Patent Application
20140144626
Not Applicable
The invention relates generally to a method and apparatus of producing steam and, more particularly, to a method utilizing untreated feedwater as a source for steam used in enhanced oil recovery. Superheated steam from treated water is contacted with untreated feedwater in multiple sequential stages to allow for a higher fraction of untreated water to be vaporized.
Conventional oil reserves are preferred sources of oil because they provide a high ratio of extracted energy over energy used for the extraction and refining processes it undergoes. Unfortunately, due to the physics of fluid flow, not all conventional oil can be produced. Additionally, as conventional oil sources become scarce or economically non-viable due to depletion, unconventional oil sources are being explored as a potential supply of oil. However, unconventional oil production is also problematic because it consists of extra heavy oils having a consistency ranging from that of heavy molasses to a solid at room temperature and heavy oils may also be located in reservoir rocks. These properties make it difficult to simply pump the unconventional oil out of the ground. Thus, its production is a less efficient process than convention oil.
As a result, enhanced oil recovery (EOR) techniques are often employed to increase the amount of heavy crude oil extracted. Using EOR, 30-60% or more of the original oil in place can be extracted. Additionally, EOR techniques can be applied in both conventional and unconventional oil reserves.
During EOR, compounds not naturally found in the reservoir are injected into the reservoir to assist in oil recovery. Simply stated, EOR techniques overcome the physical forces holding the oil hydrocarbons underground. There are many types of EOR techniques that are categorized by the injection: gas injection, chemical injection, microbial injection or thermal recovery. While there are many types of EOR techniques, reservoirs containing heavier crude oils tend to be more amenable to thermal EOR methods, which heat the crude oil to reduce its viscosity and thus decrease the mobility ratio. The increased heat reduces the surface tension of the oil and increases the mobility of the oil.
A summary of various EOR techniques is presented in Table 1.
While many EOR techniques involve injecting steam into underground formations, SAGD is the most favored form of EOR involving steam. It is especially useful for the recovery of semi-solid crude oil known as bitumen. In SAGD, steam is injected into an upper horizontal injection well, which creates a steam chamber, and mobilizes the oil at the edges of the chamber. The live oil then gravity drains to a lower horizontal production well, where the oil and water mixture is then collected. Large amounts of steam are needed for this operation, and in SAGD the steam to oil ratio (SOR) is typically about 3, and can easily go higher.
Currently, most steam generators used for EOR are small, portable, once-through type units fired with oil or gas. The most common type of SAGD boiler is the Once Through Steam Generator (OTSG), which generates steam through indirect heat transfer. As evidenced by the name, water/liquid enters the system and makes a single-pass by the heat exchanger, vaporizes as it travels and exits as a steam/vapor mix. The main advantage of OTSGs is its lower capital cost and ability to handle water with relatively high percentage of dissolved and suspended solids, and organic contaminants.
In SAGD, high-pressure saturated steam is produced in boilers and delivered to the wellpad, where it is injected into the SAGD reservoirs. However, the necessary water to oil ratio is very high. For every barrel of bitumen recovered, 2 to 4 barrels of water are needed.
Moreover, the steam condenses during contact and is coproduced with oil. Thus, both water separation and subsequent water treatment are necessary operations in heavy oil recovery. Because vast amounts of water are needed to generate the required steam, a method of recycling the produced water is essential for a cost effective, sustainable SAGD system. Furthermore, concerns about climate change have encouraged the development of ‘zero-emissions power generation.’
Produced water and brackish well water are the main boiler feedwater sources used for SAGD. But, both sources of water contain contaminants, particularly dissolved solids, which cause scaling or fouling of boiler systems. Fouling or scale from the contaminants can result in failure of boiler tubes, down time to blow-down of the boiler and/or loss of boiler efficiency.
Normally, an OTSG can produce about 75-80% quality steam from feedwater with total dissolved solid (TDS) levels of 3,000 to 8,000 ppm. This relatively low steam quality is necessary to maintain wet conditions in the OTSG tubes in order to reduce fouling and scaling, but results in high blow-down levels of 20-25%. Although OTSG feedwater has relatively high TDS levels, it still requires some treatment to reduce silica and hardness levels. This is typically accomplished by warm lime softening followed by ion exchange. This water treatment process represents a significant portion of surface facility capital costs, and has a significant economic impact on a SAGD operation. Thus, what is needed in the art is a method of recycling untreated water for steam generation without pretreatment, yet without fouling the boiler systems.
U.S. Pat. No. 4,398,603 describes a method of using low quality feedwater to produce steam. Here, feedwater is recycled and contacted with superheated steam to produce saturated steam and precipitated minerals. The precipitated minerals are removed by withdrawing a stream of waste water containing the minerals from the contacting vessel. However, this method requires a steam compressor that is not commercially available.
