Patent Grant
9809888
Field of the Invention
This invention relates in general to well production systems for subterranean resources such as oil or water, and in particular to equipment and methods for protection of metallic well casing from the corrosive effects of moist soil the well cellar area.
Description of the Related Art
Steel components of well production systems, such as the well casing, that are submerged in a corrosive environment require some form of protection to prevent corrosion. Cathodic protection (“CP”) systems, for example, are conventionally used to protect steel components of well production systems from corrosion.
One particular type of CP system is known as a galvanic anode cathodic protection (“GACP”) system. In GACP systems, steel structures can be protected from corrosion (“a protected metal”) by being positioned as a cathode in an electrochemical cell that includes an anode composed of a more highly reactive metal than the cathode. The anodes can be composed, for example, of highly reactive metals such as aluminum, zinc, or magnesium. The electrochemical cell includes an electrolyte (e.g., water or moist soil), and the anode and the cathode are positioned in the same electrolyte to provide an ion pathway between the anode and the cathode. In the electrochemical cell, the anode and the cathode are also eclectically connected, for example, by a conductive cable, to provide an electron pathway between the anode and the cathode.
When the protected metal and the anode are positioned in the electrochemical cell accordingly, the more reactive anode corrodes in preference to the protected metal structure, thereby preventing corrosion of the protected metal. Due to the difference in the natural potentials between the anode and the protected metal, by their relative positions in the electrochemical cell, when the anode corrodes, high-energy electrons flow from the anode to the cathode through the electrical connection, thereby preventing an oxidation reaction at the protected metal structure. Thus, the anode corrodes instead of the protected metal (the cathode), until the anode material is depleted. The anode in a GACP system is known as a “sacrificial anode,” and likewise, GACP systems are also known as “sacrificial anode systems.”
GACP systems are conventionally used for the cathodic protection of subsea pipeline due to the high conductivity of seawater and the ease at which galvanic anodes can be placed on the pipeline. On the other hand, GACP systems are not primarily used for cathodic protection of subterranean well casings because of the higher current output necessary to protect large metal structures surrounded by a highly resistive ground electrolyte.
Another type of CP system is known as a impressed-current cathodic protection (“ICCP”) system. In many ways, ICCP systems are similar to GACP systems—except that ICCP systems use less reactive anode metals needing to be connected to an external power source to provide greater current output. In the prior art, ICCP systems have been used for the purpose of protecting subterranean well casings in well production systems. Impressed-current cathodic protection systems employ D/C power (e.g., rectified A/C power) to impress a current between one or more external anodes (e.g., positioned in a subterranean anode bed) and the cathode surface (e.g., a well casing). The anode bed and the well casing are both buried in the earth, and are surrounded by a ground electrolyte (e.g., backfill for the anode bed or moist soil for the well casing).
Although ICCP systems are intended to protect the entire length of subterranean pipeline in typical well production systems, ICCP systems often fail, however, to adequately protect certain sections of the well casing, such as those sections that are shielded from the ground electrolyte. In particular, certain sections of the well casing, for example, those sections enclosed by a cellar (such as a circular metallic or non-metallic ring, such as a cement ring, installed at the wellhead base prior to drilling operations to secure the hole during drilling and are left in place during well production operations) at or near the earth surface, for example, are shielded from the ground electrolyte, and thus, are inadequately protected by conventional ICCP systems. Those certain sections of the well casings enclosed by a cellar can be, for example, the upper two meters of the well casing.
One conventional use of GACP for cathodic protection of subterranean well casings has been to provide a supplemental cathodic protection system to a relatively small number of well casing joints or sections at or near the surface of the well casings, leaving the remainder of the well casing to be protected by other means, such as an ICCP system. Accordingly, combined ICCP-and-GACP systems have been used to provide overall protection of the well casing as well as localized protection of the well casing sections in the cellar area. Conventional GACP systems used for this purpose have included standard cylindrical anodes, for example, two pre-packaged 60-lbs. magnesium anodes, positioned within the cellar. Because the relatively low current output of GACP systems compared to ICCP systems, and because of the relatively high level of non-homogenous electrolyte in the cellar area, there are unique disadvantages of using GACP systems in the cellar area. The electrolyte in the cellar area, which is also referred to as the “backfill,” can become polluted, for example, with various non-conductive or less conductive substances (herein referred to as “non-homogenous”) such as drilling mud, cement, or other foreign particles. A non-homogenous backfill disadvantageously increases the resistance within the electrochemical cell and reduces the effectiveness of the GACP system. A related disadvantage, is that due to increasing non-homogeneity of the backfill, the ongoing effectiveness of the GACP system is reduced over time and, eventually, the backfill must be replaced periodically to restore an adequate level of cathodic protection, which can be both time-consuming and costly, and failure to periodically replace the backfill has resulted in significant corrosion to metal structures in the cellar area, resulting in even more time-consuming and costly repairs of the upper well casing joints. There is a need in the art for improved cathodic protection systems for well casing sections in the cellar area that exhibit greater effectiveness in polluted backfill and reduce or eliminate the need to periodically replace the backfill.
