MINIMIZING FOULING IN STEAM ASSISTED OIL PRODUCTION

FIELD OF THE INVENTION

[0001] The invention relates to methods and systems for generating high-pressure steam with minimal or eliminated fouling resulting largely from boiling nucleation.

BACKGROUND OF THE DISCLOSURE

[0002] Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology for producing heavy crude oil and bitumen. It is an advanced form of steam stimulation wherein a pair of horizontal wells are drilled into the oil reservoir, one a few meters above the other. High-pressure steam is continuously injected into the upper wellbore to heat the oil and reduce its viscosity, causing the heated oil to gravity drain into the lower wellbore, where it can be pumped to the surface.

[0003] Generally speaking, high quality, high temperature and high pressure steam is required for the SAGD process. SAGD calls for 100% quality, 7,000-11,000 kPag and 238- 296°C temperature steam. Considering oil production volume, and the fact that at least 2 barrels of water are needed for every barrel of oil, the water requirements for SAGD are immense.

[0004] A once-through steam generator (OTSG), for example, generates around 75% to

80%) quality steam, which then goes through a series of liquid-steam separators (also called "flash drums") to increase the steam quality. An OTSG is a large, continuous tube type steam generator in which wet steam is produced at the outlet of the continuous tube. Feedwater is supplied at one end of the tube having low temperature, and then undergoes a preheating- evaporation cycle as it travels along the tube.

[0005] OTSG features a single pass of water through the generator coil, where the feedwater is heated and eventually vaporized. Typically an OTSG comprises a convection section (also called economizer section) and a radiant section. In the convection section, the feed water is pre-heated by heat exchange with a hot combustion gas, usually flue gas. In the radiant section, the majority of the feedwater/wet steam will be heated by the heat radiated from the furnace, resulting in about 80% quality steam, i.e. the mass ratio of water to steam at the outlet of the generator is about 1 :4.

[0006] A source of large amounts of fresh or brackish water and large water recycling facilities are required in order to create the steam for the SAGD process. Boiler feed-water (BFW) quality is critical due to organic/inorganic contaminants in the water that may cause fouling/scaling and lead to boiler tube failure.

[0007] Fouling is the contamination of the heating surface, and the build-up of contaminant will eventually decrease the heat-flux and thus the heating efficiency. Therefore the boiler has to be shut down several times a year to remove the fouling layer and/or repair the tubing. In addition to the repairing cost, the downtime increases the cost of the SAGD operation.

[0008] Nucleate boiling is considered one of the main reasons for fouling in heat transfer tubes. Nucleate boiling is characterized by the growth of bubbles on a heated surface, which rise from discrete points on a surface, whose temperature is only slightly above the liquid's temperature. In general, the number of nucleation sites is increased by an increasing surface temperature and by irregular surfaces of the boiling vessel.

[0009] Nucleate boiling takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux. For water, nucleate boiling occurs when the surface temperature is higher than the saturation temperature (T _s ) by between 4°C to 30°C. The critical heat flux is the peak on the curve between nucleate boiling and transition boiling.

[0010] If the heat flux of a boiling system is higher than the critical heat flux (CHF) of the system, the bulk fluid may boil, or in some cases, regions of the bulk fluid may boil where the fluid travels in small channels. Thus, large bubbles form, sometimes blocking the passage of the fluid. This results in a departure from nucleate boiling (DNB) in which steam bubbles no longer break away from the solid surface of the channel, bubbles dominate the channel or surface, and the heat flux dramatically decreases. Vapor essentially insulates the bulk liquid from the hot surface. [0011] DNB is also known as transition boiling, unstable film boiling, and partial film boiling. For water boiling as shown on the graphs in FIG. 7-8, transition boiling occurs when the temperature difference between the surface and the boiling water is approximately 30°C (54.0°F) to 120°C (216°F) above the Ts. This corresponds to the high peak and the low peak on the boiling curve. The low point between transition boiling and film boiling is the Leidenfrost point. See FIG. 8.

[0012] During transition boiling of water, the bubble formation is so rapid that a vapor film or blanket begins to form at the surface. However, at any point on the surface, the conditions may oscillate between film and nucleate boiling, but the fraction of the total surface covered by the film increases with increasing temperature difference. As the thermal conductivity of the vapor is much less than that of the liquid, the convective heat transfer coefficient and the heat flux reduces with increasing temperature difference.

