BACKGROUND

Field

The present disclosure relates generally to compositions, systems, and methods for identifying, preventing, and reducing iron sulfide scale formation. In particular, the disclosure relates to voltammetric identification of iron sulfide scale precursors and chemical compositions for preventing and reducing iron sulfide scale formation from said precursors.

Description of the Related Art

Downhole scale formation in oil and gas wells, such as for example sour gas wells, is a persistent problem that adversely affects operating costs and hydrocarbon production. Scales can include, and in some instances be predominantly, iron sulfides, of which iron disulfide (FeS2) is oftentimes considered to be the most difficult to remove. Though mechanical descaling treatments can be implemented in wellbore and hydrocarbon production applications, these are time-consuming and will increase operating costs.

Iron sulfides, such as for example pyrrhotite, troilite, mackinawite, and greigite, can be dissolved with acids such as 15% by volume HCl; however, this may cause damage to production systems including corrosion to downhole metallurgy, and uncontrolled H2S gas release. The solubility of iron disulfide in acids is slow and difficult according to SPE 68315, Iron Sulfide Scale: Formation, Removal and Prevention, and using acids may not efficiently remove the scales.

Iron sulfide deposition in downhole applications has caused loss of gas production and loss of access to reservoir management and surveillance. Prior art scale removal compositions and methods can cause serious corrosion to steel tubing and other metal components in a wellbore or proximate a hydrocarbon-bearing reservoir.

There are inherent problems involved with controlling iron sulfide scale formation and deposition in ultra-sour oil and gas fields across the globe, which cost billions of US dollars per year to mitigate. Heightened costs of FeS scaling and deposition prevention can be attributed to little knowledge available regarding FeS formation mechanisms and reactions in sour and ultra-sour reservoirs. Reactions of reduced hydrogen sulfide and sulfur intermediate species (for example, from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR)) with ferric/ferrous irons under reduction-oxidation (redox) environments are difficult to control, in part because of fast oxidation kinetics, and exotic thermodynamic stability in high pressure and high temperature (HPHT) conditions.

One way to overcome scale formation and accumulation is by direct in situ measurements related to the onset of FeS scale deposition during hydrocarbon production. Certain techniques must be element and species specific (for example, for iron and sulfur), environmentally sensitive, and able to detect and decipher scale precursor iron sulfide molecular cluster (FeS(aq)) formation in HPHT conditions. By detecting iron sulfide scale precursors and likely formation, appropriate inhibition and mitigation strategies can be developed.

Iron sulfide molecular clusters, also referred to herein as FeS(aq), are a group of polynuclear nano-particulate iron sulfide complexes formed under reducing conditions at different anoxic or suboxic interfaces in petroleum transmission pipelines and downhole reservoirs. FeS(aq)are necessary precursors for the formation of solid iron sulfide minerals and scale, and dissolution of FeS(aq)molecular clusters from bulk FeS minerals like mackinawite (tetrahedral FeS), pyrrhotite (octahedral Fe1-xS), and greigite (isometric octahedral Fe3S4) has been proven as a precursor for FeS scale mineralization in typified sour petroleum reservoirs.

Formation, deposition, and treatment of iron sulfide (FeS) scaling mineralization and contaminants build-up is poorly understood, and this impacts (i) the quality and quantity of natural gas (NG) produced per day from a reservoir, such as a sour reservoir; (ii) reservoir permeability downhole, as well as plugging pressure control devices; and (iii) eventual shutdown of oil or gas plants. Despite FeS occurrence being widespread in many hydrocarbon industries, iron sulfide molecular clusters FeS(aq)and final mineralized FeS scaling products (sometimes termed black powder) are not well understood in terms of formation processes and management, causing issues with metal corrosion and flow.

FeS scale paragenesis in hydrocarbon exploration and production operations in sulfur-bearing reservoirs is generally presumed to proceed from iron sulfide molecular clusters over ferromagnetic pyrrhotite (Fe1-xS) as illustrated in Equations 1 and 2. Multiple potential reaction pathways for FeS formation in petroleum reservoirs are believed to exist. These include (1) direct precipitation from the reaction of iron and sulfide to form iron monosulfide precursors (for example, mackinawite); (2) progressive conversion of solid iron monosulfide species to form iron deficient pyrrhotite; and (3) further redox reactions of iron monosulfide that involve reactive sulfide intermediate species (for example, bisulfide (HS−) and polysulfide Sn2−) that lead to the formation of stable marcasite and pyrite minerals as shown in Equations 1 and 2.
Fe2+(aq)+H2S/HS−/S2−(aq, gaseous)→Fe(HS)nx→FeS(aq)→FeS scale(stable, metastable minerals)Eq. 1
FeS(Mackinawite)→Fe1-xS(Pyrrhotite)→Fe3S4 (Greigite)→FeS2 (Polymorphic Marcasite)→FeS2 (Pyrite)Eq. 2

Formation, deposition, and treatment of solid iron sulfide scaling products that inhibit or block hydrocarbon migration pathways are poorly understood, and this ultimately reduces production and causes severe corrosion and equipment damage to both upstream and downstream facilities.

