POTENTIAL
FOR THE DESALINATION
OF PRODUCED BRINE
Produced water desalination
(PWDS) technology
Desalination refers to the process of removing salts
from brackish water or salt water to produce potable
water. It is primarily considered a technique to produce
drinking water, but desalination technology has also
been used to produce water for various industrial and
agricultural processes. Simply put, desalination
technology separates salt water into two separate
streams: desalted water with a minimal concentration of
dissolved salts and minerals, and a liquid containing
the residual dissolved solids, referred to as the brine
concentrate. For every 100 gallons of seawater,
desalination can produce between 15 and 50 gallons of
potable water[22,23,24,25,and 26]. Depending on the type
of technology used, recovery rates are even higher for
brackish water. Because of this economic advantage,
brackish water desalination will be the most common
option in areas away from the Gulf Coast of Texas. While
the average salinity of produced water in Texas from
conventional oil and gas production is roughly twice as
great as seawater; many fields produce significant
amounts of brine that can be categorized as brackish.
With respect to unconventional O&G production, recent
studies by the Environmental Protection Agency (EPA) on
brine produced from coal bed methane, identified reverse
osmosis (RO) the method of greatest promise. The key to
RO desalination is pre-treatment. Texas A&M has
performed significant research in pre-treatment and
performed pilot field projects to test the A&M
technology in the field.
Desalination of highly saline brines by other
technologies is also technically possible.
Several different methods are
available to separate salt and other solids from
seawater. The two most common methods used today are
thermal desalination and membrane desalination. Thermal
desalination uses a very simple and natural process to
separate out solids: salt water is heated to produce
water vapor that is in turn condensed to form fresh
water. Some of the more specific desalination
technologies that depend on heat to produce water vapor
include multi-stage flash distillation, multiple-effect
distillation and vapor compression. Approximately half
of the desalination facilities in the world use some
form of thermal distillation.
The
Middle East Desalination Research Center’s (MEDRC)
Research Advisory Council is conducting research on both
thermal and membrane desalination technologies (Figure 8).
Ongoing research is being funded and new research is being
considered to bring these technologies closer to
commercialization in this area of the world where population
growth and lack of fresh water resources is even more common
than in the Western U.S.
The MEDRC goals are:
1.
Decrease the cost of desalination
2.
Develop productive partnerships and cooperation
3.
Develop sustainable desalination technologies
4.
Improve communications in the desalination
community
5.
Develop human resources for application of
desalination and foster international cooperation in
research activities, particularly among regional experts
6.
Utilize limited regional and international
research resources
7.
Maximize technology transfer
The research focus in the Middle East is upon potable
water systems for increased urban populations.
Regardless, the advantage of oil field brine PWDS (by
whatever technology) for providing fresh water resources
is that the RO concentrate brine can be re-injected into
petroleum formations and so utilize Class II injection
wells.
Later sections in this will address the technical,
economic, and environmentally feasibility of this type
of concentrate disposal.
Reverse Osmosis Desalination
for Oil Field Brine
Membrane
technology is the other major method used to desalinate
salt water. Like thermal technology, membrane
desalination is based on a simple concept: salt water is
forced across a membrane, producing potable water on one
side of the membrane, and leaving behind briny water on
the other side. The two most common types of membrane
desalination used today are electro dialysis and reverse
osmosis. Electro dialysis is a voltage driven process
that uses an electrical current to draw salts and other
solids through a membrane, leaving pure water behind.
With electro dialysis, ions travel through the
electrically charged membrane, which differs from
reverse osmosis, where water molecules are forced
through the membrane. Electro dialysis is not suited for
the removal of dissolved organic constituents and
microorganisms, which represents a serious drawback.
Instead of using an electrical current, reverse osmosis
membrane desalination uses high pressure to pump salt
water through a semi-permeable membrane, which acts as a
microscopic strainer, filtering out salts, minerals,
contaminants, viruses, bacteria, pesticides and other
materials. The membrane strains salt and other molecules
because they are too large to fit through the
microscopic pores.
The
technology most adaptable to PWDS is Reverse Osmosis
(RO) membrane technology. RO lends itself to scalable
systems and is a commercial process. The chief
difference for RO design in the oilfield is the care
that must be taken with pre-treatment.
RO
desalination technology has been chosen by Texas as a
preferred option of providing fresh water supplies for
the Gulf Coast. Cost of providing water resources have
been presented by three different agencies. The Texas
Water Development Board is investigating the potential
for similar RO desalination, this time from brackish
aquifer sources (BGW) in West Texas, where water
supplies are critically low. At present however, no cost
estimate for BGW desalination have been reported.