[0013] Both forms of boiling can foul heat transfer surfaces, although the majority of fouling comes from nucleate boiling. Eventually, fouling will build to a level that it must be addressed. Boiler cleaning employs physical or chemical means to clean the water tubes, and those methods inevitably require shut off period until the cleaning is completed.

[0014] Therefore, there is the need for an improved heating/pre-heating scheme that can minimize or even eliminate nucleate boiling in the steam generating process, thereby minimizing the fouling issues and reduce the downtime and cost for SAGD operation.

SUMMARY OF THE DISCLOSURE

[0015] When fluid flows through a plain tube the fluid nearest the wall is subjected to frictional drag, which has the effect of slowing down the fluid at the wall. This laminar boundary layer can significantly reduce the tube side heat transfer coefficient and consequently, the performance of the heat exchanger.

[0016] A "turbulator" is a device inserted into the tubes of firetube boilers, shell and tube heat exchangers and other types of heat transfer equipment that helps to increase heat transfer efficiency. The heat transfer coefficient for liquids and gases flowing through pipes in heat exchangers tends to be limited due to a fluid boundary layer close to the pipe wall that is stagnant or moves at slow speed, thus acting as an insulating layer. Such heat exchangers are found, for example, in domestic central heating systems. This boundary layer can be broken or reduced in thickness if turbulators are placed in the pipe, which create a turbulent flow that reduces the boundary-layer thickness and thereby increase the heat-transfer coefficient along pipe walls.

[0017] Examples of turbulators for pipe flow are:

Twisted-tape turbulators, a twisted ribbon that forces the fluid to move in a helicoidal path rather than in a straight line;

• Brock turbulators, a zig-zag folded ribbon;

• Wire turbulators, typically an open structure of looped and/or entangled wires that extends over the entire pipe length. See e.g., US4481154.

[0018] Turbulators can also be put to use in certain internal combustion engines - particularly, a ramjet engine. A simple porous wire mesh placed in the diffuser of the ramjet can increase turbulence in the flow entering the combustion chamber, which aids in fuel mixing.

[0019] In Steam Assisted Gravity Drainage (SAGD) oil production operations, Once-

Through-Steam-Generation (OTSG) boilers are widely used to supply the steam needed to heat the hydrocarbons, thus mobilizing them for production. Because of the poor quality of boiler feed water in Surmont and other field operations, serious fouling problems have always been a problem inside OTSG economizers in the convection zone. To solve the problem, every 6 to 10 weeks, a pigging interval must be introduced to clean the fouling. Thus, the fouling problem greatly affects the productivity and economics of producing Canadian bitumen.

[0020] In some embodiments, tube inserts are in wire spring-like configurations (as shown in FIG. 1), and are a commercially available technology that has been applied inside the tubes of shell and tube heat exchangers used for heating tar oil when dealing with laminar or transient flows. Operation results have showed great success of increasing heat transfer and reducing fouling. [0021] The theory of these configurations is to introduce local turbulent secondary flows, which enhance the overall heat transfer coefficient and reduce the residence time of fouling precursors close to the hot surface. However this technology has not been applied in SAGD OTSG boilers to mitigate fouling, in which flows are already very turbulent.

[0022] In Canadian OTSG boilers, more severe fouling is often observed in economizer sections, which are composed of many heat exchangers. One of the hypotheses is fouling occurs at boiling nucleation sites with an increased concentration of foulants in economizers when preheating the boiler-feed water. Introducing this technology in economizer sections of the OTSG boilers to suppress nucleate boiling may prove to be a promising way to reduce fouling in these unique systems.

[0023] This technology may also help reduce the tube wall temperature and increase the wall shear rates, which both lead to fouling mitigation.

[0024] When using a coiled wire insert in the economizer, local flow induced by the inserts forms a highly turbulent flow near the tube wall. This will increase the shear stress at the wall, mitigating bubble formation. The nucleate boiling will be greatly reduced or even eliminated where the flow goes directly into a fully boiling regime. The higher shear stress near the wall also limits the adhesion of organics, thereby further decreasing the fouling rate.

