GAS HYDRATE INHIBITORS
FIELD OF THE INVENTION
The present invention relates to a method of inhibiting the formation and/or agglomeration of gas hydrates in a petroleum fluid stream containing water. The method uses gas hydrate inhibitors selected from the group consisting of alkoxylated amines. Specifically, preferred gas hydrate inhibitors include poly alkoxylates of ammonia, and derivatives and mixtures thereof.
BACKGROUND OF THE INVENTION
Formation of natural gas hydrates is a common problem during oil and gas production. Clathrates are crystalline compounds that occur when water forms a cage-like structure around guest molecules. The term "Gas hydrates" refers generally to clathrates (inclusion structures) of gases in a lattice consisting of water molecules. "Formation of clathrate hydrates" refers to the nucleation, growth, and/or agglomeration of clathrate hydrates. Gas hydrates are metastable solids in which hydrogen bonded water molecules (hosts) encase the low boiling hydrocarbon molecules (guests) in a cage-like entity. These entities are often described as having an ice-like structure. The cage-like structures can totally enclose or trap a gas molecule.
The mechanism of hydrate formation is believed to follow a two-step process. First, the formation of clusters of hydrogen bonded water molecules around a non- polar core; followed by joining of these clusters to form gas hydrates. Gas hydrate formation is usually favored at temperatures close to the freezing point of water. However, under sufficient pressure, gas hydrates will form at temperatures above the freezing point of water. At high pressures, multiple cages can combine to form large crystalline assemblies. These agglomerates are thermodynamically favored even at temperatures well above the freezing point of water. These hydrates can form in fluids regardless of whether the fluid is flowing or substantially stationary, but are often most problematic in flowing fluid streams conveyed in a pipeline. The
resultant solids can form plugs that restrict or block gas flow during oil and gas production.
Gas hydrates often form under conditions of elevated pressure and reduced temperature, in compositions that contain hydrate-forming hydrocarbons, i.e. low boiling point hydrocarbons, and other gasses are mixed with water. Typical gas hydrates formed in petroleum (hydrocarbon) environments are composed of water and one or more guest molecules such as methane, ethane, propane, isobutane, normal butane, neopentane, ethylene, propylene, isobutylene, cyclobutane, cyclopropane, cyclopentane, cyclohexane, benzene, and so forth. Non - hydrocarbon components of hydrate forming gaseous mixtures include, but are not limited to oxygen, nitrogen, carbon dioxide, hydrogen sulfide, sulfur dioxide, and chlorine. Many of these hydrate forming compounds as well as water are typically present in natural gas and other petroleum fluids.
In the petroleum industry, gas hydrates pose particular problems. Flow restrictions from partial or complete blockages of a fluid stream can arise as clathrate hydrates adhere to and accumulate along the inside wall of the pipe used to convey the fluid. These restriction effect the production, transport, and processing of natural gas and petroleum fluids. Gas hydrates can block transmission lines and blowout preventers, they can jeopardize the foundations of deep-water platforms and pipelines, collapse tubing and casing, and foul process heat exchangers and expanders. These and other production equipment problems associated with gas hydrates can lead to the shutdown of the well. Shutdowns obviously can result in significant financial losses. Furthermore, restarting a shutdown facility, particularly an offshore production or transport facility, can be difficult because significant amounts of time, energy, and materials, as well as various engineering adjustments, are often required to safely remove the hydrate blockage. Safety of well operators is also a big concern due to the increased risk of explosion due to the accumulation and high concentration of gas in the gas hydrate agglomerations.
The control of gas hydrate formation in production can be achieved in various ways. Hydrates can be controlled by altering the temperature or the pressure at site of production. This is however very engineering intensive and expensive. Another method is based on chemical treatments of the crude. This method is much more common due to economic reasons. The chemicals used for control of gas hydrates are commonly referred to as gas hydrate inhibitors. These inhibitors are broadly grouped into three different categories based on the way in which they function to limit hydrate formation. The three broad categories are thermodynamic inhibitors, kinetic inhibitors, and anti-agglomerates.