Applicants recognize the foregoing disadvantages of conventional cathodic protection systems with respect to enclosed sections of well casings in oil and water well production operations. Where cement surrounding the well casing at the cellar area creates a shielding effect that disadvantageously impedes the current of existing ICCP systems, enhanced supplemental cathodic protection systems are needed to more effectively protect the well casing from corrosion in the cellar area. Removal of the cement surrounding the well casing can reduce shielding effects, however, such a process is dangerous, costly, and time consuming. Accordingly, Applicants provide an enhanced supplemental cathodic protection system that is more effective than are known uses of conventional GACP systems in the cellar area.
In view of the foregoing disadvantages recognized by Applicant, Applicant herein provides an enhanced cathodic protection system using bracelet galvanic anodes for localized protection of sections of well casing in the cellar area. The enhanced cathodic protection system described herein can more effectively protect the enclosed sections of well casings in the cellar area by overcoming the unique disadvantages of conventional uses of GACP systems in the cellar area. Embodiments of an enhanced cathodic protection system advantageously provide an increased tolerance to non-homogenous backfill, eliminate the need to remove any of the cement surrounding the well casing, and reduce or eliminate the need to periodically replace the backfill within the cellar area. Embodiments of the invention, for example, provide sacrificial anodes having a shape, structure, and configuration that provides decreased anode resistance compared to the conventionally-used cylindrical anodes. According to embodiments of the invention, for example, the shape and structure of the sacrificial anodes allows for a decreased distance between the anode surface and the cathode surface, thereby beneficially decreasing the resistance of the cathodic protection circuit. Also, according to embodiments of the invention, for example, the shape and structure of the sacrificial anodes allows for increased surface area of the anode, thereby beneficially decreasing the resistance of the cathodic protection circuit. Embodiments of the invention further provide an enhanced sacrificial anode assembly that is uniquely suited for existing well-casings in cellar area by allowing for a simpler and safer installation and removal of the sacrificial anode assembly.
An exemplary embodiment of the present invention includes a bracelet anode assembly to provide enhanced cathodic protection to one or more vertical well casing sections in a cellar area, the cellar area being bounded by a cellar ring and being partially filled with an electrolytic composition surrounding the one or more vertical well casing sections, the one or more vertical well casing sections in the cellar area defining a cellar-area well casing.
In such an exemplary embodiment, the bracelet anode assembly includes a plurality of arc-shaped bracelet anodes adapted to circumferentially surround a cylindrical subsection of an outer surface of the cellar-area well casing such that the plurality of arc-shaped bracelet are operable to be mechanically connected in a substantially circular tightenable bracelet form that, when tightened, is operable to clamp the plurality of arc-shaped bracelet anodes to a fixed vertical position on the cylindrical subsection of the outer surface of the cellar-area well casing.
In such an exemplary embodiment, each respective arc-shaped bracelet anode of the plurality of arc-shaped bracelet anodes includes an arc-shaped anode frame to provide mechanical support to the respective arc-shaped bracelet anode, the arc-shaped anode frame having one or more brackets at each distal end to allow a mechanical connection to be made to one or more brackets of adjacent bracelet anodes in the substantially circular tightenable bracelet form, each of the one or more brackets having a fastener hole therein to receive a fastener.
In such an exemplary embodiment, each respective arc-shaped bracelet anode of the plurality of arc-shaped bracelet anodes further includes an arc-shaped anode core being integrally connected to the arc-shaped anode frame such that the arc-shaped anode frame is substantially embedded within the arc-shaped anode core, to allow a surface of the arc-shaped anode core to substantially circumferentially surround the cylindrical subsection of the outer surface of the cellar-area well casing to operably provide an ion pathway through the electrolytic composition between the surface of the arc-shaped anode core and the outer surface of the cellar-area well casing, the arc-shaped anode core defining an anode screw hole therein.
In such an exemplary embodiment, each respective arc-shaped bracelet anode of the plurality of arc-shaped bracelet anodes further includes a plurality of fasteners to mechanically connect each of the plurality of arc-shaped bracelet anodes to the one or more adjacent arc-shaped bracelet anodes in the substantially circular tightenable bracelet form, each of the plurality of fasteners adaptable to be positioned through the fastener hole in the bracket at each distal end of the arc-shaped anode frame of the respective arc-shaped bracelet anode and further through the fastener hole in the bracket at each distal end of the arc-shaped anode frame of the one or more adjacent arc-shaped bracelet anodes, thereby allowing the substantially circular tightenable bracelet form to be operably tightened by torque applied to the plurality of fasteners;
In such an exemplary embodiment, each respective arc-shaped bracelet anode of the plurality of arc-shaped bracelet anodes further includes one or more metallic shorting screws, each of the one or more metallic shorting screws to be positioned through the anode screw hole of a respective arc-shaped anode core for each of the plurality of arc-shaped bracelet anodes so that the respective metallic shorting screw is operable to contact the outer surface of the cellar-area well casing, each of the plurality of metallic shorting screws thereby operable to be in direct electrical contact with the respective arc-shaped anode core such that that each of the one or more metallic shorting screws is operable to complete an electrical connection between the respective arc-shaped anode core and the outer surface of the cellar-area well casing to provide an electron pathway between the respective arc-shaped anode core and the outer surface of the cellar-area well casing, the electron pathway and the ion pathway completing an enhanced galvanic anode cathodic protection circuit.