Other steam generating methods also result in large amounts of CO2 being formed and subsequently co-injected with the steam. US20120160187 discloses the use of an oxygen-fuel combustor as a steam generator, instead of a more traditional boiler system. This new steam generation system is able to use untreated water to produce 100% quality steam for EOR techniques. By using an oxygen-fuel steam generator, no chemicals are needed to treat the water, regardless of total solid content because the heating is direct, rather than indirect heating as in a boiler. The oxygen-fuel generator produces less than 100% quality steam and a brine containing contaminates. The brine is then removed via a steam separator, resulting in 100% quality steam. However, this process also generates CO2 that is used during the injection process.
Betzer-Zilevitch et al. (2010) disclose another “Direct Contact Steam Generation” system in which untreated water is heated by direct contact with combustion gases, as opposed to the non-direct heating seen in OTSGs. However, the resulting steam is again mixed with a high percentage of CO2, which is then co-injected into the well.
The presence of CO2 in the steam injection can be problematic for particular types of underground formations, and the combination of CO2 and water can produce a corrosive mixture of carbonic acids that attack the carbon steel typically used in injection pipes.
US20110061610 discloses a method of using water from waste streams. Here, the untreated water is preheated in a heat exchanger before entering a dryer, wherein input steam is used to indirectly dry (evaporate) the heated untreated water. This method reduces the amount of energy need to dry the untreated water while still producing high-quality water. The resulting steam is recycled in the dryer. However, during the drying process, the contaminants form a solid cake that, upon further processing, can be used to backfill the reservoir.
Thus, what is needed in the art is a method for generating steam that lowers water treatment costs and still avoids boiler fouling and the resultant costs, preferably a method that requires no pretreatment of untreated water is needed. Preferably, this method will also utilize current steam generator and steam/water separation methods without expensive modifications.
Embodiments of the invention describe a method of utilizing superheated steam to vaporize untreated water for use in enhanced oil recovery techniques, preferably SAGD. The vaporization occurs in stages, thus allowing for a greater fraction of untreated water to be utilized. In doing so, the water treatment cost of SAGD surface facilities are decreased.
Some embodiments meet one or more of the following objectives.
A general objective is the design of an apparatus and method for generating steam that is simple in design, economic to build, maintain and operate, and is sufficiently rugged for wellpad use.
Another objective is the design of an apparatus and method for generating steam from untreated water to reduce water treatment cost.
Another objective is the ability to reduce boiler fouling and any resulting boiler blow-down time.
Another objective is the adaptability of the present invention to steam generating systems currently in use with little modification.
In one embodiment, superheated steam, generated by a boiler or a furnace, is directly contacted with untreated water to vaporize some or all of the untreated water. The contaminants in the untreated water are removed as solids if all of the water is vaporized. Otherwise, the contaminants can be removed as a concentrated brine if only partial vaporization occurs. Both can be removed simultaneously in a suitable steam/water separator (such as a cyclonic separator) or solid and liquid separators can be used sequentially.
This results in a larger amount of steam that is significantly cooled with respect to the original superheated steam. The steam is re-superheated in an indirect heat furnace, and then directly contacted with more untreated water in a second stage. Again, the contaminants are removed. The process is repeated for multiple stages. At each stage, successively larger amounts of untreated water are contacted with superheated steam.
The initial superheated steam is heated to about 900-1000° F. before mixing with the initial untreated feedwater. The boiler or furnace used to generate superheated steam can be any commercial available unit capable of superheating steam.
Later stages of superheated steam (initial steam+steam from untreated water) are reheated to 900-1000° F. via a furnace. The superheated steam can either by produced in superheater coils placed in the radiant section of a boiler (common practice in power generation boilers), or in a stand-alone fired steam superheater.
In another embodiment, the mixing of steam and untreated water results in a wet steam plus liquid. Contaminants are then removed as a concentrated brine. This concentrated brine, removed at each step, is vaporized in a single mixer and solids are removed in a single filter. The use of a single mixer and solid filtration device can lower overall costs.
The contaminants are removed using well-known methods. In particular, for solids removal devices, cyclones and/or filters can be used. For a concentrated brine, liquid/gas separation devices such as gravity separators, centrifugal separators, and filter vane separators can be used.
In one aspect of the invention, untreated water with high levels of total dissolved solids can be used without any pretreatment step.
The term “boiler,” as used herein, denotes any means of indirectly producing superheated steam from feedwater before the initial contact of superheated steam and untreated water, wherein the heat source is water.
The term “furnace” as used herein implies indirect heating of steam to increase its level of superheat; wherein the heat source is a hydrocarbon such as gas or oil.
The term “untreated water” encompasses all water used for SAGD that has not undergone significant pretreatment to e.g., remove dissolved solids before being heated and includes sources such as feedwater, brackish water and water recovered from a production fluid.
The term “separators,” as used herein, mean any type of separation device used to separate components in different phases, i.e. solids/liquids, or liquids/gases.