Thermodynamic inhibitors disrupt the thermodynamic equilibrium conditions under which formation of hydrates is favored by suppressing the hydrate dissociation curves to lower temperatures or higher pressure. Several thermodynamic measures are possible to eliminate the formation of hydrates. In principal, one can remove free water, maintaining an elevated temperature, and/or reduced pressure, or can add freezing point depressants (antifreeze). In the latter case, large quantities of thermodynamic inhibitors are required in order to provide an effective treatment. As a practical matter, adding freezing point depressants such as lower alcohols and glycols has been most frequently utilized. However, in order for such substances to be effective, it is necessary that they be added in substantial amounts, e.g., 30% by weight of the water present. Not only is this expensive, it also poses additional problems in handling large amounts of flammable solvent (in the case of methanol treatment) and in the recovery of these additives prior to further processing of the fluid. Consequently, these procedures are ultimately uneconomical. Examples of this type of gas hydrate inhibitor include tripropylene glycol bottoms (Halliday, et al. U.S. Patent Nos. 6,165,945; 6,080,704); Polyethylene glycol (Collins, et al. U.S. Patent No. 6,225,263); and glycerol-based organics (Hale, et al. U.S. Patent Nos. 5,076,364, 5076373, 5083622, 5085282, 5248665).
Kinetic inhibitors are typically polymeric chemicals that are added to the gas- water mixture at low concentrations to prevent or delay nucleation and crystal growth. Kinetic inhibitors are cost effective but fail to inhibit agglomeration of the crystals once nucleation occurs. Examples of this type of gas hydrate inhibitor include: N-substituted acrylamides (U.S. Patent No. 5,600,044 CoIIe, et al.); CIE polymers (polyalkyleneimines, U.S. Patent No. 5,583,273 CoIIe, et al.); copolymer of vinyl caprolactam and vinyl pyridine, (U.S. Patent No. 6,281 ,274 Bakeev, et al., U.S. Patent Nos. 6,242,518; 6,117,929); vinyl caprolactam and a polyoxyalkylenediamine copolymers (Bakeev, et al., U.S. Patent No. 6,180,699) of N-acyldehydroalanine derivatives (U.S. Patent No. 6,222,083 CoIIe, et al); alkylacrylamides (U.S. Patent No. 6,107,531 CoIIe, et al; U.S. Patent No. 6,028,233; 5,874,660; 5,600,044); caprolactam derivatives in combination with quaternary ammonium salt (Duncum , et al. U.S. Patent No. 6,251 ,836); Cationic polymers (U.S. Patent No. 5,981,816 Sinquin et al.); maleimide copolymers (U.S. Patent No. 5,936,040 Costello, et al.); meth)acrylate derivatives (U.S. Patent No. 6,232,273 Namba, Takashi); "polysurfactants" (Peiffer, et al. U.S. Patent No. 6,194,622); Acrylamide polymers and maleic esters (Klug, et al. U.S. Patent No. 6,177,497); acrylate polymers and amine quaternary salts (Duncum, et al. U.S. Patent No. 6,063,972); vinyl polymers and non-ionic, anionic, cationic, zwitterionic organic compounds (CoIIe , et al. H1 ,749).
Anti-agglomerates are typically polymeric, or are surface-active chemicals, such as surfactants, that prevent the growth of hydrate crystals and allow the transportation of the gas-water mixture through the pipelines. Anti agglomerate mechanism for hydrate inhibition are considerably different than kinetic inhibitors. It is believed that anti-agglomerates have a dual mode of activity. While they allow hydrates to form, they limit the growth of these hydrates thus minimizing the risk of pipeline plugging. These additives are believed to inhibit further growth of the hydrate by binding to the surface of the initially formed hydrate thus altering the structure of the clathrate. Anti-agglomerates second mode of hydrate inhibition is achieved by their dual behavior as dispersants. These molecules allow the
previously formed hydrates to disperse in the oil phase. The effectiveness of a given anti agglomerate has been shown to be dependent on the hydrocarbon fluid composition, the level of salinity and the amount of water (water cut) in the formation. Anti-agglomerates are typically added at low concentrations (<2%) and are consequently quite economical. Anti-agglomerators are believed to function at the gas-water interface. Examples of some surfactants used as anti- agglomerators include esters of polyols and substituted or unsubstituted carboxylic acids (Sugier, et al., U.S. Patent No. 5,244,878); Shell Quat (U.S. Patent No. 6,214,091and U.S. Patent No. 6,152,993, Klomp); Amine oxide (U.S. Patent No. Klug et al., 6,102,986); polyether ammonium (Pakulski et al., U.S. Patent Nos. 6,025,302, 6,025,302); Amido diamine alkoxylates (Pakulski et al., U.S. Patent No. 5,741 ,758); alkyl aryl sulfonates (Muijs et al., Canadian Patent No. 2,036,084); alkylphylalkoxylates (Chem Eng. Sci., 50 (5), 863, 1995); and alkyl polyglycocides (Reijnhout et al, European Patent No. 526,929).