Another exemplary embodiment of the present invention includes an enhanced cathodic protection system for a subterranean well casing having an upper vertical well casing section thereof in a cellar area and one or more lower well casing sections below the cellar area, the cellar area being bounded by a cellar ring, the vertical well casing section in the cellar area defining a cellar-area well casing.
In such an exemplary embodiment, the enhanced cathodic protection system includes an impressed current cathodic protection system comprising a subterranean anode bed surrounded by a ground electrolyte in communication with the one or more of the lower well casing sections, the subterranean anode bed adapted to provide a first ion pathway through the ground electrolyte to the one or more of the lower well casing sections, the subterranean anode bed further being electrically connected to the subterranean well casing through one or more cables and one or more cathodic protection rectifiers to provide a first electron pathway to the subterranean well casing, the impressed current cathodic protection system providing primary cathodic protection to the subterranean well casing.
In such an exemplary embodiment, the enhanced cathodic protection system further includes an enhanced galvanic anode cathodic protection system comprising one or more bracelet anodes being circumferentially mounted to the cellar-area well casing and being surrounded by a cellar electrolyte, the cellar electrolyte being bounded by the cellar ring and also surrounding an outer surface of the cellar-area well casing, the one or more bracelet anodes circumferentially surrounding the cellar-area well casing and providing a second ion pathway through the cellar electrolyte to the cellar-area well casing, the one or more bracelet anodes further being electrically connected to the cellar-area well casing through one or more shorting screws to provide a second electron pathway to the subterranean well casing, the enhanced galvanic anode cathodic protection system providing secondary cathodic protection to the subterranean well casing.
Yet another exemplary embodiment of the present invention includes a method for providing enhanced cathodic protection to a subterranean well casing having an upper vertical well casing section thereof in a cellar area and one or more lower well casing sections below the cellar area, the cellar area being bounded by a cellar ring, the vertical well casing section in the cellar area defining a cellar-area well casing.
In such an exemplary embodiment, the method includes the step of completing an impressed current cathodic protection circuit comprising a subterranean anode bed surrounded by a ground electrolyte in communication with the one or more of the lower well casing sections, the subterranean anode bed adapted to provide a first ion pathway through the ground electrolyte to the one or more of the lower well casing sections, the subterranean anode bed further being electrically connected to the subterranean well casing through one or more cables and one or more cathodic protection rectifiers to provide a first electron pathway to the subterranean well casing, the impressed current cathodic protection system providing primary cathodic protection to the subterranean well casing.
In such an exemplary embodiment, the method further includes the step of completing an enhanced galvanic anode cathodic protection circuit comprising one or more bracelet anodes being circumferentially mounted to the cellar-area well casing and being surrounded by a cellar electrolyte, the cellar electrolyte being bounded by the cellar ring and also surrounding an outer surface of the cellar-area well casing, the one or more bracelet anodes circumferentially surrounding the cellar-area well casing and providing a second ion pathway through the cellar electrolyte to the cellar-area well casing, the one or more bracelet anodes further being electrically connected to the cellar-area well casing through one or more shorting screws to provide a second electron pathway to the subterranean well casing, the enhanced galvanic anode cathodic protection system providing secondary cathodic protection to the subterranean well casing.
So that the manner in which the features and benefits of the invention, as well as others which will become apparent, may be understood in more detail, a more particular description of the embodiments of the invention may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which form a part of this specification. It is also to be noted, however, that the drawings illustrate only various embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it may include other effective embodiments as well.
The present invention relates to cathodic protection for one or more well casing sections enclosed by the cellar area of an oil or water well. As can be shown with reference to
As can further be shown with reference to
Below the cellar area 100 and supporting the backfill 104 is the earth subsurface 105, which includes earth media such as mud, rock, sand, reservoirs, and like subterranean earth media or structures. The cellar ring 101 extends through the earth surface 107 so that the cellar-area well casing 108 is substantially surrounded by backfill 104, atmosphere 112, or by backfill 104 and atmosphere 112. As can be shown with reference to
A conventional ICCP system used to protect the well casing can be shown with reference to
A conventional GACP system used to protect the enclosed section 108 of the well casing 106 in the cellar area 100 can be shown with reference to
Exemplary embodiments of the present invention providing enhanced galvanic anode cathodic protection to the cellar-area well casing 108 can be shown with reference to
Those having skill in the art will appreciate, however, that the invention is not limited to embodiments having only two substantially semicircular anodes being positioned horizontally opposed. Certain embodiments, for example, may have three or more arc-shaped anodes that can be positioned horizontally along the circumference of the well cellar-area well casing 108 and mechanically connected to surround a well casing and form a substantially circular anode assembly. Those having skill in the art will understand that the degree to which the anodes are required to be arc-shaped, in different embodiments of the present invention, will depend on the number of anodes used, the spacing between the anodes, and the circumference of the cellar-area well casing 108. Accordingly, certain embodiments of the present invention having many anodes, e.g., more than six anodes, can incorporate anodes that are flat or substantially flat. Furthermore, those having skill in the art will appreciate that the invention is not limited to embodiments in which the two or more arc-shaped anodes are equal in size or equal in arc length. The substantially circular anode assembly 400, as, perhaps, can best be shown with reference to
In other embodiments, however, the substantially circular anode assembly can include two arc-shaped anodes having a much shorter arc length, for example, approximately one-quarter of the outer diameter of the cellar-area well casing, and the arc-shaped anodes can flank the cellar-area well casing 108 like parentheses, leaving more substantial gaps along the circumference of the outer surface of the cellar-area well casing.
