Deep well anodes for electrical grounding
Two related approaches are provided to controlling the relative permeability of ground water and gases developed in deep well anodic installations. The central metallic anode is encased and protected by a semi-permeable conductive cementaceous sheath, using as the sheathing material any of a number of compositions having the desired properties of permeability and conductivity. Alternatively, a conductive but impermeable anode is used in conjunction with a semi-permeable backfill material in a bore hole of a deep well anode installation, thereby achieving the same desired ends of controlling the relative permeability of water and gases in the installation.
This application is a continuation-in-part of U.S. patent application Ser. No. 10/643,149 filed on Aug. 19, 2003 and entitled CONDUCTIVE CONCRETE COMPOSTIONS AND METHODS OF MANUFACTURING SAME, which claims priority based on U.S. provisional patent application No. 60/404,129, filed on Aug. 19, 2002, both of which are hereby incorporated herein in their entirety, by reference.
BACKGROUND OF THE INVENTIONElectrical Grounding Techniques Generally
Various electrical grounding techniques are utilized throughout the world for the prevention of electrical damage to buildings and equipment. Such grounding techniques find numerous applications in such diversified areas as power and telecommunication systems, electronic equipment, fuel storage tanks, industrial installations, commercial and residential buildings as well as buried equipment such as pipelines. The grounding techniques are also used to protect the buildings or equipment from a variety of electrical hazards ranging from the rapid and intense, such as a lightning strike, to the slow degradation caused by electrochemical corrosion.
The established grounding techniques commonly involve the use of wires or rods of copper or other electrically conductive metals being attached to the installation requiring protection, after which the metallic rod is buried in a narrow trench or driven into the earth. In various such techniques, known as “cathodic protection,” an electrical circuit is established such that a direct electrical current flows into buried electrodes. In this art, the flow of electricity is such that the anode in the circuit is placed within the well, while the objects to be protected such as metallic structures or pipes etc., become the cathode. The art of electrical grounding may be thus be conveniently divided into two classes: “shallow trench” and “deep well” applications, with the latter technology being primarily used for cathodic protection. The shallow trench grounding method is described in greater detail in co-pending U.S. patent application Ser. No. 10/643,149, the entire disclosure of which is incorporated herein by reference, as are the disclosures of the prior U.S. patents of others specifically identified in the present specification.
One particularly valuable material used in both shallow and deep well applications consists of the various forms of carbon. Carbon is allotropic and is found widely in its crystalline and amorphous forms. It is found in coke in its amorphous form, while graphite and diamond provide examples of the crystalline form. Graphite, carbon black and coke breeze are all allotropes of carbon that conduct electricity, “breeze” being defined as small cinders or spherical particles which are formed as a by-product of the processing of coal or petroleum. While such forms of carbon are sometimes used in shallow trench and deep well applications as backfill material without further processing, it is also known to modify carbon by means of various cementitious materials to improve its strength and structural integrity. Of the various types of materials which can be used thus improve the properties of the carbon, hydraulic cements such as Portland cement, blast furnace slag, fly ash etc., are known. Concrete and other cementitious compositions are normally prepared by mixing required amounts of hydraulic cement with fine and course aggregates and other additives known to the art, with required amounts of water. The terms ‘paste’, ‘mortar’ and ‘concrete’ are used in the art: pastes are mixtures composed of an hydraulic cement binder, usually, but not exclusively Portland cement, which itself is a mixture of calcium, aluminum and ferrous silicates.
In the conductive concretes relevant to the present invention, the sand, stones and other minerals normally employed as aggregate are replaced by carbon in one of its forms. Optionally the various forms of carbon can be admixed with the aggregates and other additives commonly known in the art, providing the concentration of carbonaceous material is sufficient to provide the necessary electrical conductivity.
Such carbonaceous cements may be formed in place, or attached to the electrodes in the deep well cathodic process. Thus U.S. Pat. No. 6,121,543 (Hallmark) has described a groundbed electrode comprising a horizontally-oriented copper, or other electrically-conductive metal conductor, embedded in a cementitious sheath containing approximately equal parts of Portland cement and powdered crystalline carbon. The cementitious sheath may contain from approximately from 45 parts to 55 parts crystalline carbon powder, with the balance being Portland cement.
In a related type of application U.S. Pat. No. 3,941,918 (Nigol) discloses a conductive cement for use with electrical insulators in which graphite fibers are used to form a conducting network within a combination of Portland cement, graphite fibers and high structure carbon black to provide an electrically conductive cement with high compressive strength. Related applications of carbonaceous materials in a concrete matrix for use on various surfaces walkways, floors roadways and the like are described in U.S. Pat. Nos. 3,573,427 and 3,962,142, while U.S. Pat. No. 5,908,584 (Bennett) has described an electrically conductive building material comprising a mixture of graphite, amorphous carbon, sand, and a cement binder to shield building materials against electromagnetic radiation.
