Hydrate inhibition

Details Hydrate inhibition Hydrate inhibition

2000-04-04

2002-08-20

Tucker, Philip (Department: 1712)

Earth boring, well treating, and oil field chemistry

Preventing contaminant deposits in petroleum oil conduits

C507S123000, C507S128000, C507S129000, C507S135000, C507S939000, C585S015000, C585S950000

Reexamination Certificate

active

06436877

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 offs hore 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. Thus according to the present invention, there is provided a method of inhibiting or retarding hydrate formation, which method comprises adding additives (hereinafter called Additives) which comprise (i) a corrosion inhibitor and (ii) a salt which is of formula [R
1
(R
2
)XR
3
]
+
Y
−
, I 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, NR4 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, the additives being added in an amount effective to inhibit or retard hydrate formation, to a medium susceptible to hydrate formation.
The Additive (i) 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 activity and especially surface wetting activity.
It is especially a nitrogenous compound with 1 or 2 nitrogen atoms. The corrosion inhibitor may be a primary, secondary or tertiary amine, or a quaternary ammonium salt, usually in all cases with at least one hydrophobic group, usually a benzene ring or a long chain alkyl group eg. of 8-24 carbons. It may be a quaternary ammonium salt, a long chain aliphatic hydrocarbyl N-heterocyclic compound or a long chain amine. The quaternary salt may be an (optionally alkyl substituted) benzyl trialkyl ammonium halide, in particular when at least 1 and especially 1 or 2 alkyl groups is of 1-20, in particular 8-20 carbons such as cetyl and the other alkyl groups are of 1-6 carbons such as methyl or ethyl; examples are benzyl alkyldimethyl ammonium chloride and Benzalkonium chlorides.
Other quaternary ammonium salts may be of formula [R
5
R
6
NR
7
R
8
]
+
Z
−
, wherein Z is an anion eg. a halide or sulphate, R
5
is an alkyl or alkenyl group of at least 8 carbons, R
6
is an alkyl or alkenyl group, each of at least 2 carbons or a N-heterocyclic group, and R
7
and R
8
, which may be the same or different represents an alkyl group, with the proviso that at least one of R
6
-R
8
has less than 4 carbon atoms. R
5
may be of 8-24 carbons, such as 10-18 carbons, especially, dodecyl, lauryl, cetyl, palmityl, stearyl or oleyl, while R
6
may be selected from the same groups as R
5
, or may be ethyl, propyl, isopropyl, butyl or hexyl. R
7
and R
8
may be selected from the same groups as R
6
but preferably represent methyl groups. Examples of these quaternary salts are cetyl trimethyl ammonium, dodecyl trimethylammonium and lauryl trimethylammonium halides, eg. chlorides or bromides.
Other quaternary salt corrosion inhibitors are of formula [R
9
NR
10
R
11
]
+
Z
−
, where Z is a anion eg. as defined above, R
9
N or R
9
NR
10
forms a quaternizable N heterocyclic ring, and R
11
represents an alkyl or alkenyl group each of at least 8 carbons eg. as described for R
5
. The R
9
N group may be N-heterocyclic group with 1 or 2 ring N atoms, especially with 1 or 2 heterocyclic rings, eg. of 5 or particularly 6 ring atoms; examples of the rings are saturated ones eg. piperidine. The group R
9
NR
10
may also be such an N heterocyclic group but with the R
9
and R
10
groups combined with the N atom to which they are bonded to form an unsaturated ring or fused N bridged ring system such as a pyridine ring. R
10
if present may otherwise be an alkyl or alkenyl group eg. as described for R
8
. Examples of these quaternaries are cetyl pyridinium halides, such as the chloride.
The corrosion inhibitor may also be a long chain aliphatic hydrocarbyl N-heterocyclic compound, which is not quaternised. The aliphatic hydrocarbyl group in the heterocyclic compound usually has 8-24 carbons in the hydrocarbyl group, preferably a linear saturated or mono or diethylenically unsaturated hydrocarbyl group; cetyl-, stearyl and especially oleyl-groups are preferred. The N-heterocyclic compound usually has 1-3 ring N atoms, especially 1 or 2 which usually has 5-7 ring atoms in each of 1 or 2 rings; imidazole and imidazoline rings are preferred. The heterocyclic compound may have the aliphatic hydrocarbyl group on an N or preferably C atom in the ring; the ring may also have an amino-alkyl (e.g. 2-amino ethyl) or hydroxyalkyl (e.g. 2-hydroxyethyl) substituent, especially on an N atom. N-2-aminoethyl-2-oleyl-imidazoline is preferred.

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