As can be shown with reference to
Returning to
Minimizing the horizontal length of the ion pathway can be shown in the cutaway plane with reference to the ion pathway 330 in
Accordingly, the substantially circular anode assembly can advantageously reduce the horizontal length of the ion pathway with respect to all points along the outer circumference of the well casing, which beneficially reduces the resistance through the backfill 104 to points along the outer circumference of the well casing thereby enhancing cathodic protection thereto. By reducing the length of the ion pathway, e.g., ion pathways 420, through the backfill 104 between the anode and cathode, the absolute quantity of pollutants in the ion pathway, for any given density of pollutants or degree of non-homogeneity, can be reduced. Further, by reducing the length of the ion pathway, e.g., ion pathways 420, through the backfill 104 between the anode and cathode, any given quantity of pollutants in the ion pathway has less effect on the resistivity of the backfill 104 through the ion pathway. Accordingly, the effectiveness of the enhanced cathodic protection system is increased, thereby increasing the degree of cathodic protection provided to the cellar-area well casing 108 and better protecting the cellar-area well casing 108 from corrosion.
Advantageously, the bracelet anodes 401 and 402 can be positioned so as to circumferentially surround the subsection of the cellar-area well casing that is most susceptible to corrosion, thereby most substantially minimizing the ion pathway length between the bracelet anodes 401 and 402 and the subsection of the cellar-area of the well most susceptible to corrosion. Accordingly, cathodic protection can be beneficially enhanced especially for a specific cylindrical subsection 410 as well as for the overall surface of the cellar-area well casing 108.
As can be further shown with reference to
According to embodiments of the invention, anodes used in the anode assembly 400 are preferably composed of magnesium or zinc. Although aluminum anodes are available—such as those used in applications for protecting sub-marine pipelines—aluminum anodes perform unfavorably in soil applications having low chlorides. Once exposed to oxygen in the atmosphere, aluminum anodes develop a hard, permanent, and intact surface oxide film (aluminum oxide). Where the application involves water rich in chlorides, the aluminum oxide will be removed in a chemical reaction with the chlorides. For soil applications, magnesium anodes can be either standard or high-potential magnesium, and zinc anodes can be of any type intended for soil applications.
In further detail, as can be shown with reference to
The structure of an exemplary bracelet anode 401 according to one embodiment can be shown with reference to
Band 503h, rod 503f and plates 503g and 503e, and brackets 503a and 503c can further provide a surface for the anode core 501 to mechanically connect. In the embodiment illustrated in
The construction of the bracelet anode assembly 400 can be shown with reference to
In even further detail, an elevation view illustrating the substantially circular shape of the anode assembly 400, can be shown with reference to
The actual torque on fasteners 507a-b and 509a-b will be a function of the well casing outer diameter, which also determines the diameter of the bracelet anode. In general, adequate torque requirements for a ¾ inch bolt having 10 threads per inch will be in the following range: plain steel 350-425 ft-lbs, galvanized steel 438-531 ft-lbs, and waxed steel 175-213 ft-lbs. The clamping force on the well casing will also be a function of the anode weight. For an exemplary anode weight for a bracelet anode assembly of 110 kg net, the shear force will be 110 kg for all four fasteners which will be 27 kg of weight per fastener. In an exemplary embodiment as can be shown with reference to
In an exemplary embodiment as can be shown with reference to
Embodiments of the invention provide a bracelet anode assembly for a cathodic protection system (a GACP system) including a vertical well casing, the bracelet anode assembly being tightenable around the well casing to advantageously allow for mounting the one or more bracelet anodes at a fixed vertical position on the vertical well casing. Embodiments of the invention also provide a bracelet anode assembly having one or more bracelet anodes that advantageously allow for mounting of the one or more bracelet anodes at a minimal distance from the outer surface of the vertical well casing. The effective distance between the bracelet anodes can be substantially equidistant from the outer circumference of the outer surface of the vertical well casing (e.g., for any circular cross-section of the outer surface of the vertical well casing), which advantageously provides an enhanced ion pathway between the one or more bracelet anodes and all points of the well casing thereby beneficially to decrease the resistance of the cathodic protection circuit and to provide a higher tolerance for non-homogeneity of the backfill.
Embodiments of the present invention beneficially position the anode to be as close as possible to the well casing so that current will discharge most directly from the anode to the cathode, minimizing the ion pathway distance through the electrolyte surrounding both the anode and the cathode. For any given density of pollutants or degree of non-homogeneity in the ion pathway, as can be shown with reference to
In other words, minimizing the distance between the anode and the cathode allows the enhanced GACP system to function more effectively in a non-homogenous electrolyte. By minimizing this distance, the resistance through the ion pathway is reduced for any given density of pollutants or degree of non-homogeneity of the electrolyte. Further, the effectiveness of the cathodic protection circuit can be maintained for higher densities of pollutants or higher degrees of non-homogeneity.