There have been certain pilot plant scale FeS treatment studies conducted to mitigate iron sulfide deposition issues; however, very few technologies have been deployed in the field to detect FeS in situ. In oil and gas field settings, FeS detection and mitigation technologies are generally hampered by the fast reactivity of iron and sulfide species, the low sensitivity of sensors, and the dynamic range of FeS concentrations and precursors at HPHT reservoir conditions.

Current control and mitigation practices for FeS scale in the oil industry have focused on treating or removing scale via a multi-stage process using organic and inorganic solutions. However, with continuous or direct addition of acidic solution: (i) acid only removes metastable phases of iron sulfide scale; (ii) acid can produce high H2S concentrations from the reservoir that usually exacerbate pipeline corrosion; and (iii) acid can stimulate microbial growth through microbial sulfur oxidation and reduction pathways. Improved detection and inhibition formulations and methods are needed to provide preventive approaches to mitigate FeS deposition, and to generate little to no H2S, leading to lower operational costs associated with corrosion and microbial treatment in oil and gas production.

SUMMARY

Applicant has recognized a need for systems, processes, and compositions for the accurate detection of and environmentally-friendly prevention and removal of FeS(aq)molecular clusters and solid FeS scale in hydrocarbon production and processing operations.

Disclosed here are sensitive electrochemical systems and methods for detecting iron sulfide scale precursors that eventually aggregate to form iron sulfide scale deposition contaminants in sour oil and gas fields. Additionally, embodiments disclosed here present novel chemical formulations, along with methods for producing the same, that can be used throughout the petroleum industry, or others, to inhibit nano-particulate iron sulfide precursors (FeS(aq)) prior to or during the onset of stable FeS scale mineralization and deposition in oil and gas field production operations. While certain prior art electrodes are strictly designed to detect trace element species for biogeochemical processes from microbial mats, cultures, and waters emanating from hydrothermal systems, voltammetry electrodes described here are able to detect iron, sulfide, and iron sulfide indicative species, including precursor species, in mixed petroleum, hydrocarbon, and aqueous HPHT systems and environments.

A reservoir can be considered “sour” when the H2S content in produced reservoir oil and gas ranges from about 3-25 wt. %, and H2S content can run as high as 90 wt. % in gas fields classified as “ultra-sour” reservoirs.

The costs and negative impacts of FeS scale deposits, generically termed as Black Powder (BP) in the hydrocarbon industry, can be attributed, in part, to the scant knowledge available on iron sulfide formation in low to high concentration sulfur-bearing reservoirs. Disclosed here are systems and methods for detection of scale precursors called nano-particulate FeS(aq)molecular clusters, which were detected by real-time in situ electrochemical methods applicable for oil and gas fields.

Understanding the geochemical formation mechanisms and timing of FeS(aq)molecular clusters will enable the oil and gas industries to inhibit or control the onset of FeS scaling and deposition in HPHT conditions.

Modified high temperature in situ solid state glass gold-amalgam electrochemical sensors are disclosed, and embodiments allow monitoring and quantifying of minimum amounts and forms of iron and sulfide species required to initiate iron sulfide molecular clusters, FeS(aq), and subsequent buildup of FeS scale deposition under hydrocarbon production conditions.

Additionally disclosed are new solvent formulations that can be assimilated into oilfield operations to inhibit iron sulfide molecular clusters, which nucleate to form FeS scaling products in oil and gas fields. Rapid detection systems and new chemical formulations are therefore provided to inhibit and curb FeS precursors to ensure smooth operations and uninterrupted flow of oil and gas, in either production or processing where iron sulfide scale is likely to form.

As noted, little is known about the role of FeS(aq)molecular cluster formation as part of the reactions governing the pathways of FeS scale mineralization in petroleum reservoirs. Without being bound by any theory or practice, dynamic conditions between the formation of the clusters, as well as other thermodynamic species illustrated in the reaction pathways, serve as critical components for inhibiting FeS scaling and deposition in upstream and downstream hydrocarbon production and processing systems.

