and also to avoid potentially explosive gas
mixtures).
Depending upon the method used to inject
gas into the water stream, we may have Dissolved
Gas Flotators (DGF) or Induced Gas Flotators (IGF),
the latter being less effective but easier to operate and
more widely used for Produced Water treatment.
Unidro IGF available technologies include
alternatively the use of injectors or turbines to mix the
gas into the water. Both systems being reliable and
efficiency proven, the choice to be made considering
the conditions at site (type, quantity and conditions of
the available gas, disposal facilities for the used gas
such as flare, safe location and so on).
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Tertiary separation: Deep bed multilayer filters
Unidro Multimedia Filters can remove more than 95% of suspended solids, down to 2 microns
size, as requested by the most stringent specifications for water injection. Also most traces of
insoluble hydrocarbons (up to 90%) can be removed.
As all Unidro units, Multimedia filters are not "shelf products" as they are 100% custom-designed,
to fit all needs of process as well as arrangement and space requirements of our customers.
Filter regeneration includes gas scouring (use of fuel or nitrogen gas is advised to prevent oxygen
to be added to water) and backwash with filtered water. In all cases where water consumption is to
be optimised, waste water may be recovered after backwash by treatment on Unidro Lamella
Settlers.
Material selection is made according to the handled fluids and ambient conditions and includes CS
with different types of lining as well as Stainless Steel.
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Tertiary separation:
Continuous sand filters
Where the water specification at injection point is not
so stringent as far as suspended solids content and
size, the more simple and cost saving Continuous
Sand Filters can be applied.
These filters do not
require the usual backwash facilities as pumps and
water hold-up and are driven by pressurised air or
gas, usually available on the spot.
The filters do not
require sequencers nor PLC/DCS inputs as their
operation is continuous and self adjusting.
Material of wetted parts may be selected to meet the
requirements of the handled fluids and of the ambient
atmosphere: SC, SS or FRP for the tanks, CS, SS, PP
or Ti for internals.
Both fresh water, sea water or
produced brine may be treated without any corrosion
problem.
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..and finally, back to well: Injection manifolds
Unidro experience in water system treatments has
been applied to the design and construction of the
high pressure distribution manifolds, thus providing
the complete package at the well water side.
The manifolds are fully preassembled on self
supporting skids, equipped with motorised valves
and, as required, flow meters to monitor flowrates to
each well.
The manifolds may be for water distribution to wells
or, where required, both for water distribution and oil
collection.
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