Note: Descriptions are shown in the official language in which they were submitted.
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HYBRID SYSTEM AND METHOD FOR TREATING PRODUCED WATER AND
SEA WATER TO BE RE-INJECTED INTO A SUBSEA OIL RESERVOIR
FIELD OF THE INVENTION
[0001] This invention relates to water treatment systems in offshore oil
production installations. More specifically, this invention relates to
treatment
systems of produced water and sea water for secondary recovery in oil wells.
BACKGROUND OF THE INVENTION
[0002] It is well known that, in offshore oil installations, one of the
techniques used in secondary oil recovery is the injection of treated
seawater. In
this context, it is known that seawater contains significant amounts of
sulfate ions
(SO4-2), around 2,800 mg/L. When seawater is injected into fields whose
formation
water (cognate water) contains enough Barium (Ba+2), Strontium (Sr+2) or
Calcium
(Ca2+) ions in solution, the contact between these two fronts normally causes
the
precipitation of its sulfates: Barium Sulphate (BaSO4), Strontium Sulfate
(SrSO4) or
Calcium Sulphate (CaSO4). These salts are extremely insoluble and cause
formation damage due to clogging of the pores with the precipitated salts.
They can
also precipitate in the production lines and equipment of the process plant.
[0003] Depending on the barium and strontium contents in the formation
water, it may be necessary to deploy a sulfate removal unit (URS) for seawater
treatment for injection into the reservoir, as shown in Figure 1. In the URS,
membranes of nanofiltration (which may be ceramic or polymeric) are used to
remove sulfate ions from seawater. Since seawater has solid particles as well
as
marine flora and fauna components, it is necessary to install filters upstream
of the
URS unit to improve its performance. The filtration is done initially with
coarse filters
and later cartridge filters of smaller flow diameter.
[0004] In the URS, water permeates through nanofiltration membranes
while a fraction, typically 25%, is concentrated in sulfate ions and separated
to be
future discarded at sea. To achieve the design specification for sulfate ions
in the
treated water, two sets of membranes are used in parallel followed by a third
set in
series, according to the schematic shown in Figure 2.
[0005] Once the water is treated by URS, it acquires the necessary
specification and can already be injected into the oil reservoir for secondary
recovery.
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[0006] Additionally, it is further known that the produced water
arriving at
the treatment unit is treated for removal of oil droplets. Conventional
techniques for
this type of treatment have, in general and simplified form, the configuration
shown
in Figure 1.
[0007] In particular, the produced water undergoes a treatment process
for
separating the aqueous phase from the oil phase composed of gravitational
separation, hydrocyclones and floats and is then specified for disposal at sea
in
accordance with current Environmental Legislation. Water that is not specified
for
disposal in some platforms has the possibility of being directed to a tank
called "Off-
spec Tank", where it will have a longer time to separate the oil phase, and in
some
cases may be reprocessed in the treatment plant.
[0008] This produced water treatment equipment, however, has a reduced
efficiency in the removal of solids particles and oil droplets of less than
5.0 pm. Such
conditions limit the overall efficiency of the treatment and, consequently,
the
obtaining of an effluent stream with characteristics suitable for more
restrictive
reservoirs in terms of suspended solids content, oils and greases. Therefore,
after
treatment, the produced water is specified for disposal at sea, and not
specified for
reinjection due to its content of suspended solids, oils and greases.
[0009] This way, nowadays the destination of produced water at offshore
oil production facilities after treatment is only the disposal. The low
efficiency of the
conventionally produced water treatment plants employed to obtain solids and
oil
contents according to the requirements required for the reinjection in the
more
restrictive reservoirs contributes, among other factors, to the infeasibility
of the
reinjection. Thus, in recent projects for secondary recovery this alternative
is still
ignored.
[0010] It is noted, however, that the development of a treatment system
that allows the reinjection of the produced water is a very interesting option
for the
oil production sector mainly due to the tendency of environmental legislation
to
become increasingly restrictive, in addition to moving towards increasing the
sustainability of industrial practices in this action area.
[0011] In this sense, the micro/ultrafiltration membrane splitting
technology
(with ceramic membranes) have proved to be an interesting option for this
challenge, since, when applied to the treatment of produced water, it results
in water
with low oil and solids contents.
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[0012] In the
micro/ultrafiltration membrane separation process, as known
in the state of the art, water permeates the membranes while a fraction of the
fed
volume accumulates the non-permeated oil and returns to the system in the
recycle
form.
[0013] The paper
entitled "Ceramic Ultra-and Nanofiltration Membranes for
Oilfield Produced Water Treatment: A Mini Review", by Ashaghi, K. Shams et
al.,
discloses a review study regarding the use of micro/ultrafiltration ceramic
membranes for treatment of produced water (removal of solids and particles of
oil).