Deep Well Grounding Technique for Cathodic Protection
Deep well beds provide an effective method of increasing the life of subsurface metallic structures and the use of metallic anodes in combination with various carbon and graphite electrodes is now widespread. In this procedure the cost of electrode replacement is an important consideration, the rate of anodic consumption being dependent on the current density at the interface of the anode and soil medium. It has been found that a more uniform flow of current can be achieved if the anode is completely surrounded by a uniformly conductive carbonaceous backfill material.
According to the deep well technique a hole is drilled in the soil near the structure to be protected to an approximate depth of 150 to 450 feet, with a diameter of six or more inches. An anodic chain is then lowered into this hole and the hole is then filled with the backfill material, either in the dry form or optionally as an aqueous slurry. Typically the central anodes are composed of expensive materials such as mixed metal oxides or silicon-iron alloys, and under normal working conditions they may last from 5 to 10 years before suffering erosive failure.
It is well known to those skilled in the art that appropriate use of a suitable carbon backfill material can double the lifetime of the system. This is accomplished because when carbon is present it is sacrificially oxidized by the electrical current in preference to the central anode. The rate of loss of the carbon can be quantitatively estimated from known rate of oxidation during the electrolytic process.
It is also a common practice to further extend the lifetime of such deep well anodic systems by employing a reservoir of excess carbon above the anodic bed, which carbon flows under the influence of gravity as the backfill is consumed. If such a flow does not occur there is a danger that the electrical conduction of system can be lost by a break in the circuit as the result of a void. As will be apparent, formation of a void in one of the expensive sheaths disclosed in the prior art can result in the premature failure of the entire deep well installation.
Existing procedures for deep well electrical grounding, as described above, suffer from a number of shortcomings, one of the most serious being environmental contamination. An unavoidable by-product of the electrolytic reaction is the generation of gas, since the passage of one joule of electrical current results in the formation of one mole of gas. The most prevalent gas formed at the anode is oxygen, but if chlorides are present in the ground water, highly corrosive chlorine is also be generated. If these gases are not able to escape from the system, cavitation and ultimately interruption of the circuit can result.
It is therefore important that the construction of a deep well anode be such that the anodic gases are able to escape. This is usually accomplished by utilizing the conductive backfill material to close the gap between the anode and the wall of the well. Utilization of backfill also allows the dimension of the hole to be large enough for easy emplacement of the central anode. There is however a serious problem associated with this common practice. Since coke breeze is very permeable to water, its presence in deep well installations results in large quantities of water migrating between geological layers with unacceptable environmental consequences.
It is therefore essential that the material used for backfill purposes have both suitable rheological properties and the requisite degree of electrical conductivity.
In recent years various disclosures have attempted to solve this problem. One approach has involved covering the central metallic anode with some kind of sheathing material, or for example, by use of pre-packaged anodes emplaced in special containers or rigid cartridges (U.S. Pat. Nos. 3,725,699 and 4,400,259), or a more flexible construction which retains its shape and is thus more readily transported and installed. U.S. Pat. No. 4,544,464 (Bianchi) uses a perforated disk filled with coke and to facilitate electric current between the central anode and the external casing, combined with backfill composed of graphite and coke such that the anode is homogeneously surrounded by backfill in order to provide consistent current flow as the corrosion continues. U.S. Pat. No. 4,786,388 (Tatum) discloses a low resistance non-permeable backfill for cathodic protection of subsurface metallic structures consisting of a mixture of carbonaceous materials, lubricants, Portland cement and water. In that process the slurry is pumped into an anode bed. To this end Tatum describes a method of pumping an electrically conductive cementitious backfill into the well in such a way as to produce a groundbed construction with a non-permeable concrete annulus in contact with the earthen bore, improvement that is said to avoid water quality degradation while at the same time achieving a low resistance ground contact. U.S. Pat. No. 5,080,773 (Tatum) describes an electrical ground installed in the earth comprising an electrical conductor, a bore hole and a conductive non-porous carbonaceous cement composition surrounding said conductor and in contact with said rod by means of earth. These compositions are said to have enhanced conductivity, decreased porosity and a rate of set similar to that of conventional concrete.
