Patent Application
20190299162
The present disclosure relates generally to the field of water treatment methods, particularly for removing sulfate from water. The present disclosure further relates generally to the field of water injection into an oil and gas reservoir to enhance production.
In offshore waterflood operations, seawater is often injected into an oil and gas reservoir to increase the reservoir pressure and thus enhance production of oil and gas from the reservoir. When the seawater being injected has a high sulfate content and the formation water present in the formation contains barium, barium sulfate scale can be formed. When sulfate reducing bacteria are being introduced into the reservoir, injecting seawater having a high sulfate content can also result in severe reservoir souring issues. In some cases, very stringent requirements for sulfate content must be applied to seawater for use in waterflood operations. In some cases, for example, sulfate content is limited to less than 10 ppm in seawater injection water at all times. To achieve this, two-pass sulfate removal membrane (SRM) units are used. The use of a two-pass system increases both operating expense and capital expense as well as footprint on the offshore platform as compared with a one-pass membrane system. The use of a two-pass system undesirably involves additional membranes, an additional feed pump, lower recovery, additional chemical dosage, membrane cleaning and maintenance.
There exists a need for methods and systems that remove sulfate from water for offshore waterflood operations in a simpler, less costly manner.
In one aspect, the disclosure relates to a method for treating water high in sulfate. The method includes passing a feed stream of water having an initial sulfate content greater than 100 ppm at a temperature of between 10° C. and 45° C. through a nanofiltration membrane module and a reverse osmosis membrane module in series such that a first retentate stream from the nanofiltration membrane module is fed to the reverse osmosis membrane module. A first permeate stream produced from the nanofiltration membrane module has at least 90% lower sulfate content, at least 50% lower magnesium content and at least 30% lower calcium content with respect to a sulfate, magnesium, and calcium content, respectively, in the feed stream. The first retentate stream is passed to the reverse osmosis membrane module to produce a second permeate stream having at least 95% lower sulfate content, at least 90% lower magnesium content and at least 90% lower calcium content with respect to a sulfate, magnesium, and calcium content, respectively, in the first retentate stream. The first and second permeate streams are combined to form a treated stream containing less than 40 ppm sulfate. A second retentate stream from the reverse osmosis membrane module is removed as a reject stream.
In another aspect, the disclosure relates to a system for treating water high in sulfate. The system includes a nanofiltration membrane module for receiving a feed stream of water having an initial sulfate content greater than 100 ppm and forming a first permeate stream and a first retentate stream; a reverse osmosis membrane module located such that the first retentate stream from the nanofiltration membrane module is fed to the reverse osmosis membrane module and wherein the reverse osmosis membrane module forms a second permeate stream and a second retentate stream; and a conduit in which the first permeate stream and the second permeate stream are combined to form a treated stream containing less than 40 ppm sulfate.
These and other objects, features and advantages of the present invention will become better understood with reference to the following description, appended claims and accompanying drawings. The drawings are not considered limiting of the scope of the appended claims. Reference numerals designate like or corresponding, but not necessarily identical, elements. The drawings illustrate only example embodiments. The elements and features shown in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the example embodiments. Additionally, certain dimensions or positionings may be exaggerated to help visually convey such principles.
Typically, in sulfate removal units, there are two or three stages in each membrane pass depending on the target percent recovery. By “percent recovery” is meant the percentage of feed water which becomes permeate.
Referring to
Standard seawater sulfate removal membranes (SRM), which are nanofiltration (NF) membranes, cannot meet sulfate requirements in a single pass (one-pass) system. Therefore, a two-pass system is typically required. Referring to
In one embodiment, a nanofiltration membrane module, also referred to as a nanofiltration SRM, is used as a first stage and a seawater reverse osmosis (RO) membrane having higher sulfate rejection than the nanofiltration SRM is used as a second stage to improve sulfate rejection. Referring to
A reverse osmosis membrane module 120 is located downstream and in series with the nanofiltration membrane module 102 such that the first retentate stream 105 from the nanofiltration membrane module 102 is fed to the reverse osmosis membrane module 120. In one embodiment, a booster pump 114 is located between a retentate outlet of the nanofiltration membrane module 102 and an inlet of the reverse osmosis membrane module 120.
