Patent Grant
10626506
The invention refers to an anode composition for providing cathodic protection to underground metallic structures. The composition comprises a slurry comprising a fluid carrier containing a granulated electrical conducting material.
Cathodic protection is one of the methods used to reduce corrosion problems in metallic structures exposed to aggressive aqueous environments. It is one of the most effective techniques for corrosion control, applied in a number of industrial fields. The application thereof was first reported by Humphrey Davy in 1824, disclosing a sacrificial system for protecting copper components employed in ship hulls comprising zinc or iron plates.
On one hand, cathodic protection systems with sacrificial anodes employ metals with electronegative electrochemical potential, like zinc, aluminum, magnesium or alloys thereof to protect more noble or electropositive metals and alloys, like iron, steel, copper, titanium, etc. The potential difference between the anodic metal and the structure to be protected (i.e. cathode) provides the driving force that creates a charge flow or protection current.
A cathodic protection system with sacrificial anode comprises four main components: an anode (a metal or alloy with electronegative potential), a cathode (a structure to be protected which has a more electropositive potential than that of the anode), an electrical contact between the anode and the cathode and an electrolyte (or corrosive medium) in which the anode and the cathode are immersed.
On the other hand, impressed current cathodic protection systems employ an external source of electric power to generate a potential difference between anode and cathode that enables to provide a protection current. In this case, a metal or conductive material with high corrosion resistance, like silicon-iron alloys, graphite, MMO (Mixed Metal Oxides), and stainless steel, is used as an impressed current anode, so as to ensure proper protection system durability.
Therefore, to provide cathodic protection to a structure it is necessary to install a predetermined anodic metal mass close to the cathode (i.e. structure) to be protected. The electrochemical potential difference between the anode and the cathode will provide a system protection current. This current will depend not only on the electric potential difference between the anode and the cathode but also on the electric/electrolytic resistance of the circuit, according to Ohm's Law.
I=(Ea−Ec)/R [1]
In turn, resistance R depends on the electric resistivity of the medium and on the geometry and proximity of the anode to the structure to be protected. The higher the value of R, the lower the current provided by the protection system. Accordingly, in order to achieve proper protection for the metallic structure, the sacrificial anodes should be located so as to obtain a protection current distribution as homogeneous as possible. In this regard, for cathodic protection of oil producing wells or water injectors/producers it is complicated to achieve a uniform current distribution along the casing length. Although for this kind of structures impressed current cathodic protection systems are usually employed, enabling to produce larges currents, the high variation of formation electric resistivity across the well often causes that the protection current cannot reach the deep casing areas exposed to corrosive formations and aquifers.
The current distribution problems shown in
The present invention provides an impressed current anode system that provides a solution of these kinds of technical problems, as it is disclosed below.
In a first aspect, the present invention provides a cathodic protection composition applicable to underground metallic structures, preferably for casings of hydrocarbon producing wells or water injecting/producing wells. The composition acts as a liquid anode, in the form of a slurry comprising a granulated conducting material and a carrier fluid. The slurry may further comprise a filler material with high electric conductivity, hereinafter referred to as “backfill”, as well as viscosifiers and other additives commonly used in well completion fluids.
The granulated electrical conducting material may be selected according to the kind of protection system to be applied to, i.e. sacrificial anode or impress current system. The carrier fluid comprised in the slurry has an adequate viscosity so as to carry all particulate solid materials.
In a second aspect, the present invention provides a method for cathodically protecting underground metallic structures, preferably for casings of hydrocarbon producing wells and water injectors/producers that employs a liquid anode composition in the form of a slurry that can be pumped into the well down into the underground formation and located to a specific depth where protection is needed.
Therefore, it is an object of the present invention, an anode slurry composition comprising a solid material in a carrier fluid, usable in cathodic protection systems for underground metallic structures, comprising a granulated electrical conducting material as anode.
In a preferred embodiment of the present invention, the anode slurry composition further comprises a granulated high electrical conductivity backfill.
In another preferred embodiment of the present invention, the concentration of the granulated electrical conducting material in the slurry is in the range of 10-100% based on the total weight of solid material.
In another preferred embodiment of the present invention, the concentration of the granulated high electrical conductivity backfill is up to 90% based on the total weight of solid material.