Pre-Treatment of Oil
Field Brine
The oil
industry refers to water pre-treatment as “water
conditioning” and routinely performs this process as a
necessary step to water re-injection. Since several
billion gallons of water per day are re-injected, the
practice of water pre-treatment is well established. A
water flood engineer faces the same concerns facing
those who are designing membrane treatment systems. Such
issues such as scale removal, biofilm suppression and
solids control must be handled in a cost effective
manner, otherwise the injection well plugs,
necessitating a costly workover.
Comparing
the cost of desalinating brackish oil field brine with
the costs of desalinating BGW shows that pre-treatment
of the oil field brine will be more expensive, but
concentrate disposal will be less expensive. Newer
desalination technology is also expected to reduce these
costs.
Pre-treatment to accommodate saline oil field brine
desalination is critical. The characteristics of the
materials, particularly oily water make pre-treatment
mandatory. Several methods of oil and solids removal
have been tested at the A&M facility.
Powered centrifuges are routinely used in offshore oil
production operations to remove oil and solids from
water before it is discharged into sea. Siddiqui [1]
tested the use of a centrifuge to reduce oil
concentration from the produced water as a pre-treatment
for desalination but found the power requirement to be
too high. Hydrocyclone separators have been developed
for more efficient oil/brine separation [27,28,and 29].
Effective hydrocyclones impart more than 100 g
centrifugal force at maximum efficient flow rate.
Systems are best for fluids with significant density
difference. Hydrocyclones work best over a narrow flow
range but have proven to be effective in high pressure
and medium pressure oil systems. This technology is now
considered to be the most reliable for offshore
applications in meeting the required level of oil for
discharge. Hydrocyclones have limitations in
low-pressure systems. The efficiency of oil removal with
a hydrocyclone unit becomes less due to the fact that
there is not enough pressure in the system to drive the
water. Consequently the water has to be pumped and as a
result the produced water becomes more difficult to
clean. Small oil droplets and the use of different
chemicals, makes the hydrocyclone option not very
effective in a number of gas condensate systems. Also
small density difference between the oil and water phase
solid particles present in the feed reduced the
efficiency of hydrocyclones.
Doyle [30] studied the
use of organoclay for the removal of dispersed oil from
water by adsorption and performed limited field tests
with this technology. For onshore operation,
vaporization of water using large surface area exposure
of water on water ponds is another option. Boysen et al.
[31] looked into the commercial feasibility of using
freeze thaw and evaporation process to treat produced
water. This approach may cause environmental impacts
relevant to the atmosphere as well as life around the
ponds.
Removal of Dissolved
Oil from Produced Water:
The technology for removing soluble components from
produced water has not been fully assessed and utilized
till date. Such technology does exist offshore, and it
has been used onshore only with a certain degree of
success. The technology for removing soluble components
can be based on extraction, precipitation, oxidation
process, or by per-vaporation system. All these
technologies require relatively large facilities to
handle the large volume of produced water offshore. Most
of these technologies involve the use of other chemicals
and solvents, use of additional power as well as
producing a concentrated waste stream. Activated carbon
has been used in the chemical industry for a long time
for the removal of dissolved organics from waste
streams. Some of the new technologies that are available
today for the removal of dissolved hydrocarbon
components from the produced water are MPPE system from
Akzo Nobel (www.akzonobel.com),
“Pertraction” technology (www.tno.nl)
and surfactant modified zeolites.microfiltration
Table 2 contains data from a test of pre-treatment of an
oily water stream with heavy biological contamination
using both oil absorbent and a new type of membrane
microfilter. This data was collected at Texas A&M
University using a specially designed portable unit that
monitors power usage as a function of treatment type,
water quality, and treatment time. Test results found
that contaminants could be removed for less than $1.00
per 1,000 gallons of raw water processed (power cost
only). Power cost is typically the largest expense in
membrane plant operations thus measurement of this cost
under field conditions should provide more accurate
estimation of a full size facility’s cost.Table 2. Pre-Treatment Costs:
Removing Contaminants from Waste Water
| Type of Pre-Treatment |
Kw Used |
Fresh Water Produced |
Power per
1,000 gal |
Cost* per 1,000 gal |
oil + biofilm
removal |
2.80 |
199.4 |
14.04 |
$0.98 |
| oil removal |
0.94 |
99.4 |
9.46 |
$0.66 |
| * = Power cost @ $.07
per Kwh |
Disposal of Materials
Removed from Brine during Desalination
Any form of
desalination treatment will include some means of
handling byproducts and waste removed during the
purification process. In
addition to brine concentrate, a desalination project
may generate solid waste in the form of sand, silt and
other debris found in the brine that must be filtered
out before it is desalinated by the reverse osmosis
membranes. The amount of solid waste generated by a
large-scale desalination facility is considerable. At
the Tampa facility, the pretreatment process produces
approximately 14 wet tons a day of organic material,
suspended solids and metals found in the source water. However, it is also possible to handle slurries
produced from the pre-treatment process with the brine
discharge directed to re-injection into the oil field.