Even further, according to embodiments of the invention, the arc-shape of the sacrificial anodes allows for increased surface area of the anode, thereby beneficially decreasing the resistance in the cathodic protection circuit. By way of example, the formulas below such decreased resistance, assuming the use of sweet sand as the backfill 104 (sweet sand can be assumed to have an electrical resistivity of approximately 10,000 ohm-cm in its pure state). As a preliminary matter, those having skill in the art will appreciate that the any backfill 104 can be selected to most advantageously reduce the cost of repairs and work-over due to casing corrosion inside the cellar area or otherwise.
The difference between embodiments of the invention and known off-shore subsea applications of bracelet galvanic anodes becomes immediately apparent, using the McCoy formula as is set forth in equation [1], as the resistance through the electrolyte would be approximately 22.2 ohm for an anode bracelet 30 inches long for a well casing having a 30 inch outer diameter.
In contrast, for bracelet anodes of the same size in a subsea environment, the resistance through the seawater would be approximately 0.036 ohm, which can be shown using the McCoy formula as shown in equation [2] and assuming that the seawater has an electrical resistivity of approximately 16 ohm-cm. Accordingly, the anode resistance is approximately 600 times greater in the cellar area, where the electrolyte is one of a backfill soil rather than sea water.
The difference in resistance exhibited in the cellar area and the subsea environment illustrates the different considerations in structuring anodes as bracelets (i.e., being arc shaped and circumferentially surrounding the casing or pipeline) in the cellar environment on the one hand and in the subsea environment on the other hand. In the cellar area, embodiments of the present invention overcome high levels of resistance through the electrolyte for short distances, and the shape and positioning provided by bracelet anodes best overcomes highly resistive electrolytes. In the subsea environment, by contrast, the electrolyte is significantly more conductive and the shape and positioning of the anode as a bracelet is much less significant to the overall resistance of the circuit (due to the much longer lengths of pipeline protected). As is discussed below, a greater consideration in using bracelet anodes in the subsea environment may be for ease of off-site installation, which is irrelevant to cellar area applications.
Turning attention to the cellar area, the difference in resistance exhibited by conventional GACP systems and embodiments of the present invention further illustrate the advantages of using bracelet anodes therein. Discrete cylindrical anodes conventionally used in the cellar area, which are assumed to a length of 150 cm and a diameter of 10 cm, for example, would provide a resistance through the electrolyte is approximately 40.15 ohm, as can be shown by Dwight's Equation, as is set forth in Equation [3]:
It is evident, therefore, that the overall resistance of the galvanic protection circuit is related to the shape and dimensions of the anode, as well as the resistivity of the electrolyte around it (which is directly proportional to resistivity and inversely proportional to size). A bracelet anode assembly mounted directly to the well casing effectively minimizes the distance between the anode and the cathode and, therefore, minimizes the resistive effect of the electrolyte as a charge carrier. The shape of the bracelet anodes also results in a lower anode resistance compared to pre-packaged cylindrical anodes, which advantageously increases the effectiveness of the GACP system when used in non-homogenous backfill. Accordingly, the type of anode used is less significant in a homogenous electrolyte. The anode resistance provided by standard cylindrical magnesium anodes, for example, reduces the efficiency of conventional GACP systems operating in a non-homogenous electrolyte, such as in cellar-area applications, and has created the need to replace the electrolyte to restore efficiency. Accordingly, embodiments of the present invention eliminate any need to replace the cellar-area backfill once it becomes polluted. Likewise, embodiments of the present invention minimize the risk that such replacement may be necessary in order to ensure adequate cathodic protection is provided to the cellar-area well casing.
Embodiments of the invention further provide enhanced anode assemblies that are uniquely suited for installation on existing well-casing in the cellar area without the need for heightened installation precision or any electrical cable connections. For example, embodiments of the invention provide a means of making a direct electrical connection using a shorting screw, which is uniquely suited for installation on existing well casings in the cellar area.
As can be shown with reference to
Making a direct electrical connection through a shorting screw, such as shorting screw 522, is uniquely suited to cellar area applications, where it is critical to ensure that the anode and cathode are electrically connected using the shortest path and the path of least resistance. Making a direct connection using the shorting screws is advantageous because direct physical contact between the anode core 501 or the anode frame 503 (which is directly electrically connected to the anode core) and the well casing 108 does not provide an adequate or reliable electrical connection. For example, there may be gaps and accumulated matter between the anode core 501 or the anode frame 503 and the well casing 108.
Shorting screws are suitable for the cellar area because there, unlike in subsea pipeline applications, current-throw—which is the ability for the anode to throw current outwards to cover a large surface area of the cathode—is not a concern. For bracelet anodes installed on subsea pipelines, however, current-throw concerns can affect the quantity of anodes needed over a certain pipe length. For example, in subsea environments, the bracelet anodes are typically spaced approximately 150 meters apart (intended to protect 75 meters of pipeline in two directions) and they are fully and permanently surrounded with highly conductive seawater. Accordingly, for subsea pipelines, the operative distance of cathodic protection provided by a single bracelet anode is a function of the current demand by the pipe, which is governed by the quality of the coating on the surface of the pipe. In contrast, to cathodically protect well casing in the cellar area, only one bracelet anode assembly is required—the remainder of the pipeline outside of the cellar area is protected by an impressed-current cathodic protection system. Current-throw, therefore, is not a concern in the cellar area, and the direct electrical connection provided by shorting screws advantageously reduces the length and the resistance of the electrical path, thereby providing a more effective CP circuit uniquely suited for cellar-area applications.