SUMMARY OF THE INVENTION
The invention relates to a process and a family of inhibitors for inhibiting the formation of gas hydrates in gas-productive drilling or work-over wells, producing wells and facilities (may be subsurface), and onshore and offshore, from both fixed platforms and floating platforms, by injecting a suitable additive to the fluid. The present invention provides an improved method of gas hydrate inhibition using ammonia-based alkoxylates as the gas hydrate inhibitor family.
The process comprises adding into the fluid an effective amount of a gas hydrate inhibitor selected from the group consisting of poly alkoxylated amines including, but not limited to, alkoxylates of ammonia, alkanolamine, i.e., for example, MEA
(Monoethanol amine), DEA (Diethanol amine, and TEA (Triethanolamine), and the like. The base products may be converted to salts with organic and mineral acids, or to their corresponding quats with alkyl halides or alkyl sulfates. The base products may also be converted esters of organic or inorganic acids. These products can be utilized both in salt and non-salt forms as mixtures with other
anionic, non-ionic, cationic and amphoteric compounds. These materials can be utilized to inhibit clathrate formation, growth and/or agglomeration in flowing or even substantially stationary fluids. In particular, they may be suitable for use during the transportation of fluids comprising gas and water. They may also be suitable for use in drilling muds to inhibit hydrate formation during drilling operations.
DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS Using the method of the present invention, a solution, either concentrated or diluted, of one or more of the desired compounds can be introduced into a petroleum fluid stream. The gas hydrate inhibitors of the present invention include poly alkoxylated ammonia that may be represented by the following general formula (I). (I)
where R is H or a group represented by general formula (II):
(H) wherein each of R2 is, independently, H, or a short chain alkyl group such as methyl, ethyl, propyl or butyl, R3 is H or a straight or branched chain, saturated or unsaturated fatty acyl group having 2 to 22 carbon atoms, and x is an integer of from 1 to 50.
Specific examples of gas hydrate inhibitors according to the invention include, but are not limited to the following structures:
Amines such as TEA 14.9 PO.
wherein a + b + c = 14.9;
Polyoxyalkylated ammonia or their derivatives may be prepared by direct alkoxylation of ammonia, or other starting materials derived from ammonia such as alkanol amines including, but not limited to, ethanol amine, diethanol amine, triethanol amine and the like. These highly branched materials may also be converted to their salts with organic or mineral acids, or to their corresponding quats by treatment with an alkylating agent. Preferred alkylating agents include, but are not limited to, alkyl halides, including but not limited to methyl chloride, ethyl chloride, ethyl bromide, butyl bromide and the like, and alkylsulfates, including but not limited to methyl sulfate, and the like.
The polyalkoxylated derivative of the invention is preferably alkoxylated to a total level of 3 to 210 units combined of propylene oxide and/or ethylene oxide and derivatives thereof; and mixtures thereof. In another embodiment said
polyalkoxylated derivative is alkoxylated to a total level of 3 to 60 units combined of propylene oxide and/or ethylene oxide and derivatives thereof; and mixtures thereof. In still another embodiment the polyalkoxylated derivative is alkoxylated to a total level of 3 to 21 units combined of propylene oxide and ethylene oxide and derivatives thereof; and mixtures thereof.
Preferred polyoxylated ammonia derivatives useful as a gas hydrate inhibitor of the present invention are illustrated by general formula (III): (III)
where R has been defined above, R4 is hydrogen, a straight or branched chain, saturated or unsaturated alkyl group having from 2 to 22 carbon atoms, or a substituted or unsubstituted alkyl aryl group having at least from 6 carbon atoms, preferable from 6-30 carbon atoms, and still more preferably from 6 to 18 carbon atoms, and X" is any counter ion that may be used with quaternary ammonium compounds including but not limited to halides; oxo ions of phosphorous, sulfur or chloride; and various organic anionic molecules. The inhibitors of the present invention can be prepared using techniques known to those of skill in the art. As noted above, this preparation generally involves either direct alkoxylation, or the preparation of intermediate compounds (i.e. the alkoxylate) followed by further manipulated to salts, quats or esters.
The inhibitors may be added neat, directly to the petroleum fluid, or they may be dissolved in a solvent or carrier and added as a concentrated solution or even as mixtures with other additives. Solvents include, but are not limited to, water, brine,
sea water and produced water; alcohols, particularly lower alcohols of Ci to Cs, including glycols and poly glycols, alkyl esters such as methyl esters; and mixtures thereof. While the use of a carrier solvent is not required, it is a desirable way of introducing the inhibitors to the fluid.