In a preferred embodiment of the present invention, the granulated electrical conducting material is a granulated metallic electrical conducting material.
In a more preferred embodiment of the present invention, for application to a cathodic protection system with sacrificial anode, the granulated metallic electrical conducting material is a metal selected from the group comprising Al, Zn, Mg and alloys and mixtures thereof.
In yet another preferred embodiment of the present invention, for application to an impressed current cathodic protection system, the granulated metallic electrical conducting material is a metal showing high corrosion resistance, selected from the group comprising silicon-iron alloys, stainless steel, titanium, platinum and combinations thereof.
In yet another preferred embodiment of the present invention, the granulated electrical conducting material is a granulated non-metallic electrical conducting material.
In a preferred embodiment of the present invention, for application to an impressed current cathodic protection system, the granulated non-metallic electrical conducting material consists of a non-metallic material, selected from the group comprising graphite, Mixed Metal Oxides (MMO) and combinations thereof.
In an embodiment of the present invention, the granulated high electrical conductivity backfill is selected from the group comprising coke, activated carbon or coke, graphite and combinations thereof.
In another embodiment of the present invention, the concentration of the granulated high electrical conductivity material (backfill) is up to 90% based on the total weight of solid material.
In another preferred embodiment of the present invention, the anode slurry further comprises viscosifier agents and other additives commonly used in well completion fluids.
It is also an object of the present invention a method for providing cathodic protection to an underground metallic structure comprising the injection and pumping of an anode slurry consisting of a solid material in a carrier fluid, comprising at least one granulated electrical conducting material as anode into an underground formation containing said metallic structure.
In an embodiment of the method of the present invention, the metallic structure is part of a hydrocarbon producing well or a water injecting/producing well.
In a preferred embodiment of the method of the present invention, the metallic structure is a casing.
In a more preferred embodiment of the method of the present invention, when applied to sacrificial protection of hydrocarbon producing wells or water injector/producing wells, the slurry is injected into the formation through punched holes made in the casing.
In a yet preferred embodiment of the method of the present invention, the injection and pumping is performed at a hydraulic fracture regime or rate so as to ensure packing and electric contact between the solid material contained in the slurry and the structure to be protected.
In a most preferred embodiment of the method of the present invention, the injection and pumping is performed at a pressure higher than the fracture gradient of the underground formation containing the metallic structure.
The present invention provides an anodic liquid composition in form of a slurry comprising at least a carrier fluid and a granulated electrical conducting material.
For the purpose of the following detailed description, the anode slurry composition of the invention may be also referred to simply as “slurry” and the granulated electrical conducting material may be also referred to as a “granulated anode material” or “anode”.
The slurry of the invention may further comprise high electric conductivity backfill, also referred to herein as “backfill”, preferably graphite or activated carbon or coke and viscosifiers to improve viscosity and thus the carrying ability of the solid materials contained in the slurry.
The slurry of the present invention has suitable fluidity and viscosity so as to be pumped into a subterranean formation allowing the transport of all solid materials (e.g. granulated anode material and backfill) that provide anticorrosion protection to a metallic structure, especially hydrocarbon producing wells or water injecting/producing wells.
In the case of sacrificial cathodic protection systems, the granulated anode material contained in the slurry preferably is a metal selected from the group comprising zinc, aluminum, magnesium and alloys thereof.
In the case of impressed current cathodic protection systems, the granulated anode material consists of corrosion resistant materials, metallic or non-metallic, like iron-silicon alloys, stainless steel, graphite and/or MMO.
When applied to sacrificial cathodic protection systems in hydrocarbon producing wells or water injecting/producing wells, the anode slurry composition of the invention is injected into the formation through perforations made in the casing, as shown in
When applied to impressed current cathodic protection systems, the anode slurry composition of the invention is pumped by means of an ad hoc installation reaching the formation into which the granulated anodic metal is being located, as shown in
In both cases above, pumping is performed at a hydraulic fracture regime or rate so as to achieve a suitable anode geometry and electric contact between the solid material contained in the slurry and the metallic structure to be protected. The pumping operation may be performed as batch-frac, to which end the slurry is prepared in a mixer and then pumped into the well at a hydraulic fracture regime or rate by means of at least one high pressure pump. The pressure and pumping regime or rate will depend on slurry rheological properties, pipe diameter, type and number of punched holes and formation fracture gradient.