Several techniques using micro/ultrafiltration ceramic membranes are presented
in
this scientific article, so that their description is incorporated herein by
reference.
[0014] The paper
entitled "Evaluation of membranes for the treatment of
water from the oil extraction process", by Weschenfelder, Silvio E. et al.,
one of the
inventors of this invention, discloses a study evaluating the performance of
membranes for the treatment of produced water by long-term tests with real
effluent,
taking into account the evolution of the permeate flow and the characteristics
of the
generated effluent. The results indicate that by using membranes with pore
size
equal to 0.1 mm it is possible to obtain a permeate stream with solids
contents of
less than 1 mg L-1 and contents of oils and greases in the range of 1 to 3 mg
L-1.
Further, such document discloses that with the chemical regeneration process
95%
reestablishment of the original permeability of the micro/ultrafiltration
ceramic
membrane is possible. The disclosure of this document is also incorporated
herein
by reference.
[0015] In a
current approach, if it were decided by the application of the
micro/ultrafiltration membrane separation process to complement the
conventional
treatment of the produced water to enable the reinjection, for example, an
additional
system would be required in the treatment plant, as described in prior art
documents
cited above. This brings significantly higher deployment, operation and
maintenance
costs and greater operational difficulty, as well as greater weight and area
occupied
on the platform.
[0016] Thus, it
is clear that the prior art lacks a produced water treatment
system which allows for reinjection without the need for an additional
treatment
system as known in the prior art.
[0017] As will
be better described below, this invention seeks to solve the
above-described problems of the prior art in a practical, efficient and cost-
effective
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manner.
SUMMARY OF THE INVENTION
[0018] This invention has the main object of providing a hybrid system
and
process for the treatment of seawater and production which allow the
reinjection of
the produced water without the need for an additional treatment system on the
platform.
[0019] In order to achieve the above-described object, this invention
provides a hybrid system for the treatment of produced water and seawater for
reinjection into an offshore oil reservoir, comprising (i) at least one inlet
of water to
be treated, (ii) at least two modules of micro/ultrafiltration water
treatment, each
module comprising (ii-a) at least one set of micro/ultrafiltration membranes
adapted
for removal of oils and solids from the water to be treated or (ii-b) at least
one set of
nanofiltration membranes adapted for removal of sulfate ions of the water to
be
treated, (iii) at least one treated water outlet, wherein the volume of water
to be
treated is directed to a water treatment module comprising membranes
micro/ultrafiltration or to a water treatment module comprising nanofiltration
membranes depending on the quality of the water in relation to the oils and
solids
content or the sulfate ion content.
[0020] This invention further provides a hybrid process for treating
produced water and seawater for reinjection into an offshore oil reservoir,
comprising basically the steps of (i) directing the water to be treated to a
water
treatment module comprising at least one set of micro/ultrafiltration
membranes
adapted for removal of oils and solids from the water to be treated or (ii)
directing
the water to be treated to a water treatment module comprising at least one
set of
nanofiltration membranes adapted for removal of sulfate ions of the water to
be
treated, wherein the volume of water to be treated is directed to the water
treatment
module comprising micro/ultrafiltration membranes or to the water treatment
module
comprising nanofiltration membranes depending on the quality of the water with
respect to the content of oils and solids or content of sulfate ions.
BRIEF DESCRIPTION OF THE FIGURES
[0021] The detailed description given below refers to the attached
figures
and their respective reference numerals.
[0022] Figure 1 shows a schematic diagram of a seawater treatment
system and produced water for injection and disposal, respectively, as known
in the
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status of technique.
[0023] Figure 2 shows a schematic diagram of an example of seawater
treatment for injection into an oil reservoir through a sulfate removal unit
(URS), as
known in the status of technique.
[0024] Figure 3 shows a schematic diagram of a treatment module
comprising nanofiltration or micro/ultrafiltration membranes in accordance
with the
preferred embodiment of this invention.
[0025] Figure 4 shows a schematic diagram of one of a hybrid treatment
system of seawater and produced water for reinjection according to the
preferred
embodiment of this invention.
[0026] Figure 5 shows a schematic diagram of a complete system for
treating seawater and produced water for reinjection comprising the hybrid
system
of this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] In the foregoing, it will be appreciated that the following
description
will depart from a preferred embodiment of the invention. As will be apparent
to one
skilled in the subject, however, the invention is not limited to that
particular
embodiment.
[0028] Figure 4 shows a simplified schematic diagram of one of a hybrid
seawater treatment system and produced water for further reinjection according
to
the preferred embodiment of this invention. Such a figure basically
contemplates
two intakes of water to be treated, namely one of produced water 2, with high
contents of oils and solids, and one of seawater 4, with a high content of
sulfate
ions.