Water tightness is the ability of a cementitious material to hold back or retain water without visible leakage. Permeability refers to the amount of water migration through concrete when the water is under pressure, the value of good quality concrete typically being in the range of 10−8 to 10−10 cm per second. For the purpose of cathode protection systems such a low degree of permeability is undesirable because although a cementitious sheath of that permeability would prevent the vertical transmission of ground water, it would be too low to allow the gases to escape. By contrast, the permeability of carbon backfill is about 0.05 cm/second, which allows both the gases to escape and water to migrate.
Although various conductive sheath compositions consisting of carbon in combination with cementitous binders have been directed to the problem of vertical water flow between aquifers, none has yet proved to be technically or economically suitable. One significant problem is the high cost of manufacture and installation, but more importantly none has described a composition or method which simultaneously allows the gases to safely escape, while preventing the ground water from migrating. As a consequence porous carbonaceous backfill such as coke breeze is still widely used in deep well cathodic systems, in spite of the fact that the vertical transmission of ground water has become so serious that consideration is being given to banning the procedure.
In short, there has not been available to date a simple, inexpensive method for controlling the relative permeability of water and gases in deep well anodic installations.
SUMMARY OF THE INVENTIONThe principal objective of the present invention is to provide a deep well anode for electrical grounding in which the central metallic anode is encased and protected by a cementitious sheath and backfill affording sufficient permeability that by-product gases developed in use can escape at a rate which permits long-term operation of a cathodic protection system, while minimizing migration of groundwater. The sheath/backfill protection for the metallic anode according to the invention exhibits the attributes of:
- (i) a level of electrical conductivity affording the desired cathodic protection.
- (ii) sufficient physical strength to withstand placement down a well and the pressures encountered by the deep well anode in the course of its useful lifetime.
- (iii) performance of the deep well anode under a wide range of current densities and rates of gas evolution.
- (iv) resistance to any corrosive gases encountered in ordinary use.
- (v) cost of manufacture and use comparable to existing deep well anode electrical grounding systems.
- (vi) minimal environmental impact.
With a view to achieving these advantageous properties and overcoming the aforementioned disadvantages of known deep well anodic installations, the present invention according to a first embodiment provides carbonaceous cementitious materials for forming a sheath for the central metallic anode in a deep well anode system which possess sufficient electrical conductivity while exhibiting a degree of permeability that allows by-product gases to migrate to the surface of the well while minimizing or eliminating the vertical flow of groundwater.
Preferred compositions of such semi-permeable conductive cement comprise Type 10 Portland cement at a concentration of 20 to 60% by weight and from 80 to 40% of carbon having particle size between 50 and 150 mesh. The carbon additive can be, for example, coke dust or carbon black. Alternatively or in addition to finally divided carbon, a number of other materials may be used to reduce the permeability of the cement.
According to a second embodiment of the invention, the provision of a deep well anode system having the desired electrical conductivity in combination with a level of semi-permeability allowing by-product gases to migrate to the surface of the well, while minimizing or eliminating the vertical flow of groundwater is achieved by forming an impermeable conductive sheath around the central metallic anode, but using as the backfill in the well or hole a backfill material which itself is semi-permeable and conductive. The preferred backfill material to achieve this objective comprises a flowable granulated carbon such as coke breeze admixed with Type F fly ash, the latter having the surprising effect of reducing water permeability of the backfill to the selected degree. The anode is composed of a granular electrically conductive form of carbon compounded with a binding/sealing agent to render it impermeable. A preferred binding/sealing agent comprises from 10 to 40% of a long chain fatty acid and a metal oxide or hydroxide cross-linking agent such as quicklime or slaked lime.
According to a further aspect of the invention, there is provided a method for preparing deep well anodes of the kind having a central metal core encased in an electrically conductive carbonaceous cement composition comprising the steps of providing a mold, aligning a ground anode in the mold to receive a protective sheath of the carbonaceous cement composition, encasing the metallic ground anode in the mold with a dry granular carbonaceous cement and tamping the dry mixture above the anode until fully settled and shaped, slowing adding sufficient water to fully saturate the sheath of carbonaceous cement and, fully, curing the carbonaceous cement to hardness. The encased anode is then ready for installation in a conventional deep well anode bed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTSDeep Well Anodes with Semi-Permeable Conductive Cement Casing.