Detection systems and methods along with chemical formulations disclosed here allow effective inhibition and control of the onset of stable/metastable FeS scale deposits in hydrocarbon transmission pipelines and downhole environments at HPHT reservoir conditions. Suitable HPHT temperature and pressure ranges for in situ detection and treatment systems include between about 75° F. to about 315° F. and about 1,000 psi to about 2,300 psi, respectively.

Disclosed geochemical surveillance and monitoring of the following FeS scale precursors is enabled using real-time electrochemical detection systems and methods during hydrocarbon exploration, production, and processing operations, for example in sulfur-bearing carbonate reservoirs: Fe2+ions; molecular FeS(aq)clusters; hydrogen sulfide (H2S); and reactive sulfide intermediate species (HS−/S2−/Sn−).

Once molecular FeS(aq)clusters are detected in a production or flow line system or process, a mixed solvent comprising any one of or any combination of methyl trifluoromethanesulfonate (CF3SO3CH3) (methyl triflate), zinc acetate (Zn(CH3CO2)2), and copper chloride (CuCl2) can be applied to prevent, reduce, or remove FeS scale mineralization.

Modified HPHT glass Au-amalgam electrodes were used to detect and quantify electroactive species of FeS scale precursors via cyclic voltammetry in petroleum producing environments. Inhibition of nano-particulate FeS(aq)molecular clusters in oil and gas fields can be achieved. Nano-particulate FeS(aq)molecular clusters are inhibited by newly-formulated mixed organic-ionic solvent systems. Core flood experiments using sulfur-bearing petroleum source rocks were performed for the first time to observe the formation and build-up of nano-particulate FeS(aq)molecular clusters in petroleum systems. In addition, embodiments have been designed to investigate, detect, and monitor solution-phase electroactive species such as FeS(aq), Fe3+, Fe3+S2−, O2, H2S, including sulfide intermediates such as HS−, S2−, and Sn2−, which are responsible for forming iron sulfide, FeS, solid scales in petroleum production and processing systems.

In other words, modified HPHT glass Au-amalgam electrodes for direct in situ electrochemical detection methods directly monitor solution-phase and nano-particulate FeS(aq)molecular clusters at reservoir conditions. Rapid geochemical surveillance and monitoring of FeS scale and deposit precursors in oil and gas fields is enabled. In some embodiments, an 80%, 90%, or 95% inhibition of nano-particulate FeS(aq)molecular clusters is observed prior to the onset of formation of stable FeS scaling and deposition in petroleum systems. Complete scavenging of sulfides and up to 70% or 81% reduction in certain reactive species under reservoir HPHT conditions are achieved.

Therefore, disclosed here are methods for iron sulfide scale precursor detection, reduction, and removal, one method including the steps of detecting at least one component indicative of an iron sulfide scale precursor, the at least one component selected from the group consisting of: H2S; HS−; S2−; FeS(aq); Fe2+; Fe3+; and combinations of the same; preparing a composition to react with the iron sulfide scale precursor, the composition comprising at least one compound selected from the group consisting of: a methylating agent; a metal operable to react with sulfide species; a compound to increase the oxidation state of Fe2+; and combinations of the same; and applying the composition to the iron sulfide scale precursor to consume the iron sulfide scale precursor. In some embodiments, the step of detecting comprises the use of a solid state glass Au-amalgam electrode comprising activated carbon particles. In other embodiments, the Au-amalgam electrode comprises a gold wire about 100 μm in diameter. In certain embodiments, the method includes the use of a Pt counter electrode, and a Ag/AgCl reference electrode with the Au-amalgam electrode.

Still in other embodiments of the method, the composition comprises the methylating agent, the metal operable to react with sulfide species; and the compound to increase the oxidation state of Fe2+. In some embodiments the methylating agent comprises methyl trifluoromethanesulfonate, the metal comprises zinc, and the compound to increase the oxidation state of Fe2+comprises chlorine. In yet other embodiments, the step of preparing comprises mixing methyl trifluoromethanesulfonate with copper chloride and zinc acetate. In some embodiments, the molar ratio of methyl trifluoromethanesulfonate to zinc acetate is about 1:1 and the molar ratio of methyl trifluoromethanesulfonate to copper chloride is about 10:1. In some other embodiments, the step of mixing includes the use of an organic solvent. In certain embodiments, the organic solvent comprises ethanol.