Even further, using a screw to make a direct electrical connection between the anode and the well casing is particularly advantageous in the cellar area due to the shape of a screw. For example, sand or other solid particles (e.g., dirt, sludge, other pollutants) within the cellar area could, potentially, obstruct an electrical connection to the well casing. In particular, it is likely that such solid particles may accumulate on the well casing or be introduced during installation or the ongoing operation of the bracelet anode assembly. A shorting screw, for example, is beneficially thin in cross-sectional form, which thereby minimizes the surface area presented to make the direct electrical connection to the well casing, and thereby minimizes the risk of such an obstruction. In certain embodiments, the shorting screw can have a pointed tip, which advantageously increases the pressure that can be applied by the screw on contact with the well casing, thereby operably displacing or penetrating accumulation of foreign matter. Accordingly, using a shorting screw in embodiments of the present invention minimizes the likelihood of such an obstruction being an issue during the installation and operation of the bracelet anode assembly. Furthermore, the portion of the threaded shaft of the screw positioned within the anode core can beneficially increase the contact surface with the anode core to overcome accumulation of foreign matter (including any solid particles introduced to or present in the threaded holes of the anode core).
Even further still, the use of a shorting screw advantageously imposes minimal structural requirements for the bracelet anode to receive the shorting screw. The only structural requirement to accept the shorting screw is that the anode core and, optionally, the anode frame be drilled and tapped to receive the shorting screw. Such minimal structural requirements advantageously allows conventional equipment (e.g., the anode frames themselves as well as any molds, presses, jigs used in constructing the bracelet anodes) that is conventionally used for subsea pipelines to be used also to construct the anode core and the anode frame for the bracelet anode assembly.
Even further yet, the advantages of using a shorting screw to make a direct electrical connection between the anode core and the well casing include the low cost and the ease of replacing a shorting screw in the event that the shorting screw fails. Advantageously, the shorting screws should be readily available, and the shorting screw provides a single, integral component both to make the electrical connection (metal composition) and to secure the connection (threads and screw head).
According to an exemplary embodiment, shorting screws 521 and 522 are provided as shown in
Accordingly, in embodiments of the invention comprising shorting screws, such as shoring screw 522, the electron flow in the galvanic cathodic protection circuit can be shown with reference to
In certain embodiments, such as for water wells, the bracelet anodes may also be electrically connected to the cellar-area well casing 108 using a cable connection as a fail-over. As can be shown in
Embodiments of the present invention also include methods for installing the bracelet anodes 401 and 402 that are distinguished from conventional methods, for example, with respect to installing conventional bracelet anodes on subsea pipelines. For subsea pipelines, for example, bracelet anodes are conventionally mounted around a pipeline joint in a workshop, before the greater pipeline is installed in the sea. For subsea pipelines, conventional bracelet anodes are installed over insulation and are electrically connected to the pipeline by a cable connection. The anode cable is welded and tested in the workshop, and the finished product is a protected pipeline assembly including the pipeline joint, the anodes, and the anode cable. Conventional pipeline anodes are typically spaced on the pipeline so there is one anode assembly at every tenth pipe joint. The protected pipeline assembly is then transported to the installation location on an installation barge, where multiple pipeline joints, including one or more protected pipeline assemblies, are welded together to form a pipeline chain, which is then lowered into the water as a chain.
According to embodiments of the present invention, bracelet anodes 401 and 402 are installed on a pre-existing well casing in the cellar area and are not installed on the well-casing prior to installation at the well site. Installation is performed by first removing existing backfill 104 from the cellar area so that the bracelet anodes 401 and 402 can be positioned around the cellar area well casing 108 at a location of the cellar-area well casing where corrosion is most persistent. The bottom of the enclosed section 108 of the well casing 106 is, for example, is where the presence of water is typically most persistent and where corrosion is often most severe. Embodiments of the present invention do not require removal of any cement from surrounding the well casing 106. Absent embodiments of present invention, the cement surrounding the cellar-area well casing 108 and shielding the ICCP ionic current 200 from the conventional deep anode bed 202 disadvantageously leaves bare metallic well casing in the cellar area 100 unprotected. One known solution has been to remove cement to reduce or eliminate the shielding effect. Using embodiments of the present invention, however, the disadvantage is overcome because adequate localized protection can provided within the cellar area. Embodiments of the present invention therefor eliminate the need to remove cement, which can pose safety hazards for personnel operating in the cellar. Those having skill in the art will appreciate, however, that partial removal of cement may be desirable, for example, to facilitate installation of the bracelet anodes 401 and 402 at the bottom of the enclosed section 108 of the well casing 106.