The first embodiment of this invention involves the discovery of a series of compositions with desirable permeability properties that are capable of being formed into a semi-permeable conductive sheath around the central anode. The compositions are such that permeability of the system can be adjusted as required so that the by-product gases are allowed to migrate to the surface of the well, while vertical flow of the groundwater is minimized or eliminated. In the course of conducting this research we realized that this process is made possible by the fact that when a carbonaceous cementitious sheath is employed, the by-product gases form on the surface of the individual carbon particles which are an integral part of that composition. They are therefore generated at sufficient distance from each other that the build up is widely distributed, such that a suitably designed material will allow the gases to escape. It will be appreciated that because of the many different variables encountered in the field no single composition will meet the requirements of the many different Deep Well applications used today. This is because the quantity of gas produced, and the rate of migration of the water depends on many different factors such as current density, depth of the anode, conditions of the water table etc. Thus although a number of compositions which provide this result are here described as different examples, these are not considered to limit the invention. In the course of this research a large number of promising avenues were explored before being abandoned. We discovered that the requirements described above may be accomplished by means of carbonaceous concretes of which several ranges of composition are here described. We discovered that permeability is a function of a number of factors including the type of cement, type of carbon, and utilization of different additives. Thus permeability can be reduced by utilizing finely divided carbon, such as coke dust or carbon black, or by higher concentrations of Portland cement. Very low permeability can also be attained by cross-linking soluble alkali silicates with suitable di- or tri-valent metal such as calcium, zinc or aluminum. Another important aspect of this invention was the discovery that the permeability of Portland cement based compositions can be significantly reduced by incorporation of the alkali metal soaps of long chain fatty acids.
The preferred concentration of cement and carbon to be used in the compositions is determined by a number of constraints which are set by the performance requirements. Thus we found that the optimum properties are obtained if Type 10 Portland cement is used between a concentration of about 20 to 60%, and the carbon between about 80 and 40%, with the carbon having a particle size between about 50 and 150 mesh. If less than about 20% cement is used, the composition has unacceptable strength, whereas if less than 40% carbon is employed the final product has insufficient electrical conductivity. The preferred permeability is believed to be between about 10−4 and 10−6 cm/sec. The preferred strength is greater than about 300 pounds per square inch (psi). The variation in strength, permeability and conductivity of some typical compositions are provided in the Examples. The application here described is, however, not limited to the ingredients shown in the examples, because it is anticipated that various additives known to those skilled in the art of Portland cement based concrete may also be used in this system. Thus it is anticipated that similar results might be obtained if such well known additional materials as blast furnace slag, fly ash, silica fume, aggregates, water reducing superplasticizers, accelerators, retarders, defoamers or air entraining agents could all be employed in these systems.
The invention here disclosed and illustrated, is that certain additives not previously known for this application are particularly valuable as a means of modifying the permeability of these systems. It is known that waterproofed Portland cement based concrete can be manufacture by means of addition of small quantities of the calcium or aluminum soaps of long chain organic compounds such as stearic or oleic acids, esters such as butyl stearate, and such proprietary formulations such as those containing silicones. After numerous experiments, however, we determined that most of these known cement waterproofing agents are of marginal usefulness when dealing with carbonaceous cements. This is due to the fact that the carbon particles are so porous as to render these additives generally ineffective. In the course of this work we discovered to our surprise that the soluble alkali metal soaps of long chain fatty acids are efficacious in modifying the permeability of the carbonaceous cements here described. We found that the permeability of the carbonaceous concrete may be greatly reduced if a fatty acid was introduced to the uncured composition in the form of its soluble alkaline soap. Although the mechanism is not fully understood, it is conjectured that these soluble soaps react with the lime produced during the cement curing process in such a way as to form a uniform dispersion of water insoluble calcium soaps. We discovered that the alkali soaps of both oleic and stearic acid and distilled tall oil, are all effective in this process, and it may be speculated that numerous other fatty acids might also be so employed. As illustrated in the examples, the reduction in permeability is directly proportional to the concentration of soap, up to an 80% reduction being realized. Since the reduction in permeability is not sufficient to prevent the migration of gases through the composition, it is thus possible to control the permeability of water and gas by manipulation of the quantity of fatty acid soap which is added.
The examples provided are intended to illustrate the concept, that by correct choice of the sheathing and backfill material it is possible to design a deep well anodic system which will allow the gases to escape, while greatly reducing the migration of the ground water. As illustrated in one of the examples given below, such performance has now been demonstrated in a field installation that has been in continuous operation for a period of three years.
Deep Well Anodes Encased in Impermeable Conductive Cement Used in Conduction with Semi-Permeable Backfill Material
This second embodiment describes utilization of a completely impermeable conductive cement in combination with a semi-permeable conductive backfill material. This aspect of the invention differs from the previous embodiment in that the electrochemical reactions take place at the surface of the conductive sheath, and in the volume occupied by the backfill material, whereas in the first embodiment the by-product gases formed within the semi-permeable conductive cement. The advantage of this technique is that the sheath completely prevents migration of water, and is very much less expensive than the central anode. The other advantage is that the compositions disclosed in this embodiment are more resistance to corrosive gases such as chlorine or hydrochloric acid which may form in certain saline ground waters. As is well known, concrete derived from Portland cement can be corroded by such acids. It will be realized that because the sheaths in this embodiment are impermeable to water, it is necessary to combine them with a backfill material which itself is semi-permeable, allowing free flow of the by-product gases but reducing the migration of groundwater.
