Technical field

The present invention relates generally to the field of enhanced oil production and recovery. More specifically, it relates to the recovery of crude oil from emulsions of polymer enhanced oil recovery floods.

Background of the invention

Crude oil is a limited resource and thus it is important to maximise its recovery from existing oil reservoirs. Conventional recovery methods include primary production and secondary water flooding, however these result in a significant quantity of crude oil remaining in the reservoir. Enhanced oil recovery (EOR), also known as tertiary recovery, refers to various techniques for increasing the amount of crude oil that can be extracted from an oil reservoir. Such techniques include thermal recovery, gas injection and chemical injection. Using EOR, 30 to 60% or more of a reservoir's original oil can be extracted compared to 20 to 40% using primary and secondary production methods.

Chemical enhanced oil recovery (cEOR) is expected to play a major role in the future of global crude oil production. cEOR methods include the use of polymer floods, e.g. surfactant-polymer (SP) floods and alkali-surfactant-polymer (ASP) floods, in which a combination of materials including a water-soluble polymer is injected into the reservoir, typically in a brine solution. The precise nature of the polymer is generally not relevant provided this can increase the viscosity of the injected water. Amongst the most commonly used polymers in cEOR methods are acrylamide-based polymers, e.g. polyacrylamide (PAM) and hydrolysed polyacrylamides (hPAM).

Polymer flooding is a well known technique which has been used for many years. It can result in a significant increase in oil recovery compared to conventional water flooding techniques. Compared to other EOR methods, it is also simple, cost effective, low risk and has the advantage that it can be used over a wide range of oil reservoir conditions. Polymer is dissolved in the injected water to increase its viscosity and to increase the sweep efficiency in the reservoir. When water is injected into an oil reservoir it finds the path of least resistance. Where the remaining oil has a higher viscosity than the injected water, the water will finger through this oil and effectively bypass it. This results in a low sweep efficiency and a loss in recovery of oil. By decreasing the mobility ratio between the water and oil, sweep efficiency is enhanced. This results in a higher recovery of oil from the reservoir.

Extensive research and development and field practices have demonstrated improvements in oil recovery using polymer flooding, however the development of effective methods for processing of the produced emulsions lags behind. The polymer in the injection fluid, which alters its physical and chemical behaviours, ultimately breaks through with the produced emulsion and changes its physical and chemical characteristics. This can result in tighter emulsions, as well as increased produced water viscosities, and limitations on processing methods and process conditions, e.g. reduced efficiency of current equipment technologies, and in some cases equipment failures and damage. In particular, the presence of the polymer makes it difficult to process the polymer-containing emulsions using conventional production technologies. Added to the processing difficulties is the uncertainty of the nature of the breakthrough fluid - as the polymer passes through the reservoir its physical and chemical properties change due to formation adsorption, mechanical degradation, and chemical reactions (which can alter the degree of hydrolysis of the polymer). The polymer which is present in the produced emulsions may thus differ from the polymer material injected into the reservoir and may be constantly changing during the production cycle. This can make it more difficult to predict the condition of the polymer material in the breakthrough fluid.

The result of methods using polymer floods is a produced emulsion that contains at least crude oil, water and polymer (this may be in addition to other materials such as surfactants and alkaline agents). Because the injected polymers are anionic, electrostatic and steric effects can result in enhanced emulsion stability thus preventing agglomeration and settling. The stability of the emulsion presents difficulties in its separation, in particular with respect to the quality of the produced oil (this should be suitably 'dry' for sale and further processing in an oil refinery). Key to the success of oil recovery methods involving the use of polymer floods is thus the ability to break the produced emulsions in order to produce a dry oil and produced water with a sufficiently low content of oil.

As discussed, the presence of the polymer as a component of the produced emulsions may also impact on other downstream processing methods and equipment. Where polymer EOR flooding is used, heating equipment, e.g. heat exchangers and in-vessel heating elements such as firetubes, can suffer severe fouling. This decreases outlet temperatures of heat exchangers and can also lead to equipment failures.

It is therefore important not only to be able to break the produced emulsions, but in certain cases also to effectively remove the added polymer materials to minimise any negative impact on downstream processing methods and equipment.

Various chemicals, e.g. surfactants and flocculants, have been proposed for use in demulsifying emulsions containing crude oil and water, however, these do not always provide the desired degree of separation of these components in the presence of breakthrough polymer. For example, they may produce oil which contains an unacceptable level of water, or water which is still contaminated with unacceptable levels of oil.