The amount of inhibitor delivered into the aqueous phase of the petroleum fluid is typically between about 0.01 percent by weight to about 10 percent by weight of the water present in the petroleum fluid, more preferably about 0.1 wt-percentage to about 5 wt-percentage, and most preferably about 0.5 wt-percentage.
Any convenient concentration of inhibitor in the carrier solvent can be utilized providing that it results in the desired final concentration in the aqueous phase of the petroleum fluid. The concentration selected depends on a variety of factors including the inhibitor chosen and its chemical structure, the solvent selected, and the solubility of the inhibitor in the carrier solvent at application conditions.
The inhibitor mixture is introduced into the aqueous phase of the petroleum fluid using mechanical equipment, such as, chemical injection pumps, piping tees, injection fittings, and other devices, which will be apparent to those skilled in the art. However, such equipment is not essential to practicing the invention. To ensure an efficient and effective treatment, all that is necessary is that the hydrate inhibitor disperses through the mixture sufficiently enough to be able to interact with any water within the mixture, thus inhibiting hydrate formation, growth and/or agglomeration. The inhibitor should be introduced into the petroleum fluid prior to formation of the clathrate hydrates. The inhibitor primarily serves to inhibit formation and growth ratherthan reversing their formation. It is therefore important to treat the fluid prior to substantial formation of the hydrates.
As a wet petroleum fluid cools, it eventually reaches a temperature known as the hydrate equilibrium dissociation temperature orTe<7, below which hydrate formation is thermodynamically favored. The Teq of a petroleum fluid will shift as the
pressure applied to the fluid and its composition change. Various methods of determining Teq at various fluid compositions and pressures are well known to those of skill in the art. Preferably, the fluid should be treated with the inhibitor when the fluid is at a temperature greater than its Teg. It is possible to introduce the inhibitor when the temperature is at or slightly below the fluid's Teq but preferably before the clathrate hydrates have begun to form.
There are many modifications and embodiments of the present invention possible, and one of ordinary skill in the art would understand that it is possible to make such modifications without altering the scope of the present invention.
The following examples are intended to be further illustrative of the present invention but are in no way intended to limit the scope of the present invention in any manner.
Evaluation procedure for Low Dose Gas Hydrate Inhibitors
Testing Procedure: To a high pressure 600 ml Parr reactor, modified with a whisk like stirrer, and with the internal cooling coil and dip tube removed, was added 500 ml of a 3.5% synthetic sea salt (Instant Ocean). To the aqueous solution was added 5g (1 % by weight) the specific Gas Hydrate Inhibitor. The headspace of the reactor was then purged with a gas composition employed to form gas hydrates. The Parr reactor was then sealed and pressurized to approximately 3500 psig with the gas mixture. The pressure of the reactor was maintained at 3500 psig, and the contents mixed with a high rate of stirring for 2 hours. The reactor was placed in a cooling bath set to 62° F. After this equilibration stage, the reactor was re-pressurized to 3500 psi, sealed completely, and the stirring rate reduced to approximately 20 RPM. The automated computer interface was enabled, and the data logger was set to collect temperature, pressure and torque measurements every 2 seconds. The temperature of the reactor was held at 62° F for an additional hour. The automated chiller program was then initiated to cool the reactor down to 32° F (at a rate of 0.1 F per minute). The reactor was held at
the final temperature of 32° F, for 5 hours. The reactor was the warmed back to 62° F and maintained there for 4 hours.
Data Analysis: The data obtained from the automated data logger was graphed. All signals are unfiltered except the torque. The torque signal was smoothed by a 9 pt Weighted-moving average mathematical transformation. The transformation is as follows: the standard deviation and average of the 4 points previous and post to each individual point were calculated. The S. D. was divided by the absolute value and multiplied by the true signal. This was then averaged with it self and the eight surrounding values, and divided by the sum of S. D. over the sum of the average.
The TEA family with various level of propoxylation was evaluated using the procedure above. A typical run for the blank (i.e. salt water) is shown along with the run for TEA 14.9 PO. The graphs below were evaluated to indicate the point at which hydrates formed. This point was taken to be the region at which an exotherm, a slight drop in the pressure, and a steep increase in torque was observed.
Blank TriaM 00% Water
6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time (hrs.)
TEA + 14.9 PO 1%, 100% Water
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Time (hrs.)
The data corresponding to the TEA test set is tabulated below.
TEA PO # being the average overall amount of propyleneoxide-derived units per molecule, the starting material being triethanol amine.
The Effects of the # of PO Units on Formation Depression
5 10 15
# of PO Units