Removal and replacement of backfill 104 for existing well casings is critical due to the likely presence in the backfill 104 of pollutants, such as cement rubble remaining after the cementing job in drilling the well. Cement rubble, for example, may contain numerous cavities, thereby impeding the flow of ions in the cathodic protection system. Because pollutants in the backfill 104 are, generally, not conductive, and because the size, distribution, or composition of the pollutants in the backfill 104 cannot known with certainty, replacement of the backfill to remove the pollutants advantageously allows the conductivity of the backfill, and the effectiveness of the GACP system, to be provided with greater certainty. After the installation of the bracelet anodes 401 and 402, which is described further herein, the backfill 104 can be replaced with sweet sand, for example, having no more salt content than 0.1% by weight and no more free-moisture content than 2.0% by weight (for example, the sweet sand shall be dried in preparation and screened through 2 mm mesh and handled to ensure mixture remains free from foreign matter). Using sweet sand as the backfill advantageously reduces the cost of repair and work-over due to casing corrosion inside the cellar area or otherwise. Even properly prepared and handled sweet sand, however, can become polluted over time; and embodiments of the present invention advantageously allow efficient, localized cathodic protection to the well casing in such a circumstance.
The bracelet anodes 401 and 402 can be mounted directly on the bare well casing, without any insulation between the anode and the well casing. One bracelet anode assembly 400 is sufficient to protect a single cellar-area well casing section. To install the bracelet anode assembly, a field operator performs the following steps: (i) excavate a half meter deep and half a meter wide “donut hole” around the well casing at the cellar area, for example, using a shovel; (ii) fix and clamp the two halves of the bracelet anode to circumferentially surround the well casing at an area of the well casing most susceptible to corrosion—this area will receive the strongest degree of cathodic protection, but cathodic protection will protect the entire length (approximately 2 meters) of the well casing section within the cellar area and surrounded by the backfill electrolyte; (iii) tighten the bolts to an appropriate torque, as described herein and as will be appreciated by those having skill in the art, in accordance with the bolt size, as shall be governed by the well casing outer diameter and the corresponding anode size; (iv) tighten the shorting screws (e.g., one for each bracelet anode, and two for the entire bracelet anode assembly) to make the direct electrical connection (i.e., a short) from the anode to the cathode; (v) add backfill to the excavated area around the anode using sweet sand as described herein; (vi) moisten the area around the anode using 10 liters of distilled water; (vii) test the potential of the anode and the cathode, independently, using a Cu—CuSO4 reference electrode and report/confirm the readings as described herein. Accordingly, the compressive force provided by the tightening of the bracelet anode assembly around the well casing can clamp the bracelet anode assembly at a fixed vertical position on the cellar-area well casing 108. Fasteners 507a-b and 509a-b, having been tightened, thereby secure the vertical position of bracelet anodes assembly 400 on the cellar-area well casing 108, and shorting screws, such as shorting screws 521 or 522, provide the primary electrical connection between the anode core 501 and 502 and the cellar-area well casing 108.
In further detail, it is notable that precision is not critical in positioning and installing the bracelet anodes 401 and 402, provided that the bracelet anodes 401 and 402 are positioned substantially near the bottom of the cellar-area well casing 108 where corrosion is most likely. Installation precision is not essential because the existing well casing 106 is already in place, and there is no risk of movement by the well casing 106 that could cause the bracelet anodes 401 and 402 to slip or to be jarred from the well casing, as is often the case for pre-installed bracelet anodes on subsea pipeline joints as they are laid into the sea. The application of bracelet anodes in the cellar area advantageously lacks precision mounting requirements, for example, as the application can tolerate gaps between the anode assembly and the well casing due to the generally static nature of materials and operations in the cellar area. Bracelet anodes installed on subsea pipelines, however, require highly precise mounting, as any gaps between the anode assembly and the pipeline could allow sliding of the anode when the pipeline assembly chain (including the anodes that are pre-installed) is lowered to the sea floor. Any sliding of the anode in subsea environments could be detrimental, for example, as the anode itself could be damaged or the cable connection could snap. Further, in embodiments of the present invention, any gap between the anode assembly and the well casing or any sliding of the anode assembly is tolerable because the shorting screws ensure a direct electrical connection regardless of the position of the bracelet anode along the length of the casing in the cellar area. The shorting screws, being readily removable, also allow the bracelet anode to be efficiently removed for periodic maintenance or anode replacement, avoiding the need to break or re-weld any cables. More importantly, the shorting screws beneficially allow avoiding a thermite weld near the wellhead, which could be dangerous due to the presence of combustible materials. Accordingly, subsea installation of conventional bracelet anodes on pipelines is a dynamic process that raises concerns different than those faced in the cellar area. In the cellar area, for example, the primary concerns in installation are efficiency and safety of on-site installation on static, existing well production operations.
According to embodiments of the present invention, the operation of an enhanced galvanic anode cathodic protection system is not critically dependant on the homogeneity of the electrolyte in the backfill 104 of the cellar area 100, and therefore, there is no need to periodically refurbish, restore, replace, or refresh the electrolyte in the cellar-area backfill. Because embodiments of the present invention position the anode assembly directly (physically) on the well casing, any variance in conductivity of the cellar-area electrolyte is insignificant, and does not affect the operation of the protective electrochemical cell. Furthermore, according to embodiments of the invention, enhanced cathodic protection of the enclosed section 108 of an existing well casing 106 can be provided using only one bracelet anode assembly for each well casing 106, as can be shown with reference to
An enhanced cathodic protection system and method to provide cathodic protection to a well casing, according to embodiments of the invention, can be shown with reference to
The primary cathodic protection circuit is provided by an impressed-current cathodic protection system, which includes a power source 201, a deep anode bed 202, the unenclosed section 111 (as the cathode), and an electrolyte in the earth medium 105 that provides one or more ion pathways between the deep anode bed 202 and the unenclosed lower section 111 of the well casing 106. The primary cathodic protection circuit includes transmitted ion current 200 through the ion pathway in the electrolyte in the earth medium 105, which, in part, surrounds the unenclosed lower section 111 of the well casing and, in part, the deep anode bed 202. The primary cathodic protection circuit also includes transmitted electron current through an electrical connection, such as the wire 203, between the power source 201 and the well casing 106. The primary cathodic protection circuit is provided, therefore, as a first line of defense against well casing corrosion for the entire well casing 106, despite acknowledged disadvantages in the cellar area caused by the cellar ring 101.