The term “filter” refers to a device that separates solids from liquids (or solids) on the basis of particle retention and thus is size based.
The terms “mixer” and “contacting vessel” are used interchangeable and refer to the vessel wherein the untreated water and superheated steam are contacted.
As used herein, the term “superheated steam” means a water vapor that is 100% vaporized and at a temperature higher than its boiling point or at least 482° C.
As used herein, “steam” refers generally to water vapor although there may be some amounts of liquid water, water mist and solids therein.
“Saturated steam” is steam at the temperature of the boiling point which corresponds to its pressure; the term is sometimes also applied to wet steam, and the terms are used interchangeably herein. “Slightly saturated steam” is steam at a temperature 2.5-16° C. higher than its boiling point.
The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims or the specification means one or more than one, unless the context dictates otherwise.
The term “about” means the stated value plus or minus the margin of error of measurement or plus or minus 10% if no method of measurement is indicated.
The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.
The terms “comprise”, “have”, “include” and “contain” (and their variants) are open-ended linking verbs and allow the addition of other elements when used in a claim.
The phrase “consisting of” is closed, and excludes all additional elements.
The phrase “consisting essentially of” excludes additional material elements, but allows the inclusions of non-material elements that do not substantially change the nature of the invention.
The following abbreviations are used herein:
Embodiments of the invention provide a novel method of producing steam to be used in enhanced oil recovery techniques. In general, steam produced from a treated water source is superheated, and the superheated steam then used in one or more stages to directly vaporize untreated water. The resulting steam is easily separated from any solid contaminants using well-known solid filtration devices.
The method thus uses multiple stages for superheated steam/untreated water contact. Essentially, the initial vaporized untreated water and steam are superheated after the solids are removed and then directed to a second stage to mix with more untreated water. Again, the resulting vaporized untreated water is superheated and contacted with more untreated water in a third stage. In some embodiments, this process repeats multiple times, for a minimum of 3 stages, preferably a minimum of 5 stages. By using a staged vaporization, a higher fraction of untreated water can be converted to steam, thereby reducing water treatment cost associated with SAGD surface facilities.
In more detail, a treated water source is superheated to about 482-538° C. using a boiler or fired steam superheater. This initial superheated steam is then mixed with an untreated feedwater stream in a 2.5 to 4.5 ratio in a contactor vessel. This mixing results in a less heated steam and solid minerals or concentrated brine. The brine and solid minerals are removed from the less heated steam using a solid/liquid separating device or a liquid/gas separating device. The less heated steam is then re-superheated to about 482-538° C. using a furnace. This larger volume of steam is then mixed with a new amount of untreated feedwater in another contacting vessel. The process repeats at least two times, resulting in larger quantities of untreated feedwater being converted into less heated steam. After the final mixing, the less heated steam is injected in a well for mobilizing heavy oil.
The present invention is exemplified with respect to
Exemplary results, generated by process modeling, of the basic steam generation system depicted in
In this particular example, 20,000 barrel per day (bpd) of treated feedwater was converted into 538° C. superheated steam in a conventional boiler. That superheated steam is mixed with 6,000 bpd of untreated water in the first contacting vessel (‘mixer’). The resulting mixture is 26,000 bpd of slightly saturated steam that is about 316° C. and solid contaminants. As the slightly saturated steam is directed to the furnace, it passes a solids separation device (‘filter’). Once the solids are removed, the slightly saturated steam is re-superheated in the furnace to about 538° C. This superheated steam is directed into a second mixer with 7,900 bpd of fresh untreated water. The process repeats, with increasing amounts of untreated water being utilized at each stage.
Table 2 displays the fraction of treated and untreated water as a function of mixing stages. As shown, increasing the number of stages decreases the amount of treated feedwater needed. As such, more untreated water is utilized, thus reducing traditional water treatment cost of the facility.
The concentrated brine is separated out using a gas/liquid separator (‘Sep’). The brine can then be vaporized in a single mixer with the solids being removed via a single filter afterwards. Note, this differs from
After the brine is separated out, the remaining steam is directed into a furnace to be reheated. A second stream of superheated steam is added to the wet steam before it enters the furnace. This second stream is added to vaporize any droplets carried over from the mixers, which prevents the droplets from drying and fouling the furnace tubes. After being reheated, the superheated steam is streamed into a second contacting vessel with a new batch of untreated water and the mixing/separation process is repeated.
Some embodiments allow for the removal of both solids and concentrated brine. This design is similar to
The following references are incorporated by reference in their entirety.
Betzer-Zilvitch, M. “Integrated Steam Generation Process and System for Enhanced Oil Recovery,” Conference Paper, Society of Petroleum Engineers, GSUG/SPE 137633, 2010.
This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/731,242 filed Nov. 29, 2012, entitled “Superheated Steam Water Treatment Process,” which is incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61731242 | Nov 2012 | US |