In the course of the research here disclosed we discovered that an impermeable, conducting sheath suitable for this embodiment of this invention can be prepared by binding carbon with different types of water resistant binders other than Portland cement. Indeed Portland cement was found to be unsuitable for this application because the water resistant additives described in the first embodiment do not sufficiently reduce the flow of water to prevent acidolysis of the cement by high salinity groundwater. We did determine that this goal can be achieved with various types of thermosetting binding agents widely used as industrial grouts and adhesives such as urethanes, urea-or melamine-formaldehyde resins, epoxies and the like. While such products were investigated in the course of this research all were rejected either because they were considered too expensive for this application, or because of difficulties involved in their utilization, or because of concerns about environmental leaching. Hot melt adhesives based on such thermoplastic polymers as polyolefins, asphalts and the like were similarly considered and eliminated, both for reasons of cost and difficulty of production.
The compositions which were found to be of particular value, and are here described as a novel invention, were formulated by combining particulate carbon with various combinations of long chain fatty acids and metallic cross linking agents such as the oxides or hydroxides of di- or tri-valent metals. As illustrated in the examples, we have found that compositions with acceptable electrical conductivity and physical properties may be prepared by admixing between 50 and 90% carbon with 10-40% long chain fatty acid and 1-10% metal oxide or hydroxide. The preferred carbon is coke breeze with a particle size of 30-70 mesh, the preferred fatty acids consist of distilled tall oil or crude tall oil, and the preferred metallic cross-linking agent is calcium oxide, either in the anhydrous form (quicklime) or as the hydrate (slaked lime).
The second aspect of this embodiment derives from the discovery that the permeability of the flowable, conductive carbon which is used as backfill, can be greatly reduced by mixing this type of carbon with a class of compound known as Type F fly ash. The importance of this embodiment is due to the fact that when the anodic sheath is placed within the hole there is inevitably a gap between the sheath and the outer diameter of the hole through which water can migrate. Typically this gap is filled by a carbonaceous backfill material which is permeable to water. As discussed above, erosion of the backfill during operation of the cathodic protection system results in the formation of voids which are filled over time by a source of carbonaceous material placed above anodic bed from which the carbon flows under the influence of gravity. Typically the forms of carbon that are known to perform this function well consist of petroleum coke breeze consisting of spherical particles with acceptable flow properties whether dry or saturated with water. Like other forms of carbon, however, this petroleum coke breeze is very permeable to water, and thus does not prevent the migration of groundwater. The discovery here disclosed is that when petroleum coke breeze of that type is admixed with small quantities of a material known as Type F fly ash, the permeability to water is significantly reduced, while the electrical conductivity is largely unaffected. Fly ash is an inexpensive waste by-product of coal combustion, and Type F is differentiated from other grades such as Type C by the level of free lime. Although other additives such as clays and other types of mineral fillers can also be used to lower the permeability of such carbon to water, the rheological properties of such compositions are such that they do not flow freely. The ability of the carbon to flow from the reservoir is an essential performance aspect of the conductive backfill. The chemical properties of Type C fly ashes are such that when admixed with water they form an hydraulic cement, the nature of Type F fly ash however is that it does not form a cement.
Thus an important aspect of this disclosure is the discovery that although various additives are capable of reducing the permeability of coke breeze, none has yet been found which both provides the required properties at an acceptable cost and does not affect the rheology of the backfill. After various experiments it was determined that an optimum ratio of type F fly ash to coke breeze is within the range of 50 to 90% coke breeze and between 10 and 40% fly ash. As illustrated in Example 6 admixtures of this carbon and type F fly ash from Sammis Ohio were blended and tested. As shown the effect on permeability of mixtures containing less than 100% fly ash is minimal, while compositions containing less than 50% coke breeze have inadequate electrical conductivity. It is to be noted, that although an optimum ratio of 30:70 was obtained when using Sammis Fly ash and Hickman-Williams petroleum coke breeze in the Example, different optima may be anticipated when other materials are employed. It is to be expected however that the same principal will apply.
Tests included flowability, electrical conductivity and permeability to water. The samples were then subjected to electrolysis under the standard conditions described above for a period of two months. As shown, a composition consisting of 70% petroleum coke breeze and 30% Type F fly ash had acceptable conductivity and flow properties, but was 90% less permeable to water than the coke breeze alone.