[0029] The produced water is preferably stored in at least one tank 10
before being directed for disposal or treatment through the hybrid system of
this
invention.
[0030] Preferably, the seawater collected for treatment and subsequent
injection passes through a sequence of filters, being the first one provided
with
filtering elements with a thicker mesh and the latter having finer mesh
filtration
elements. Preferably, a first filter 12 retains particles up to 500 pm, a
second 14
retains particles up to 25 pm and a third up to 5 pm.
[0031] Preferably, both the produced water and the seawater collected
respectively arrive at least one manifold 18 consisting of a plurality of
water control
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valves which will enter each of the treatment modules 20.
[0032] Each treatment module 20 comprises at least one adapted set of
micro/ultrafiltration membranes (ceramic membranes) for removal of oils and
solids
from the produced water or at least one set of adapted nanofiltration
membranes
(ceramic or polymer membranes) for the removal of sulfate ions from seawater.
Thus, at least one manifold 18, through its control valves, directs the
produced water
into the modules comprising micro/ultrafiltration membranes and the seawater
drawn into the modules comprising nanofiltration membranes. Preferably, the at
least one manifold is subdivided into two manifolds, one for controlling the
inlet of
produced water in the modules comprising micro/ ultrafiltration membranes and
another for controlling entry of seawater into the modules comprising
nanofiltration
membranes.
[0033] Preferably, at least one manifold 18 is fluidly connected to the
two
water inlet ducts to be treated, namely one for produced water 2 and one for
seawater 4. Each of these inlet ducts, separately, is subdivided into a
plurality of
secondary ducts in parallel, a secondary duct for each treatment module. The
secondary ducts of produced water and seawater, prior to entering each
treatment
module 20, flow into a single inlet duct per module, downstream of each of the
control valves.
[0034] The control valves are positioned upstream of each treatment
module 20, so that each of the valves controls the entry of one type of water
to be
treated, namely produced water or seawater from each of secondary ducts.
[0035] Preferably, there is no mixing between produced water and sea
water prior to entering the treatment modules 20. That is, if the produced
water inlet
control valve is open, the seawater inlet control valve should preferably be
closed.
[0036] Preferably, each treatment module 20 comprises only one type of
membrane, namely nanofiltration or micro/ultrafiltration. Thus, preferably, if
a
particular treatment module 20 comprises only nanofiltration membranes, only
seawater will be directed thereto, the produced water inlet control valve
being
closed. Likewise, the produced water will be directed to a treatment module 20
comprising only micro/ultrafiltration membranes.
[0037] Each treatment module 20 is designed to allow interchangeability
between nanofiltration membranes and micro/ultrafiltration membranes. In other
words, each module may have its nanofiltration membranes replaced with
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micro/ultrafiltration (and vice versa) depending on the demand for treatment
of each
of the waters.
[0038] By way of example, it is to be expected that soon after the
implementation of the hybrid system of this invention there will be only
demand for
treatment of seawater through nanofiltration membranes, since there will still
be no
produced water. Thus, practically all the treatment modules 20 may be equipped
with only nanofiltration membranes. As the produced water is generated, the
demand for treatment of seawater decreases. In that case, the nanofiltration
membranes of the treatment modules 20 are being replaced by
micro/ultrafiltration
membranes.
[0039] Figure 3 shows in a schematic diagram details of a treatment
module 20 according to this invention. As mentioned, the treatment module 20
may
comprise nanofiltration membranes or micro/ultrafiltration membranes depending
on the type of water (produced or sea) that will pass through that particular
module.
Each module comprises at least one set 20 of micro/ultrafiltration or
nanofiltration
membranes. Preferably, like the URS of the status of technique, each module
comprises two sets of parallel membranes 20a, 20b followed by a third set of
series
membranes 20c.
[0040] Preferably, in the case of a treatment module 20 provided with
nanofiltration membranes, for the removal of sulfate ions from seawater, the
water
to be treated passes through the first two sets of nanofiltration membranes in
parallel, so that the largest fraction of the volume of treated water becomes
low
sulfate ions concentration and are sent to the injection in the reservoir.
[0041] The remainder of the water passing through the first sets of
membranes, concentrated in sulfate ions, is directed to the third set of
membranes
20c in series with the first two. This third set treats this more concentrated
water and
also generates a larger portion with a low concentration of sulfate ions,
which will
be mixed with the water treated by the first two sets of membranes, and a
smaller
portion extremely concentrated in sulfate ions that is normally discarded in
the sea.