The secondary cathodic protection circuit is provided by a galvanic anode cathodic protection system, in particular, to overcome the disadvantages caused by the cellar ring 101. The secondary cathodic protection circuit includes bracelet anodes 401 and 402, the enclosed upper section 108 of the well casing, and an electrolyte in cellar backfill 104 present in the cellar 100. The electrolyte in the cellar backfill 104 provides one or more ion pathways 700 between the bracelet anodes 401, 402 and the enclosed section 108 of the well casing 106 through the electrolyte in the cellar backfill 104. The secondary cathodic protection system transmits ion current 700 through the cellar backfill between the well casing 108 and the bracelet anodes 401, 402 and transmits electron current through the shorting screw 522 (as can be shown with reference to
The secondary cathodic protection circuit differs from conventional galvanic anode CP provided for subsea pipelines in that the entire well casing is also protected by the primary cathodic protection circuit. Accordingly, the secondary cathodic protection circuit is not intended to provide protection to the entire well casing or even to a substantial length of well casing (in most cases, the length of the cellar-area well casing to be protected is approximately 2 meters or less). Because the secondary cathodic protection circuit is intended only to provide protection to the part of the well casing within the cellar area, the shape, structure, mounting assembly, and installation procedure can be simplified and/or enhanced specifically for cellar-area applications. The secondary cathodic protection circuit is further enhanced, for cellar area applications, over bracelet galvanic anode systems in subsea environments, for example, by incorporating a shorting screw to improve the efficiency and safety of installing the galvanic anode assembly on existing well casings, i.e., well casings installed and in operation as well as to improve the efficiency and safety of ongoing operations. In the cellar environment, a shorting screw can provide a direct electrical connection to the cathode that is simpler, more efficient, and safer to install and uninstall than alternative means of electrical connection, such as a welded cable. Likewise, disadvantages of using a shorting screw in subsea environments, such as the potential for installation breakage or reduction of current-throw, are not encountered in the cellar environment. The use of a shorting screw, furthermore, also distinguishes embodiments of the invention over the conventional use of galvanic anode CP in the cellar area, which, for example, employ discrete anodes not directly mounted to the pipeline. Moreover, the secondary cathodic protection circuit differs from conventional galvanic anode CP provided in the cellar environment in that an anode assembly is provided which can be mounted directly on the well casing to minimize the distance of the ion pathway between the anodes and the cathode. Minimizing the distance of the ion pathway, and thereby reducing the resistance thereof, advantageously increases the tolerance of the enhanced galvanic anode CP system to non-homogeneity in the backfill. Accordingly, for any shorter distance of the ion pathway, the enhanced galvanic anode CP system can tolerate greater resistivity of the backfill, for example, as a result of increased non-homogeneity.
In exemplary embodiments, the expected lifespan of the anode core is approximately five (5) years. Monitoring the performance, and therefore, life expectancy, of the anodes can be accomplished by taking pipe-to-soil potential measurements using a Cu—CuSO4 reference electrode placed in the soil. The anode core, according to embodiments of the invention, is not expected to encounter drastic changes in its current output during operation absent any drastic change in the operating environment. Examples of such drastic environmental changes include, for example, the soil around the casing being replaced with a very conductive/corrosive soil. In such a circumstance, the anode core would be expected to discharge more current and, hence, have a shorter lifespan. Absent any drastic changes, the anode is expected to have a relatively linear consumption rate over most of its operating life. Changes in the temperature of the fluid inside the casing, for example, are unlikely to significantly affect the anode consumption. Operatively, the anode is expected to undergo a fast polarization with a higher consumption rate over an initial period shortly after installation, and then the anode current output is expected to reach an equilibrium in which the anode current remains linear over the life of the anode material.
In the drawings and specification, there have been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation; for example, words related to numbering used herein—such as “primary,” “secondary,” “first,” “second,” “third” or other ordinal numbers—are merely descriptive and do not define or connote an order, sequence, or degree of importance. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification and as defined in the appended claims.
This application is a divisional of U.S. patent application Ser. No. 13/628,621, titled “System, Apparatus, and Method for Utilization of Bracelet Galvanic Anodes to Protect Subterranean Well Casing Sections Shielded by Cement at the Cellar Area,” filed on Sep. 27, 2012, which claims priority to U.S. Provisional Patent Application No. 61/540,849 filed on Sep. 29, 2011, the disclosures both of which are incorporated herein by reference in their entireties.
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| Number | Date | Country | |
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| 20150329974 A1 | Nov 2015 | US |
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 13628621 | Sep 2012 | US |
| Child | 14807255 | US |