[0025] The present disclosure provides a method of minimizing the fouling caused by nucleate boiling and/or transition boiling of the feed water in a steam generator. The current invention may reduce fouling by inserting a push-to-fit coiled wire insert into the pipes of e.g., the economizer section of the OTSG in the direction of flow, wherein the wires are coiled either regularly or randomly.

[0026] Coiled wire turbulators are easily fitted and removed. They are flexible along their length for ease of installation even where access is restricted. Elements are simply guided into place along the axis of the tube, although the exchanger tubes may require being in a clean condition before the elements are installed, as it is important that the loops on the elements have contact with the tube wall. [0027] Various coiling patterns are possible, and a few are shown in FIG. 2. However, a wire turbulator where the wire forms concentric circular loops that are twisted around a flexible central wire spine is preferred. These circular loops all slant in one direction, opposing the flow of the fluid. The slant of the loops makes it very easy to pull the turbulator through the tube in one direction, but is difficult to pull through the tube in the opposite direction. The fluid in the tube also presses the loops against the wall of the tube providing a tight pressure fit. The wire loops in the face of the fluid flow provides good turbulation at reasonable pressure drop given the low resistance of the cylindrical wire redirects the fluid flow within the tube and maximizes its impact with the tube wall.

[0028] Such turbulators can also be provided with a hook at one end for pulling through the tube. At the other end, depending on the fluid flow characteristics, an anchor can be provided. However, depending on the coiling pattern and flow characteristics, turbulators typically need no fixing in the tubes as wires grip quite tightly and do not tend to move with a normal flow. Also, where long lengths are needed, a tail can be left at the other end for joining on any follow up length. If needed, the coiled wire inserts can be removed, and the tubes pigged as needed. The coiled wire inserts themselves can also be cleaned in a chemical bath, if needed.

[0029] Wire can be any suitable metal, but should be nonreactive with the hydrocarbon and brine in the reservoir, which can contaminate feedwaters, as well as with any dissolved solids, cations, anions, and the like present in the feedwater. Typically, stainless steel wire turbulators are used, but copper, brass, galvanized steel, galvanized mild steel, or monel metal is also possible.

[0030] Because the feedwaters in SAGD applications are recycled and can have significant dissolved total solids, as well as a number of reactive contaminants, it may be beneficial to protect the wire from reacting therewith. Wire can thus be coated with a non- reactive, protective polymeric coating if needed. As another alternative, the turbulator could be made with a flexible polymeric wire or polymeric mesh, instead of metal.

[0031] The method therefore may minimize the fouling caused by nucleate boiling in the boiler. This can minimize the downtime of the boiler for repairing or removing the fouling, thereby increases the operating time. [0032] The invention comprises one or more of the following embodiments, in any combination thereof:

[0033] — A steam generator system for oil production, comprising a once through steam generator (OTSG) having: an economizer section for pre-heating feedwater said economizer fitted with a wire matrix turbulator; a radiant section for converting said pre-heated feedwater to steam; and an injection system for injecting steam into an oil reservoir; wherein elements a through c are fluidly connected.

[0034] — A steam generator system for oil production, comprising a once through steam generator (OTSG) having: an economizer section for pre-heating feedwater said economizer section fitted with a wire matrix turbulator; a radiant section for converting said pre-heated feedwater to steam said radiant section fitted with a wire matrix turbulator; an water and steam separator; and an injection system for injecting separated steam into an oil reservoir; wherein elements a through d are fluidly connected.

[0035] — An improved method of producing steam for heavy oil production, the method comprising heating feedwater in an OTSG sufficiently to make steam to pump into a wellbore and use in mobilizing heavy oil, the improvement comprising outfitting tubes in said OTSG with a turbulator and thus reducing fouling in said tubes as compared with the same OTSG lacking said turbulator.

[0036] — An improved method of producing steam for heavy oil production with reduced fouling of steam generator systems, the method comprising heating feedwater sufficiently in a steam generator system to make steam to pump into a wellbore and use in mobilizing heavy oil, the improvement comprising outfitting tubes in said steam generator system with a wire matrix turbulator and thus reducing fouling in said tubes.