EXAMPLES Example 1 Permeability and Strength of Various Carbonaceous Cement Compositions To obtain these results carbon and cement of a combined weight of 200 grams were dry blended and sufficient water admixed to obtain a smooth paste. This paste was then packed into standard 2″×4″ test cylinders. The different compositions were allowed to cure under moist conditions for 28 days after which they were removed and tested. The table below illustrates some typical results that may be obtained by using various types and ratios of carbon with siliceous cements. The strength shown refers to compressive crush in lbs per square inch (psi). Permeability was obtained by immersing the samples in cold water and measuring the weight increase. The value shown is the slope of the linear curve obtained by plotting % increase against the square root of time. These particular examples were obtained using two grades of coke breeze supplied by Hickman Williams, Canada in which the particle size distribution varied from 100 to 30 mesh. The siliceous cement used in these examples was Type 10 Portland (Lafarge, Woodstock Ontario Canada). As shown the permeability, product strength and electrical are directly related to the ratio of cement to coke breeze. On the basis of these results it is concluded that the preferred ratio of cement to carbon is between 60/35 and 20/80. Below about 20% cement binder the compositions have insufficient strength, while above about 65% the electrical conductivity is inadequate. As shown however, within these limits however a wide range of permeability to water may be obtained.
Cement compositions prepared and compacted into a 2″×4″ test cylinder as described in Example 1. A 7 strand, 8 gauge copper wire was then positioned in the centre of the cylinder such that the lower end of the wire was 1″ from the bottom of the cements, and held in that position until the samples were hard. In each case the upper exposed portion of the wire was then coated with a water resistant rubber composition, and after curing for 28 days, each sample was placed within a 5 litre container containing a solution of 10% sodium sulfate. An electrical circuit was then completed by using a Topward Model 3306 variable current power supply, in such a manner that the test sample was the anode, and a 24″×½″ steel rod was the cathode. Sufficient voltage was then applied such that a direct current of 0.5 amps would flow through the circuit. It was found that suitable current could be generated with all samples containing more than about 400% by weight of carbon, but below this level the electrical resistance was unacceptably high. The current was allowed to flow and results observed.
In this experiment samples were prepared and cured as described above for 28 days. The results shown here were obtained using the sodium soaps of Pamak C4, a distilled tall oil fraction manufactured by Hercules Canada (Burlington, Ontario). In this experiment a 25% solution of soap was admixed with the water used to prepare different slurries of the carbonaceous cement. These were then transferred to standard 2″×4″ cylinders for curing. The test cylinders were then removed from the moulds, dried under ambient conditions for 7 days and weighted. Each was then immersed from water for four hours after which it was removed from the water, dried with a paper towel and weighted again. The results tabulated below show the increase in weight due to absorption of water for samples containing different quantities of soap. In each case the soap content is expressed on a dry basis. The results demonstrate that the uptake of water is inversely proportional to the concentration of soap in the concrete.
The anodes were constructed in a cylindrical shape with a diameter of approximately 3-4″ and a length of 4-5 feet. Embedded in the centre of the anode was a conductive component bonded to an insulated conductor, which in turn was connected to the rectifier. In this application a 2½″ i.d. thin wall PVC conduit was used to form the anode. The lead wire was connected to 4/0 19 strand bare copper wire by means of an exothermic weld, and placed in the form. The form was then filled with the cementitious concrete described and allowed to cure in a moist environment. Once hardened the conduit was removed from the form and placed in position in the well.
Example 5 Laboratory Preparation of Impermeable CementsTo 210 grams (70% w/w) petroleum coke breeze (Loresco Int., Hattiesburg Miss., Type SC-3) was added 30.0 grams (10% w/w) slaked lime and the two powders were mixed well at room temperature. To this mixture was added 60 grams (20% w/w) distilled tall oil (Pamak C4, Hercules Canada, Burlington Ont.), and the components were mixed well. The resulting paste was compressed into a 2″×4″ cylinder and allowed to cure for 48 hours. After removal from the cylinder the product was tested for electrical conductivity, water absorption and compressive strength. The conductivity of the cylinder was 150 Ω, and the compressive strength was 300 psi. When subjected to the permeability test described in Example 3, no uptake of water was detected after 120 hours at room temperature.