Flocculants proposed for use in the oil and gas industry include cationic alkyl ammonium halide surfactants and amphoteric surfactants. For example, n-octyl trimethyl ammonium bromide (C ₈ TAB) has been proposed for use as a demulsifier in cEOR. Organic cations charge- compensate the negatively charged polymer promoting flocculation and separation of the emulsion. However, the commonly used organic cations are toxic to the environment and are expensive.

A need thus exists for alternative methods for separating crude oil and water from emulsions produced in polymer flood processes. A need further exists for such methods for demulsifying the produced emulsions to produce a substantially clean separation of the crude oil and water and, preferably, removal of the polymer material.

The present invention is intended to solve or at least alleviate the problems identified above. An object of the invention is thus to provide a process for enhancing the separation of a crude oil and water emulsion stabilised by a water-soluble polymer. It is also desirable to further provide a process in which the water-soluble polymer can be removed from the emulsion.

Specifically, the present inventors now propose that a multivalent (e.g. trivalent) metal- containing compound is added to the emulsion produced following a polymer-enhanced oil recovery process. Although not wishing to be bound by theory, it is believed that the multivalent metal cations interact with carboxylic acid groups in the polymer molecule which curls the polymer and thereby reduces its ability to form hydrogen bonds to oil droplets and to water molecules. This disrupts the polymer/droplet-stabilising interaction and so promotes

coalescence and separation of the emulsion. The addition of an excess of the metal-containing compound may result in precipitation of the polymer from the oil and water emulsion and thus enable its separation. However, it need not be separated to achieve the benefit of enhanced separation of the oil / water emulsion as herein described.

The processes herein described represent a significant improvement in the separation of emulsions compared to the methods in the art. Additionally, by using inexpensive and low toxicity multivalent metal-containing compounds these represent a cost-effective solution to the problems in the art, as well as having a low impact on the environment.

Summary of the invention

In one aspect the invention provides a process for demulsifying an emulsion comprising oil, water and a water-soluble polymer, said process comprising contacting the emulsion with an effective amount of a multivalent metal-containing compound.

In another aspect the invention provides a method for removing a water-soluble polymer from an emulsion comprising oil and water, said process comprising the following steps:

(a) contacting the emulsion with a multivalent metal-containing compound whereby to cause the formation of a precipitate between said polymer and metal cations present in said compound; and

(b) recovering the resulting precipitate.

In a further aspect the invention provides a method of recovering crude oil from a crude oil- containing formation, said method comprising:

(a) providing a composition comprising water and a water-soluble polymer to a crude oil- containing formation;

(b) allowing the composition to contact with at least a proportion of the crude oil in said formation;

(c) recovering from said formation an emulsion which comprises crude oil, water and said polymer; (d) subjecting said emulsion to a demulsifying process which comprises contacting the emulsion with an effective amount of a multivalent metal-containing compound; and

(e) recovering crude oil from the resulting separated emulsion.

In a further aspect the invention provides the use of a multivalent metal-containing compound as a demulsifying agent, for example as a demulsifying agent to enhance the separation of an emulsion comprising oil (e.g. crude oil), water and a water-soluble polymer.

In a further aspect the invention provides the use of a multivalent metal-containing compound to remove a water-soluble polymer from an emulsion comprising oil (e.g. crude oil) and water.

In a further aspect the present invention provides purified crude oil obtained or obtainable by any of the processes herein described.

Detailed description of the invention

The present invention describes a process for treating a crude oil-containing stream which contains a water-soluble polymer added to enhance recovery of crude oil from an oil reservoir (also referred to herein as an "oil-containing formation"). The process involves contacting the stream which contains crude oil, water and the water-soluble polymer with at least one multivalent metal-containing compound.

The metal-containing compound for use in the invention may be any compound which contains at least one multivalent metal cation (e.g. one or two such metal cations). The compound should be at least partially soluble in water under the processing conditions employed for separation of the emulsion. Examples of metal compounds which find use in the invention are multivalent metal salts and multivalent metal complexes. Metal cations which may be present in such compounds include trivalent metals such as trivalent aluminium and trivalent iron.

In one embodiment the metal compound is a salt of a multivalent metal, preferably a trivalent metal salt such as one containing either trivalent aluminium or trivalent iron. Typically these may be an inorganic metal salt which term also includes mixed salts containing fixed proportions of two salts which share either a common cation or common anion. Examples of suitable inorganic metal salts include FeCI ₃ , Fe ₂ (S0 ₄ ) ₃ , AICI ₃ and AI ₂ (S0 ₄ ) ₃ . Most preferably, the metal salt is AICI ₃ . An example of a suitable mixed metal salt which may be used is iron (III) chloride sulfate, FeCIS0 ₄ .