[0037] — An improved method of producing steam for heavy oil production, the method comprising preheating feedwater in an economizer, then further heating said feedwater in an OTSG sufficiently to make steam to pump into a wellbore and use in mobilizing heavy oil, the improvement comprising outfitting tubes in said economizer with a wire matrix turbulator and thus reducing fouling in said tubes. [0038] — A method of minimizing fouling in an economizer section of an OTSG, comprising providing a wire matrix tube insert in said economizer section to alter fluid flow direction near the wall, thus decreasing the temperature gradient and minimizing nucleate boiling and fouling.

[0039] — A method of minimizing fouling in an radiant section of an OTSG, comprising providing a wire matrix tube insert in said radiant section where steam is generated to increase the heat transfer area and lead to a more uniform temperature distribution so as to minimize the risk of local burnout caused by high temperature gradients at the wall.

[0040] — The wire matrix turbulator can comprise a coiled wire, wherein subsequent coils are radially shifted around a central axis, and wherein all coils slant in a direction of fluid flow.

[0041] — The wire matrix turbulator comprises stainless steel wire, or a polymer coated wire, or a polymeric wire or mesh.

[0042] As used herein, "heat flux" is the rate of heat energy transfer through a given surface, in other words, the heat rate per unit area, whereas "critical heat flux" describes the thermal limit of a phenomenon where a phase change occurs during heating, which suddenly decreases the efficiency of heat transfer, thus causing localized overheating of the heating surface.

[0043] As used herein, "economizer" means the devices for reducing energy consumption in a steam-generating operation by preheating feedwater. Typically an economizer is in the form of heat exchanger where the thermal energy is transferred from a high temperature fluid (e.g., steam condensate, flue gas or other waste heat source) to the feedwater such that less energy is required to vaporize it. Economizers are mechanical devices intended to reduce energy consumption or to perform another useful function such as preheating a fluid. In boilers, economizers are heat exchange devices that heat fluids, usually water, up to but not normally beyond the boiling point of that fluid. Economizers are so named because they can make use of the enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler, thereby recovering more useful enthalpy and improving the boiler's efficiency. They are fitted to a boiler and save energy by using e.g., the exhaust gases from the boiler or other hot plant fluids to preheat the cold feedwater. It has been reported that approximately 35 to 50% of the total absorbed heat in OTSG is transferred in the economizer.

[0044] By "wire" herein what is meant is a long thin flexible thin flexible thread or rod.

Wires can be metal, plastic or both. Wires can also adhere to each other at crossing points, e.g., to make a "mesh."

[0045] By "wire turbulator" or "wire matrix turbulator" what is meant is that the majority of the turbulence is provided by wire, and not a zigzagged or twisted ribbon, although the central axis of a wire turbulator may be twisted wire or ribbon.

[0046] 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.

[0047] The term "about" means the stated value plus or minus the typical margin of error of measurement or plus or minus 10% if no method of measurement is indicated.

[0048] 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.

[0049] 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.

[0050] The phrase "consisting of is closed, and excludes all additional elements.

[0051] 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.

[0052] The following abbreviations are used herein: ABBREVIATION TERM

CPF Central processing facility

OTSG Once-through steam generator

SAGD Steam-assisted gravity drainage

Ts Saturation temperature

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] FIG. 1 illustrates a wire turbulator (above, right) and the resulting effects of fluid flow when a turbulator is used (below, right) as compared with a plain tube (below, left).

[0054] FIG. 2 illustrates a variety of wire coiling patterns.

[0055] FIG. 3 illustrates the operating principle for an OTSG.

[0056] FIG. 4 illustrates a typical OTSG system for SAGD, with preheater, economizer evaporator.

[0057] FIG. 5 shows an OTSG system fitted with a wire matrix turbulator. FIG 5A shows a partial OTSG system with a dashed box denoting the expanded portion used in FIG. 5B- 5C to show two embodiments of the present disclosure.

[0058] FIG. 6 shows another embodiment of an OTSG system as fitting in with other equipment typically found at or near a well-pad.

[0059] FIG. 7 illustrates the behavior of water on a hot plate. The graph shows heat transfer (flux) v. temperature (in degrees Celsius) above TS, the saturation temperature of water, 100°C (212 °F). From WikiMedia Commons.