Different variations of this formula were then prepared using various long chain fatty acids, and the oxides and hydroxides of different Group IIA alkali earths, zinc and aluminum. As illustrated below the best strength results were obtained with either calcium oxide (quicklime) or slaked lime in the range of 3 to 6%. Although acceptable results were obtained with various long chain fatty acids, for reason of cost the preferred acids derive from crude tall oil. Of the various types of carbon that were used, the best mixing properties were obtained using petroleum coke breeze such as Loresco Type SC-3 which are typically used as deep well backfill. These materials have a particle size larger than 100 mesh and spherical particles. As illustrated in Table I below, the optimum concentration range of coke breeze is between about 50 and 80%, beyond which range the conductivity is either poor or the mix is difficult to work. The optimum fatty acid range (Pamak C4 being shown here, similar data were obtained from other long chain fatty acids) is between about 15 and 30%. The preferred metal oxide cross-linking agents is quicklime or slaked lime between the range of about 3 and 10% w/w.
The composition described in Example 5 of Table I was prepared and set aside. A 7 strand, 8 gauge copper wire 6″ in length was positioned vertically in 2″×4″ plastic forming cylinder such that the wire was in the centre of the cylinder, with the lower end 1″ above the bottom of the cylinder. The composition was then compacted around the wire and allowed to cure for 48 hours. The sample was then removed from the plastic form and the copper wire protruding from the top of the formed cylinder was coated with a water resistant silicone grout and allowed to cure for 24 hours. This test cylinder was then positioned in a 5 gallon container filled with water containing 0.5% sodium chloride in such a manner that the test sample was fully submerged, but the copper wire extended to the air. The exposed end of the wire was then connected to a rectifier so that it became an anode. A steel rod introduced into the filled 20 litre pail was connected to the rectifier as the cathode. The rectifier was adjusted to supply current at the rate of 0.25 amps, for which a voltage of 0.8V was required. As electrolysis continued bubble formation was observed only at the outer shell of the test cylinder. After two weeks the sample was removed from the test apparatus and dissected. There was no evidence of penetration of water beyond the outermost layer of the cylinder.
Example 7 Permeability and Conductivity of Coke Breeze-Fly Ash Combinations Petroleum coke breeze (Loresco Int., Hattiesburg Miss., Type SC-3) and Type F fly ash (Sammis, Ohio) were dry blended in various ratios and subjected to the following tests: water permeability, conductivity and slump. As illustrated in Table II below, all compositions tested exhibited similar slump, from which it is concluded that the flow properties in the field application may be expected to be similar, while acceptable conductivity was obtained when the concentration of coke breeze was greater than about 60%. The permeability to water, expressed here as cm/sec rate penetration, was a direct function of the percentage of fly ash in the mixture. The electrical conductivity fell off when the concentration of coke breeze was below about 60%. From these results it is concluded that the preferred mixture of petroleum coke breeze to fly ash is between about 80/20 and 60/40 parts by weight.
High Silicon Cast Iron anodes manufactured by Anotec Industries (Langly B.C.) having dimensions of 1.5″ diameter by 12″ in length were used. Both control and test anodes were protected with an epoxy cap and connected to the rectifier by means of HMWPE cable. The test anode was encased in a 1.5″ layer of cementitious concrete having first been placed in a plastic mould 4″ in diameter and 12″ long. A slurry of cementitious concrete was prepared using 64 parts water per 100 parts cementitious concrete which was then poured into the mould and cured for fourteen days before commencing the test.
The test was conducted using two test cells consisting of 20 ltr plastic pales filled with 30 mesh silica sand saturated with 3% sodium chloride solution. The test anodes were in the centre of each pale while the cathodes consisted of 12″ by 12″ steel plates positioned against the wall of the pail. A variable current power supply from Spence Tek Inc. (Milpitas, Calif.) ensured that the current test anode during the course of the trial was the same and maintained within the range 0.75±0.05 amps. The uncoated and coated anodes received 0.54 and 0.31 Kamp-hours respectively and the voltage in each pail varied from 4V to 6V.
The anodes were weighed at the beginning and end of the thirty-day test, after which both were removed from their individual pales and examined after the cementitious carbon coating was removed from the test anode. The control anode appeared to be more pitted than the carbonaceous carbon anode, but both were covered with a loose black coating which was flaked off before re-weighing anodes. The weight lose of the uncoated control anode was 22 grams (0.8%) while that of the carbonaceous concrete coated anode was 15 grams (0.6%).
Example 9 Three Year Field Trial Utilizing a Semi-Permeable Anodic SheathA long term field trial utilizing the principles here disclosed commenced in January 2002 and continues. The results here presented reveal that when evaluated in January 2005, after three years of continuous operation the system continues to performing extremely well in comparison to a traditional cathodic protection system. There is moreover no evidence of a deleterious affects caused by gas formation.
In preparation for this trial, two holes 10″ in diameter were excavated to a depth of 366 feet. The first hole was used to prepare a “traditional” anodic bed, while the second hole was used for the novel semi-permeable cementitious composition. The traditional bed was prepared by installing 20 graphite rods of 4″ diameter and 80″ long. After placement of these electrodes the void space was then backfilled with 5320 pounds of metallurgical coke breeze. The test bed was prepared by introducing 12 mixed metal oxide mesh anodes 1″×40″ in size, and 3850 pounds of the novel composition.
