Hydrate inhibition

Details Hydrate inhibition Hydrate inhibition

1997-11-18

2001-06-26

Tucker, Philip (Department: 1712)

Earth boring, well treating, and oil field chemistry

Preventing contaminant deposits in petroleum oil conduits

C507S123000, C585S015000, C585S950000

Reexamination Certificate

active

06251836

ABSTRACT:

The present invention relates to hydrate inhibitors and a method for inhibiting the formation of hydrates in particular to a method for inhibiting the formation of hydrates in the petroleum and natural gas industries.
Hydrates are formed of two components, water and certain gas molecules, e.g. alkanes of 1-4 carbons, especially methane and ethane, such as those found in natural gas. These ‘gas’ hydrates will form under certain conditions, i.e. when the water is in the presence of the gas and when the conditions of high pressure and low temperature reach respective threshold values. The gas may be in the free state or dissolved in a liquid state, for example, as a liquid hydrocarbon.
The formation of such hydrates can cause problems in the petroleum oil and natural gas industries.
Hydrate formation in the field may cause blocked pipelines, valves and other process equipment.
The problem is particularly of concern as natural gas and gas condensate resources are discovered where operating conditions surpass these threshold values, i.e. in deep cold water and on-shore in colder climates.
Hydrates can also form in association with the underground hydrocarbon reservoir thus impeding production by blockage of reservoir pores.
The problem of hydrate formation is however commonest during gas transportation and processing, the solid hydrate precipitating from moist gas mixtures. This is particularly true with natural gas which when extracted from the well is normally saturated with water. Often in such a case, in a cold climate, hydrates will form in downstream transportation networks and this can cause large pressure drops throughout the system and reduce or stop the flow of natural gas.
Hydrate formation may also occur during natural gas cryogenic liquefaction and separation.
A typical situation where hydrate formation can occur is in off shore operations where produced fluids are transported in a long vertical pipeline, for example, a riser system. Such produced fluids normally include light gases known to form hydrates and water. In such a situation a temperature of 4.5° C. and a pressure of 150 psi would be sufficient for hydrate formation.
Several methods are known to prevent hydrate formation and subsequent problems in pipelines, valves and other processing equipment.
Physical methods have been used, e.g. increasing gas temperature in the pipeline, drying the gas before introduction into the pipeline, or lowering the gas pressure in the system. However, these techniques are either expensive or are undesirable because of loss of efficiency and production.
Chemical procedures have also been used. Electrolytes, for example, ammonia, aqueous sodium chloride, brines and aqueous sugar solutions may be added to the system.
Alternatively, the addition of methanol or other polar organic substances, for example, ethylene glycol or other glycols may be used. Methanol injection has been widely used to inhibit hydrate formation. However, it is only effective if a sufficiently high concentration is present since at low concentrations there is the problem of facilitation of hydrate formation. Also for methanol to be used economically under cold environmental conditions there must be early separation and expulsion of free water from the well in order to minimise methanol losses in the water phase.
We have now found certain additives which may be used as effective hydrate inhibitors at low concentrations.
The present invention provides a blend comprising Additive (i) which is a polymer of (a) an ethylenically unsaturated N-heterocyclic carbonyl compound, with 6-8 ring atoms in the heterocyclic ring and optionally (b) a different ethylenically unsaturated N-heterocyclic carbonyl compound with 5-7 ring atoms in the heteroring, the numbers of heteroring atoms in (a) and (b) differing by at least one, and at least 1 of Additives (ii) a corrosion inhibitor and (iii) a salt which is of formula [R
1
(R
2
)XR
3
]
+
Y
−
1/v, 1 wherein each of R
1
, R
2
and R
3
is bonded directly to X, each of R
1
and R
2
, which may be the same or different is an alkyl group of at least 4 carbons, X is S, NR
4
or PR
4
, wherein each of R
3
and R
4
which may be the same or different represents hydrogen or an organic group, with the proviso that at least one of R
3
and R
4
is an organic group of at least 4 carbons, especially at least 5 carbons, and Y is an anion of valency v, wherein v is an integer of 14 e.g. 1 or 2.
The present invention also provides a method of inhibiting or retarding hydrate formation and/or growth, which method comprises adding a blend of the invention in amount effective to inhibit or retard hydrate formation to a medium susceptible to hydrate formation.
The Additive (i) is a Polymer of (a) and optionally (b), each being ethylenically unsaturated N-heterocyclic carbonyl compounds. The weight proportions of structural units from (a) to (b) may be 100:0 or 100-40:0-60, such as 100-60:0-40 or preferably 100-85:0-15, preferred proportions are 100:0, 50:50 and 75:25. The Polymer has a hydrocarbon chain with pendant N-heterocyclic carbonyl groups, with the bonding to the chain preferably via the heteroring -N- atom.
The polymer may be made by polymerisation of (a) or simple copolymerisation of (a) and (b) or may be a graft copolymer, e.g. from grafting (b) onto homopolymeric (a). Each N-heterocyclic carbonyl compound may contain 1 or more than 1 e.g. 2 or 3 heterocyclic rings, but each case it contains at least 1 ring containing the specified number of ring atoms. That N heterocyclic ring may contain 1-3 ring N atoms but especially I ring N atom and 0-2 other ring hetero atoms e.g. 0 or S, but especially no extra ring hetero atom. The ring or rings may be saturated or ethylenically unsaturated. The carbonyl group may be in any position in the N heteroring, but is especially alpha to the N hetero atom, so the N-heterocyclic rings are preferably derived from lactams, such as those derived from butyric, pentenoic, pentanoic or hexanoic acid lactams (or 2-pyrrolidone, 2-pyridone, 2-piperidone or omega caprolactam). The polymer may have structural units from N-vinyl omega caprolactam (and be a homopolymer) or may also have structural units from N-vinyl pyrrolidone.
The polymer preferably consists essentially of structural units derived from (a) e.g. homopoly caprolactam or consists essentially of structural units from (a) and only ethylenically unsaturated N-heterocyclic compounds especially (b); structural units from polar ethylenically unsaturated non cyclic compounds (especially from esters of an alcohol containing more than I polar group and an unsaturated acid) and/or ethylenically unsaturated carbocyclic carbonyl compounds are preferably substantially absent.
Preferably the Polymer is water soluble or water dispersible, e.g. to an extent of at least 0.01% by weight in water such as at least 0.05% but especially at least 0.5%, such as up to 10% by weight. Its molecular weight is usually 5,000 to 1,000,000 e.g. 10,000 to 1,000,000 such as 1,000 to 50,000 or 50,000-500,000 and preferably has a K value of 10-150 especially 15-50, wherein the K value is obtained from the relative viscosity in aqueous solution via the FIKENTSCHER'S Formula, from which the average molecular weight is calculated as described in U.S. Pat. No. 2,811,499.
The polymers may be as described in WO 94/12761, the disclosure of which is herein incorporated by reference.
The blend of the invention also comprises at least one of Additive (ii) the Corrosion Inhibitor and (iii) the salt of formula I. The blends may comprise both (ii) and (iii), or may comprise (ii) in the substantial absence of (ii) or (iii) in the absence of more than 2% of (ii) (based on the combined weight of Additives (i) and (iii), preferably in the substantial absence of (ii).
The Additive (ii) is a corrosion inhibitor eg. for steel and usually one suitable for use in anaerobic environments. It may be a film former, capable of being deposited as a film on a metal eg. a steel surface such as a pipeline wall. It preferably has surfactant

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