[0042] The water with low sulfate ions concentration from the treatment
of
the nanofiltration membranes sets is used for injection into the reservoir,
but may
undergo further treatment steps.
[0043] In the case of a treatment module 20 provided with
micro/ultrafiltration membranes, for removal of oils and solids from the
produced
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water, the procedure is quite similar to the above. Preferably, the water to
be treated
passes through the first two sets of membranes 20a, 20b in parallel, so that
the
largest fraction of the volume of treated water is comprised of low
concentration of
oils and solids and is directed for reinjection into the reservoir.
[0044] The remainder of the water passing through the first sets of
membranes, concentrated in oils and solids, is directed to the third set of
membranes 20c in series with the first two. This third set treats this more
concentrated water and also generates a larger portion with a low
concentration of
oils and solids, which will be mixed with the water treated by the first two
sets of
membranes. The water with low concentration in oils and solids coming from the
treatment of all three sets of micro/ultrafiltration membranes is used for
reinjection
into the reservoir.
[0045] Depending on the quality of the water to be treated, each
treatment
module 20 may comprise more or fewer serial and/or parallel membranes sets.
Thus, it is pointed out that this invention is not limited to the
configuration of
membranes sets shown in Figure 3.
[0046] Still in the case of a treatment module 20 provided with
micro/ultrafiltration membranes, the minor portion from the third set of
membranes
20c, concentrated in oils and solids, may be directed to the inlet of the
treatment
module 20 as shown in Figure 3.
[0047] Alternatively, as shown in Figure 5 (complete diagram of the
offshore installation), the water concentrated in oils and solids (oily
recycle) may be
routed to the water treatment system for separation of the oily phase.
Preferably,
the water concentrated in oils and solids may be routed to some treatment
tank,
schematically shown in Figure 5 (treatment tank 24). This tank can be, for
example
an off-spec tank that is normally already used in produced water treatment
plants.
Alternatively, an additional tank may be provided for carrying out this step,
in
addition to the off-spec tank.
[0048] Optionally, at least one water outlet is provided in the lower
portion
of the treatment tank 24 for withdrawing water with low concentration in oil,
since
the oil, less dense than water, after a certain period will be concentrated on
top. The
water withdrawn through the outlet of water in the lower portion of the
treatment tank
24, which has relatively low or medium concentration in oils, may be
discarded, if
specified, or be directed to the hybrid treatment system according to this
invention
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where it will be routed to treatment modules 20 comprising
micro/ultrafiltration
membranes to undergo a new treatment for removal of oils and solids. The oily
concentrate remaining in the treatment tank 24, after removal of some of the
water,
is preferably directed to the oil and water separation system 23 for
utilization of the
oil in the production. This contributes to minimizing the discharge of oil
into the sea
and to a better utilization of the oil present in the produced water in the
total
production of the well.
[0049] This invention further provides for the possibility of performing
a
backwashing procedure of the membranes used in the treatment modules,
especially the micro/ultrafiltration membranes. Such a procedure can be
performed,
for example, by pumps (not shown) or manipulation of timed valves in the
treated
water line and in the feed line of each set. This procedure allows the
periodic
inversion of the flow in the membrane, cleaning it and maintaining its
performance.
[0050] Optionally, at least a first deaerator unit 28 is provided
upstream or
downstream of treatment modules 20 for deaeration of seawater, if necessary,
prior
to reinjection into the reservoir.
[0051] This invention further provides a hybrid process for treating
produced water and seawater for reinjection into the offshore reservoir,
comprising
basically the steps of:
a) directing the water to be treated to a water treatment module
comprising at least one set of micro/ultrafiltration membranes adapted for
removal
of oils and solids from the water to be treated; or
b) directing the water to be treated to a water treatment module
comprising at least one set of nanofiltration membranes adapted for removal of
sulfate ions from the water to be treated, wherein the volume of water to be
treated
is directed to the treatment module of water comprising
microfiltration/ultrafiltration
membranes or the water treatment module comprising nanofiltration membranes
depending on the quality of the water relative to the oils and solids content
or the
sulfate ion content.
[0052] It is further emphasized that all the treatment steps described
herein
detailed description apply to both the system and the process of this.
[0053] Thus, based on the above description, this invention provides a
system and process for treating seawater and production which allow the
reinjection
of produced water without the need for an additional treatment system on the
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platform. Further advantages are still achieved by this invention, such as the
reduction of offshore oil disposal by the more efficient treatment of the
produced
water and the reduction of installation, operation and maintenance costs
associated
with an additional system at the offshore installation.
[0054] Numerous
variations relating to the scope of protection of the
present application are permitted. Accordingly, the fact that this invention
is not
limited to the particular sets/embodiments described above is reinforced.