Electrical continuity between the anode particles, the high conductivity backfill and a casing or disperser, depending on the system applied (sacrificial system or impressed current system), is achieved by the closure stress of the produced fracture.
In the case of sacrificial systems, the anodic reaction corresponds to the dissolution of the metal that acts as sacrificial anode (MeA) according to the following reaction:
MeA→MeA2++2e− [2]
This way, the anode dissolution will always occur on the anode-formation interface, causing a gradual consumption of the anode pack over time. This phenomenon is experimentally verified according to the Examples below.
In the case of a slurry of the invention used for impressed current disperser anodes, with a granulated metal with high corrosion resistance, the anodic reaction is:
Aqueous media: 2H2O→2O2O2+4H++4e− [3], or
Media comprising chlorine ions (Cl−): 2Cl−→Cl2+2e− [4]
To prevent anodic materials flowback from the well and at the same time to seal the punched holes, a batch of epoxy resin or any other material able to become rigid, may be pumped at the end of treatment (see
The invention will be disclosed in further detail by means of the following non-limiting examples.
Carbon steel bars (AISI 1040) were immersed in a NaCl solution having a chloride concentration of 10 g/L, contained within cylindrical cells. Granulated anode metal (Zinc #70) is added to said solution, with and without the addition of graphite as high conductivity filler backfill.
The electrochemical potential of the steel bars with respect to a saturated Calomel electrode (SCE) was monitored during 350 days, so as to determine whether the anode material polarizes steel and protects it from corrosion. Cells with steel bars, without the addition of anode material, were used as blank. The assay conditions were as follows:
Each assay was performed in quadruplicate, potentiodinamically, at a scan rate of 0.2 mV/s.
As can be appreciated in
In case of protection with Zn (steel bars in contact with granulated Zn), the electrochemical potential of steel starts from −1.1 VECS and shows a reduction of about 100 mV at the end of the assay. When comparing this condition with the Blank solution, it can be appreciated that the anode material cathodically polarizes steel in more than 300 mV.
Finally, in case of protection with Zn+graphite, the electrochemical potential appears less stable, varying initially between −0.9±0.05 VECS, and after an exposure time of 200 days it decreases until stabilizing in about −0.7 VECS.
From the information provided in
The obtained results confirm that the addition of granulated Zn to the saline solution causes polarization and corresponding steel cathodic protection. In the case of employing Zn without high conductivity backfill (graphite), protection lasts longer than 350 days, while in the case of employing Zn with high conductivity backfill (Zn+graphite), protection lasts for about 240 days, but using only 100 g of Zn (50% less) in this case.
In order to determine the current drained by the anode (Zn) and thereby to predict the protection system durability, polarization curves were obtained for both metals (SAE 1040 steel and zinc) in the same saline solution (10 g/L Cl−) used in the assays above. Similarly to the steel case, for the zinc assay bar electrodes were employed instead of granulated zinc, due to the impossibility of precisely determining the exposed area in a granulated material. The assays in this case were galvanostatic, and applying stepped current increments.
The corrosion potential of steel identified in the polarization curve (Ecurr=−0.73 VECS) observed in
O2+4H++4e−→2H2O [5]
As from about −1.0 VECS a lineal increase of current density logarithm vs. applied potential is appreciated, due to hydrogen evolution reaction according to equation:
2H++2e−→H2 [6]
Meanwhile, the anodic behavior of zinc (broken-line curve) shows a continuous exponential increment in the current density with overpotential, corresponding to an active dissolution process (charge transference) according to Equation 1, the Zn version of which is as follows:
Zn→Zn2++2e− [7]
Corrosion potential of Zn is of about 1.050 VECS and the Tafel's slope is of about 60 mV/dec.
When overlapping both polarization curves, it can be appreciated that for a system where anode and cathode areas are similar, the mixed potential of steel-zinc cupla is of about −1.0 VECS. This potential is in accordance with the results illustrated in
According to
From the information obtained in this study, the cathodic protection of casings of hydrocarbon producing wells or water injecting/producing wells is analyzed. The mass of zinc required for protecting 100 m of 5½″ diameter casing during 10 years will be:
where in this case the protection current density i=0.2 A/m2, the casing area=3.14×5.5″×0.0254 m×100 m=43.8 m2, the use factor=0.8 and the Zn draining capacity=780 A hour/kg. By replacing said data in Eq. 8:
Required Zn anode mass=1235 Kg.