Otherwise, if pre-treatment of raw water creates
solid waste, then disposal must be addressed. Quantities
could be significant.
Since historically one
of the major impacts of desalination has been the
problem of the disposal of the salts (“concentrate”) and
other materials removed from the source water, one of
the advantages of oil field brine desalination processes
is that these materials can be re-introduced back into
the petroleum reservoir where it originated. This brine
contains concentrated dissolved salts and other
materials. However, in the oil and gas industry, high
salinity brines are routinely injected into formations
for pressure maintenance and secondary recovery by water
flooding. Since water from desalination operations may
be injected into these oil- and gas-containing
formations, the estimated cost savings can be as much as
30% of the cost of operating the desalination unit. This
represents a significant cost savings for RO technology
that offsets any added pre-treatment needed for the oil
field brine. Fresh water available is therefore
available to communities in need of this valuable
resource. This opportunity for the disposal of salts and
other materials from water treatment processes is being
considered for a number of industries [32, 33].
To
illustrate the potential for disposal of brine in an oil
field, the Spraberry Trend in West Texas was selected
for a hypothetical brine disposal project. Spraberry
reservoirs originally contained 10 billion bbls of oil
in place (more than 2,000,000 M3). Less than
10% of this oil has been recovered [34]. The reservoirs
are between 5,000 and 8,000 ft. in depth and extend over
portions of Borden, Dawson, Glasscock, Martin, Midland,
Reagan, Sterling, Tom Green, and Upton counties.
(More than 230,000 people live in this area
including the cities of Midland, Odessa, and San
Angelo.) There are more than 10,000 wells in the
Spraberry reservoirs many of them operating in fields
which are being waterflooded. A significant number of
the injection wells in the Spraberry reservoirs take
water on a vacuum (no surface injection pressure). Area
rainfall ranges from less than 10” to 18” a year. All
three of the major cities in this area are currently
under restricted use of municipal water by households
and represent potential markets for desalination
facilities. There are also several waterways in the area
considered “impaired”. Figure 9 shows the Colorado River
Headwaters watershed (No 12080002, EPA). There are
numerous oil leases producing brackish brine water in
this watershed and an extensive infrastructure of
pipelines used to carry oil and gas to gathering
facilities and pipeline connections.
Another factor favoring alternate sources of potable
water in West Texas is that many communities already
have infrastructure developed for recycling waste water
from municipal water treatment facilities. An example is
Andrews, Texas. This city recycled 100% of its discharge
from municipal water treatment into landscape irrigation
for public parks, golf course and sports fields.
Communities like Andrews have the resources to
incorporate an additional source of water into their
distribution systems if such a source became available
[35].
Desalination of oil field brine has another advantage
that being a means of disposing of the brine
concentrate. Brine re-injection into producing
formations serves as an example of alternate waste brine
disposal for desalination. Byproducts from desalination,
regardless of the technique employed, contain
concentrated dissolved salts and other materials.
Figure
9 shows one of the impaired water ways classified by EPA
as “impaired” in the Colorado
River Basin of Texas. One of the proposed uses of fresh
water produced from the Spraberry Trend is stream
augmentation to reduce chlorides.
Disposing of this brine
concentrate for traditional desalination processes can
represent a significant fraction of the cost of
operating the unit to recover fresh water. Since in the
oil and gas industry, high salinity brines are routinely
injected into formations for pressure maintenance and
secondary recovery by water flooding, water from
desalination operations could be injected into these
oil- and gas-containing formations, the estimated cost
savings are significant.
Costs
of Reverse Osmosis Desalination of Oil Field Brine
Estimated costs for several seawater desalination
facilities along the California coast range from $2.25
to $3.70 per 1,000 gallons ($711 to $1171 per
acre-foot), a substantial decrease from the 1993 cost
estimates of $3.17 to $12.70 per 1,000 gallons ($1000 to
$4000 per acre-foot). During the same period, the cost
of water from other sources in California has steadily
increased. In 1991, the Metropolitan Water District of
Southern California (“MWD”) paid approximately $27 per
acre-foot for water delivered from the Colorado River
and $195 per acre-foot for water from the California
Water Project. Now, MWD pays an average of $460 per
acre-foot for delivered water [36].