[0060] FIG. 8 illustrates the boiling curve for water at 1 atm. From WikiMedia

Commons. DETAILED DESCRIPTION

[0061] The disclosure provides a novel method for generating steam for enhanced oil recovery with minimized or eliminated fouling caused by nucleate boiling and/or transition boiling. The disclosure also provides a novel system for implementing the method. It is believed that by using the method and system of the methods described herein, fouling in the steam generator due to nucleate boiling can be greatly reduced or eliminated, thereby reducing the operational cost and downtime for repairing and maintaining the steam generator.

[0062] In general, an improved method of generating steam for SAGD and other heavy oil production uses is provided, wherein a wire matrix turbulator is used in water heating tubing, thus minimizing or eliminating nucleate boiling.

[0063] FIG. 3 illustrates the operating theory behind an OTSG. The OTSG is a continuous tube heat exchanger wherein many tubes are mounted in parallel and are joined by headers thus providing a common inlet for feedwater and a common outlet for steam. Unlike other systems which utilize a drum boiler and natural circulation of water, the water in the OTSG is forced through the tubes by a boiler feedwater pump, entering the OTSG at the "cold" end and maintaining constant flow through the tube bundle. A heated gas, flowing in the opposite direction of the water (counter current flow), heats the tube bundle. As the water flows through the heated bundle, it changes phase along the circuit as it extracts heat from the gas flow.

[0064] FIG. 4 illustrates a typical OTSG system in simplified schematic. The cool untreated water (414) flows from the storage tank (406) into a preheater (405). The preheated feedwater (415) is then pressurized by a pump (408) before being introduced into the OTSG (401). The OTSG (401) has a continuous tube (407) for water that contacts with the flue gas (arrow) generated by a burner (402). After contact with the continuous tube (407), the flue gas (413) can be recycled for use in the preheater (405). Alternatively, the preheater can have its own burner and fuel feed supply (not shown).

[0065] Because the OTSG (401) typically does not have defined economizer and evaporator sections (though it can), these areas are approximated by the dashed boxes. The actual point of evaporation can occur at any point in the OTSG and is not confined to the boxed areas. The economizer section (403) heats the water up to, but normally not beyond, the boiling point. From there, the heated water enters the evaporator (404) where most is converted to steam. The resulting steam (416) can then be injected into a hydrocarbon reservoir.

[0066] FIG. 5 shows an illustration of two wire matrix turbulator designs from expanded portion (501) of the continuous tube (407). In this section, the wire matrix turbulator extends the length of the continuous tube (407). Though not shown, such a turbulator may extend from water inlet through the economizer section and the radiant section. The turbulator typically has hooks for retrieval and for connecting lengths of turbulators, but usually nothing is required for securing the device in the pipes. If necessary, the hook or other securing member can be used for same. In FIG. 5B, the flexible spine (503) of the wire matrix turbulator has concentric wire circles (502). Though hard to see, the concentric circles (502) are slanted in the opposite direction of the water flow, such that water flow tends to open the arrow or chevron, increasing the pressure of the wires against the tube walls. FIG. 5C shows a spiraling loop configuration of wire (512) around the flexible spine (513).

[0067] Fig. 6 displays another embodiment of a typical OTSG system with equipment typically found at or near a well-pad. Here, the OTSG (100) has a burner that converts fuel and air into heated flue gas (101) for heating metals tubes (102). Untreated water from a storage facility is pressurized and introduced into the OTSG (100). The water (110) flows through the metal tubes (102), is heated, and exits the OTSG (100) as a wet steam (112). A steam/water separator (104) separates the wet steam (112) into a stream of steam (113) to be injected into a well-pad and a blowdown stream (111). The blowdown stream (1 11) is recycled but it can also be purged.

[0068] In an exemplary operation, a shell and tube heater (125 a) preheats a feedwater stream (124) after it has undergone a warm lime softening/weak acid exclusion treatment (126). The preheated feedwater (124) is then heat in exchangers (125d) and (125 c) with produced gas (122) and produced liquids (123), respectively, to form stream (127). Separately, the blowdown stream (111) from the steam/water separator (104) is flashed in a drum (120) to produce stream (121), which is then heat exchanged with stream (127) in another heater (125b) before stream (127) is pressurized and introduced into the OTSG (100). [0069] The following documents are incorporated by reference in their entirety for all purposes:

Gwak et al, A Review of Steam Generation for In-Situ Oil Sands Projects, Geosystem Engineering, 13(3), 111-118 (September 2010).

US4481154