A composition of conductive cement was then prepared using the procedure described in Example 1. Sufficient material to fill the hole was prepared by dry blending Type 10 Portland cement and Loresco Type SC-3 coke breeze in a cement mixer in the ratio of 70:30 parts by weight, with sufficient water (50 parts by weight per 100 parts of dry mix) so that the composition was converted to a paste of such consistency that it could be pumped into the cavity. After 3 weeks during which the cement was sufficiently cured, both systems were energized by means of an external direct current power source in which the electrodes in the test holes were anodic, while the cathode consisted of a steel pipe buried in the soil at a distance of 100 feet from the anodes.
In this trial the efficiency of the two systems may be determined by comparing the resistance of the two circuits, the lower the value the better. As shown in the table below, the resistance of the traditional bed increased with time, while the novel composition declined. By year three the resistance of the novel composition was about 24% lower than the control bed. Most importantly the test bed revealed no evidence of problems related to gas formation, which might have been manifest either visually or as exceptional fluctuations in current flow.
As emphasized throughout the foregoing disclosure, the invention in its broadest terms is directed to controlling the relative permeability of groundwater and gases in a deep well anode system through the use of a semi-permeable conductive sheathing material for the anode or alternatively an impermeable conductive anode in conjunction and cooperation with a semi-permeable, conductive backfill in the well bore. Those of skill in the art will appreciate that many materials might be used equivalently to achieve such control, beyond the formulations specifically exemplified above. It is intended that the appended claims cover all such equivalents.
Claims
1. A method for preparing a deep well anode, comprising the steps of:
- (i) providing a mold;
- (ii) aligning a ground anode in the mold for receiving a protective sheath of carbonaceous cement;
- (iii) encasing the anode in the mold with a dry granular carbonaceous cement and tamping the dry mixture about the anode until fully settled and shaped;
- (iv) slowly adding sufficient water to fully saturate the sheath of carbonaceous cement; and
- (v) curing the carbonaceous cement to hardness.
2. A composition for the preparation of an electrically conductive semi-permeable sheath for the encasement of a deep well anode, comprising a siliceous cement and an electrically conductive form of carbon, said anode sheath having a water permeability between 10−4 and 10−6 cm/sec.
3. A composition according to claim 2, wherein said siliceous cement is Portland cement and the composition further comprises a minor amount of an additive effective to adjust the water permeability of the anode sheath to between 10−4 and 10−6 cm/sec.
4. A composition according to claim 3, wherein said additive is a water soluble alkali metal soap of a long chain fatty acid.
5. A composition according to claim 4, wherein said Portland cement is a Type 10 Portland Cement in a concentration of between 20 and 60% by weight and said carbon is of a particle size between 50 and 150 mesh and makes up between 80 and 40% by weight of the composition.
6. A composition according to claim 5, wherein said additive is selected from the group consisting of alkali soaps of oleic acid, stearic acid or distilled tall oil and mixtures thereof.
7. In a deep well grounding system of the kind including an anode lowered into a backfilled bore hole, the improvement of providing
- (i) a metal conductive anode encased in a conductive composition which is substantially impermeable to water and to by-product gases developed in use of the system; and
- (ii) a semi-permeable conductive backfill material to allow free venting of said by-product gases while limiting undesired migration of groundwater.
8. A deep well grounding system according to claim 7, wherein said semi-permeable backfill material comprises a flowable granulated carbon admixed with a selected amount of Type F fly ash to adjust the water permeability of the backfill to a selected degree.
9. A deep well grounding system according to claim 8, wherein said conductive composition comprises a granular electrically conductive form of carbon compounded with a binding/sealing agent selected from cross-linked long chain fatty acids.
10. A deep well grounding system according to claim 9, wherein said conductive composition comprises between 50 and 90% carbon, 10 to 40% of a long chain fatty acid and 1-10% of a di- or tri-valent metal oxide or hydroxide cross-linking agent.
11. A deep well grounding system according to claim 10, wherein said carbon in the anode composition is coke breeze.
12. A deep well grounding system according to claim 9, wherein said backfill material comprises between 50 and 90% coke breeze and between 10 and 40% Type F fly ash.
Type: Application
Filed: May 9, 2005
Publication Date: Jan 12, 2006
Inventors: D. Sirola (Barrie), Graham Hagens (Hamilton)
Application Number: 11/124,406
International Classification: E21B 33/13 (20060101);