The reverse osmosis membrane module 120 receives the first retentate stream 103 and forms a second permeate stream 121 and a second retentate stream 122. The first permeate stream 105 and the second permeate stream 121 are combined in a conduit 123 to form a treated stream 124. The treated stream 124 can containing less than 40 ppm sulfate, even less than 20 ppm sulfate, and even less than 10 ppm sulfate.
Each of the nanofiltration membrane module 102 and the reverse osmosis membrane module 120 can have a plurality of membrane elements therein (not shown). In one embodiment, the reverse osmosis membrane module 120 has a number of membrane elements that is greater than 40% of a number of membrane elements in the nanofiltration membrane module 102. Typically, each membrane module will contain about 6 to 8 membrane elements depending on the permeate flux rate.
In some embodiments, shown in
In one embodiment, a method for treating water high in sulfate includes passing the feed stream of water 101 to the nanofiltration membrane module 102 at a temperature of between 10° C. and 45° C. The first permeate stream 105 produced from the nanofiltration membrane module 102 has at least 90% lower sulfate content, at least 50% lower magnesium content and at least 30% lower calcium content with respect to a sulfate, magnesium, and calcium content, respectively, in the feed stream 101. The first retentate stream 103 is passed to the reverse osmosis membrane module 120 to produce a second permeate stream 121 having at least 95% lower sulfate content, at least 90% lower magnesium content and at least 90% lower calcium content with respect to a sulfate, magnesium, and calcium content, respectively, in the first retentate stream 103. The first and second permeate streams 105 and 121 are combined to form a treated stream 124 containing less than 40 ppm sulfate. The second retentate stream 122 from the reverse osmosis membrane module 120 is removed as a reject stream.
In some embodiments, energy from the reject stream 122 can be recovered using an energy recovery turbine 126 or a pressure exchanger 127.
The percent recovery of the treated stream 124 relative to the feed stream 101 can be greater than 50%. The percent recovery of the first permeate stream 105 from the nanofiltration membrane module 102 relative to the feed stream 101 can be less than 60%. The percent recovery of the second permeate stream 121 relative to the first retentate stream 103 can be greater than 40%.
The treated stream 124 can have a salinity up to 60% lower than an initial salinity of the feed stream 101.
In some embodiments, the feed stream of water 101 can be pretreated such that the feed stream 101 contains no greater than 50,000 ppm total dissolved solids, 5,000 ppm sulfate, 2,000 ppm calcium and 2,000 ppm magnesium prior to contacting the nanofiltration membrane module 102. The pretreatment can be done by a suitable method selected from particle filtration, ultrafiltration membranes, clarifying, softening, primary, secondary and tertiary deoiling and/or desanding, using a pretreatment module selected from particle filters, ultrafiltration membranes, clarifiers, softeners, primary, secondary and tertiary deoiling equipment and/or desanding equipment. Chemical treatment including acid, caustic and anti-scalant may be used to mitigate membrane scaling.
The configuration shown in
The data in Table 1 shows that the sulfate level was reduced to about 10.6 ppm using seawater RO membranes in the second stage. By comparison, when the same Hydranautics Nano-SW membrane was used in the second stage, the lab tested sulfate in the combined permeate stream was about 18 mg/L. There was about a 41% reduction of sulfate when the Hydranautics Nano-SW membrane was replaced by the seawater RO membranes.
A simulation of a nanofiltration and reverse osmosis membrane process using the configuration shown in
Table 2 lists the TDS of various ions in mg/L for each of the feed stream, the two stage NF permeate stream and the hybrid NF and RO membrane stream. As can be seen from the data in Table 2, the sulfate concentration in the permeate was significantly reduced from about 9.5 mg/L to about 5.8 mg/L using the hybrid NF and RO membrane design (shown in
Overall low sulfate targets can be achieved through the use of the systems and methods disclosed herein. Operating expense, capital expense, footprint and/or weight can advantageously be reduced.
It should be noted that only the components relevant to the disclosure are shown in the figures, and that many other components normally part of a water treatment system are not shown for simplicity.
For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present invention. It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the,” include plural references unless expressly and unequivocally limited to one referent.
Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention.