In
Texas, the three proposed desalination facilities on the
Gulf Coast have cost estimates ranging from $3.58 to
$4.23 per 1,000 gallons ($1,000 to $1,300 per
acre-foot). These cost estimates include a
“transference” cost representing the cost to deliver raw
water to the RO facility and to delivery fresh water to
existing municipal water lines [36]. The estimates also
include amortization of the facility (~25 years) and
operation and maintenance costs.
The
economic justification for desalination of oil field
brine is entirely different than the cited examples. O&G
production savings would come from the deferred cost of
disposal of the excess brine from operating facilities.
Enhanced oil recovery processes also require water that
must have relatively low salinity. Rather than utilize
fresh water from ground water sources, the industry has
tried desalination of produced water extensively. One
large-scale program to desalinate brackish produced
water was in Crockett County Texas [32]. Marathon Oil
Company constructed and operated a facility producing
714,000 gallons per day (17,000 barrels per day) to
supply feed water for steam flooding operations. The
cost of the water treatment (no infrastructure costs)
was reportedly less than $2.50 per 1,000 gallons. The
steam flood was projected to boost oil production in the
Yates Field by more than 100,000 barrels of oil. The
facility was deactivated when more advanced oil recovery
technology was developed.
More recently, pilot tests of a produced water treatment
by membrane technology was performed in the Burgan
Field, Kuwait to test the removal of dispersed oil. Over
a five-month period the unit operated at an oil
rejection efficiency of 83% to 89%. [33].
Experience has shown that membranes can be effective
pre-treatment techniques and RO membranes can provide
desalination at less cost than the cost of brine
disposal. Testing has also shown that desalinating
brackish oil field brine is more expensive that
desalination of BGW but concentrate disposal will be
less expensive. Newer desalination technology is also
continuing its advance in the field of industrial, food,
and pharmaceutical industries.
The A&M Mobile Desalination Unit was constructed to test
both pre-treatment by membranes and RO desalination at
field sites. Different types of membranes are tested and
RO salt rejection efficiency can be determined directly.
It is equipped to run either single stage or multi-stage
membrane treatments and can be configured either for
parallel or series membrane flows. The unit is shown in
Figure 10 in Washington County, Texas in 2006.
Figure 10. The A&M Mobile Desalination Unit is shown at
a well site in
Washington county, Texas in early 2006. The unit took
brine from the fiberglass storage tank (shown on the
right of the picture) performed pre-treatment by
micro-filtration, then desalination by RO. Fresh water
was directed to the tank to the left rear of the unit.
In
addition to testing the capability of different types of
membranes, the unit has power transformers to utilize
oil field power and an electrical meter to measure power
consumption, one of the most cost factors in
desalination. The cost of desalination is directly
related to the power used to pump brine past the
filters. As salinity increases, power consumption rises.
Data from four different field sites are given for
comparison, collected on four types of saline feed
brines. Table 3 shows this comparison of electrical
power costs.
Table 3.
Representative power costs of desalination of oil field
brine.
|
Salinity of Feed Brine, tds
(ppm) |
Power Costs Kw Hr per 1,000 gal.
Permeate |
|
Pre treatment |
RO desalination |
Operating Cost. $ per 1,000
gal. |
Operating Cost. $ per bbl |
|
Contaminated Surface water
~1,500 tds. |
$.65 |
$1.25 |
$1.90 |
$0.08 |
|
Gas well produced brine ~ 3,600
tds. |
$2.50 |
$2.00 |
$4.50 |
$0.19 |
|
Oil well produced brine ~50,000
tds |
$2.20 |
$6.00 |
$8.20 |
$0.34 |
|
Gas well produced brine ~ 35,000
tds |
$2.00 (est.) |
$4.20 (est.) |
$6.20 (est.) |
$0.26 |
|
The information in the Table should be used for
estimates only. The prime performance monitor should be
salt rejection efficiency, then operating cost. Two
types of pretreatment micro-filters were used. In
addition a new low pressure RO filter was employed in
the oil well test. Salt rejection efficiency of the low
pressure membrane was lower than the filter used
earlier.
The energy cost of operating the desalination facility
represents roughly one-third of the total operating
costs. Using one of the examples given in Table 3, for
desalination on-site of brackish produced water from a
gas well, the total operating costs would be less than
$10 per 1,000 gallons of fresh water produced ($.42 per
bbl). For comparison, the operator of the well pays
approximately $1.50 per barrel to truck the water to a
commercial salt water disposal well. For this example,
the field data indicate that a dedicated desalination
unit on the site could reduce the water hauling volume
by 50% and the total water hauling costs by almost 20%.
For this example, the land owner was offered the fresh
water for no cost. Under some circumstances, the fresh
water represents income to the operator.