Said mass of granulated anode material may be pumped in a conventional operation of the batch-frac type.
This Example shows that it is possible to provide sacrificial anode cathodic protection to a metallic underground structure during a long period creating a sacrificial anode with granulated metal to be pumped into a formation in liquid form. In case of hydrocarbon producing wells or water injecting/producing wells, the protection is created by injecting a slurry containing the granulated anode metal through punched holes made in the casing zone to be protected.
Example 2. Slurry for Impressed Current Cathodic Protection Systems
As indicated above, the composition of a slurry of the invention used as disperser anode in impressed current cathodic protection systems contains a granulated anode material with high corrosion resistance and high electrical conductivity. Said material could be a metallic material, preferably iron-silicon alloys, stainless steel, titanium, platinum, etc. and/or a non-metallic material like graphite, coke or activated carbon, a mixture of metallic oxides (MMO), etc.
Similarly to the sacrificial slurry, solid materials are carried into the underground formation by means of a fluid with adequate viscosity. In a typical configuration, the disperser anode may have a design similar to a deep disperser well for impressed current cathodic protection, where the slurry of the invention replaces the conventional disperser anodes (see
When designing the impressed current system of the invention, cathodic protection conventional criteria should be taken into consideration. Besides that, certain aspects should be contemplated in order to establish the slurry composition, anode geometry as well as the methodology for placing the disperser slurry underground.
Disperser slurry composition. The proportion of granulated metallic or non-metallic, solid materials contained in the slurry may vary depending upon their electrical properties. Once pumped into the formation, the carrier fluid comprised in the slurry drains into the formation creating a compact pack of solid materials. The proportion of granulated metal with respect to the high conductivity backfill may vary between 10 to 100% v/v. The higher the load of granulated solid material in the pack, more efficient the disperser anode will be. Taking the composition of hydraulic fracture fluids as reference, where (natural or synthetic) proppants are pumped and carried by a gel of determined viscosity, the solid material load in the slurry may vary typically between 0.1 and 1 Kg/L. Viscosifiers may comprise natural (guar gum, cellulose and their derivates) or synthetic (PHPA, PVA, etc.) polymers
Disperser anode geometry. An adequate disperser anode geometry is determined by controlling the slurry pumping parameters. For obtaining an extended anode geometry like that illustrated in
In those cases where the minimum stress (σmin) acting on the formation is horizontally oriented, the fracture geometry will show two wings perpendicularly aligned with σmin, as can be appreciated in
Cathodic protection design. For designing an impressed current cathodic protection system employing the disperser slurry anode of the invention it is necessary to know the anode geometry. Length and height of the fracture produced during slurry pumping may be determined by employing general knowledge about hydraulic fracturing of hydrocarbon producing formations (M. Ecconomides and K. Nolte, “Reservoir Stimulation”, 3rd Edition, J. Wiley Edt., Schlumberger, 2000, Chap. 5 and 6.).
Knowing the fracture disposition: vertical or horizontal (see
where ρ is the medium electrical resistivity and A is the anode plate area.
By way of example, considering a disperser anode with a configuration similar to that illustrated in
RA(a)=0.016Ω
In the case of considering an anode configuration like that illustrated in
RA(b)=0.00025Ω
Finally, if the anode configuration is that corresponding to
Also by way of example, considering the anode has a diameter (d) of 25 cm (10″), and an active zone of 20 m and that the earth resistivity is the same than the previous cases, RA is:
RA(c)=1.198Ω
Said results show the great incidence of the disperser anode geometry on the cathodic protection system efficiency. For a determined electric power source, the current draining capacity decreases as the RA value increases. The disperser anode embodiment of the present invention provides RA values that are between 2 and 3 orders of magnitude lower than those of conventional disperser anode embodiments and therefore, the efficiency of the cathodic protection systems with liquid disperser anode of the invention are between 2 and 3 orders of magnitude with respect to conventional installations.
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| Number | Date | Country | |
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| 20170183784 A1 | Jun 2017 | US |
| Number | Date | Country | |
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| 62387175 | Dec 2015 | US |