In some embodiments of the method, before the step of applying the composition is cooled to below room temperature. Still in other embodiments, the method includes the step of applying voltammetry to verify a decrease of concentration in at least one component selected from the group consisting of: H2S; HS−, S2−; FeS(aq); Fe2+; and combinations of the same, or to verify an increase in concentration of Fe3+. In other embodiments, the method includes the step of applying the composition to reduce the formation of a compound selected from the group consisting of: PbS, ZnS, HgS, and combinations of the same. Still in other embodiments, the step of applying the composition to the iron sulfide scale precursor to consume the iron sulfide scale precursor comprises applying the composition in a batch of about 3 barrels to about 5 barrels to a wellbore proximate a hydrocarbon-bearing formation every about 30 to about 45 days.

Additionally disclosed are compositions for prevention and reduction of iron sulfide scale formation, one composition including a methylating agent; a metal operable to react with sulfide species; and a compound to increase the oxidation state of Fe2+, where at least one of the methylating agent, the metal, and the compound to increase the oxidation state of Fe2+react with at least one component selected from the group consisting of: H2S; HS−, S2−; Sn2−; FeS(aq); Fe2+; and combinations of the same. In some embodiments, the methylating agent comprises methyl trifluoromethanesulfonate, the metal comprises zinc, and the compound to increase the oxidation state of Fe2+comprises chlorine. Still in other embodiments, the composition comprises methyl trifluoromethanesulfonate, copper chloride, and zinc acetate. In certain embodiments, the molar ratio of methyl trifluoromethanesulfonate to zinc acetate is about 1:1 and the molar ratio of methyl trifluoromethanesulfonate to copper chloride is about 10:1. In yet other embodiments, the composition further comprises an organic solvent. In certain embodiments, the organic solvent comprises ethanol.

Still in other embodiments of the composition, the composition consists essentially of methyl trifluoromethanesulfonate, copper chloride, and zinc acetate. In other embodiments, the composition consists of methyl trifluoromethanesulfonate, copper chloride, and zinc acetate.

Additionally disclosed are methods for producing a solid state glass gold-amalgam electrode, one method including disposing a wire comprising gold in and through a high-temperature glass electrode body to expose a portion of the wire external to the high-temperature glass electrode body for detecting electrical current, the exposed portion of the wire located proximate mercury; and encasing a portion of the wire proximate the exposed portion with an epoxy resin containing gold and carbon particles. In some embodiments, the high-temperature glass electrode body comprises borosilicate glass. Still in other embodiments, the method includes the step of detecting at least one component indicative of an iron sulfide scale precursor, the at least one component selected from the group consisting of: H2S; HS−; S2−; FeS(aq); Fe2+; Fe3+; and combinations of the same. Other embodiments of the method include the steps of preparing a composition to react with the iron sulfide scale precursor, the composition comprising at least one compound selected from the group consisting of: a methylating agent; a metal operable to react with sulfide species; a compound to increase the oxidation state of Fe2+; and combinations of the same; and applying the composition to the iron sulfide scale precursor to consume the iron sulfide scale precursor.

Still other embodiments further include pulverizing activated carbon; combining the pulverized activated carbon with gold; and combining the pulverized activated carbon and gold with ultra-high temperature epoxy resin stable up to about 600° F. to form the epoxy resin containing gold and carbon particles. Other embodiments include acidifying the pulverized activated carbon; combining the pulverized activated carbon with a stabilizing functionalized amine; and sonicating the pulverized activated carbon and gold with ultra-high temperature epoxy resin stable up to about 600° F., wherein the step of combining the pulverized activated carbon with gold comprises the use of chloroauric acid. In some embodiments, the pulverized activated carbon comprises particles with a median particle size between about 5 μm to about 10 μm and the gold comprises particles with a median particle size between about 2 nm to about 8 nm.

Additionally disclosed are solid state glass gold-amalgam electrodes, one electrode including a high-temperature glass electrode body comprising borosilicate glass; a wire comprising gold disposed in and through the high-temperature glass electrode body exposing a portion of the wire external to the high-temperature glass electrode body for detecting electrical current, the exposed portion of the wire located proximate mercury; and an epoxy resin containing gold and carbon particles encasing a portion of the wire proximate the exposed portion. In some embodiments, the wire comprising gold is about 100 μm in diameter. In other embodiments, the epoxy resin once dried is stable up to about 300° F., 400° F., 500° F., or 600° F. Still in other embodiments, a portion of the wire not encased by the epoxy resin containing gold and carbon particles is encased by epoxy resin without gold and carbon particulates. And still in other embodiments, the carbon particles exhibit a median particle size between about 5 μm to about 10 μm and the gold particles exhibit a median particle size between about 2 nm to about 8 nm.