In another embodiment the metal compound may be a multivalent metal complex, or salt thereof. Such complexes may include trivalent metal ions such as trivalent aluminium or trivalent iron. An example of a metal complex which may be used in the invention is aluminium citrate.

The metal-containing compound for use in the invention may be employed in powdered form. However, typically this will be employed in the form of a solution in which the compound is dissolved or suspended. Any solvent suitable for dissolving or suspending the metal compound may be employed. Suitable solvents include water and organic solvents, e.g. an alcohol such as glycol or isopropyl alcohol. In a preferred embodiment the solvent is an aqueous solvent, e.g. water.

In the processes herein described the emulsion may be contacted with the metal-containing compound (or a solution containing the compound) in any convenient way, for example by mixing (e.g. in-line mixing), stirring, etc. The precise point in the production process at which the compound is added to the emulsion is not critical although it is envisaged that this may be injected into the crude oil / water / polymer emulsion as early in the oil recovery process as possible, e.g. as soon as this breaks through topside.

The amount of metal compound to be added is dependent on various factors, not least the amount of polymer present in the emulsion, but also the desired objective of adding the compound, for example whether this is to be used simply to 'break' the oil-in-water emulsion or whether this is intended to precipitate at least a proportion of the polymer. When used to 'break' the emulsion a minimum amount of metal compound required to achieve this would typically be used, e.g. in the range of 1 to 10 ppm. A higher amount would be required to precipitate the polymer.

The time period for contacting the emulsion with the metal compound will be dependent on factors such as the amount of polymer present in the emulsion, the amount of metal compound added, etc., but can be readily determined by those skilled in the art in order to achieve the desired degree of break of the emulsion and thus effective separation. Suitable time periods may vary, but will typically be of the order of a few seconds to a few minutes. Suitable mixing and contact times will generally be achievable by in-line mixing of the metal compound and produced emulsion.

By "oil and water emulsion" or "oil / water emulsion" is meant any emulsion which comprises at least one oil phase and at least one water phase. Typically these will be single emulsions, although they may also be double emulsions. These may comprise a water-in-oil emulsion in which the water phase is dispersed (i.e. is non-continuous) in a continuous oil phase.

Alternatively, these may comprise an oil-in-water emulsion in which the oil phase is dispersed (i.e. is non-continuous) in a continuous water phase. In most cases, the emulsion will be a single water-in-oil or oil-in-water emulsion, most typically an oil-in-water emulsion.

Although the processes herein described are applicable to any suitable oil / water emulsion which contains a water-soluble polymer, the invention is of particular utility in the recovery of crude oil. Therefore the oil will preferably be crude oil. In one embodiment, the emulsion to be subjected to the demulsifying process may thus either be a crude oil-in-water emulsion or a water-in-crude oil emulsion. Most typically it will be a crude oil-in-water emulsion.

As a result of the demulsifying processes herein described it is envisaged that the resulting oil (e.g. crude oil) and water phases will have low water and oil contents, respectively.

Water-soluble polymer materials which can increase the viscosity of an aqueous solution and which are thus suitable for use in EOR processes are well known in the art. Any of these may be employed in the methods, processes and uses herein described. These may be

homopolymers, copolymers or terpolymers. Both synthetic and naturally occurring polymer materials (i.e. biopolymers) may be used. Examples of biopolymer materials include guar gum, cellulose materials and cellulose derivatives, xanthan gum, etc.

Acrylamide-based polymers such as polyacrylamide and hydrolysed polyacrylamide (HPAM) are widely used in EOR processes and are generally preferred for use in the invention.

Hydrolysed polyacrylamides will usually be partially hydrolysed meaning that the polymer contains repeating units of both acrylamide and acrylic acid (in which the hydrogen of the carboxylic acid may be replaced by an alkali metal cation, e.g. a sodium ion). The degree of hydrolysis may vary and is not critical. Several acrylamide-based polymers are commercially available, for example those sold under the tradename Flopaam® by SNF Floerger. A particularly suitable polymer is Flopaam® 3630 which is a partially hydrolysed polyacrylamide.

When used in EOR processes water-soluble polymer materials will typically be used in combination with at least one surfactant, e.g. a mixture of surfactants. Surface active agents adsorb at the oil-water interface and serve to lower the oil-water interfacial tension. This leads to the mobilisation of trapped residual oil droplets in the reservoir. In addition to water and oil (e.g. crude oil), the oil / water emulsions herein described may thus further comprise at least one surfactant. Suitable surfactants may be selected depending on their temperature stability, and their resistance to salinity and hardness. Anionic surfactants, or mixtures of anionic surfactants, are typically used to reduce the interfacial tension at the oil / water interface and so may be present in the emulsions herein described.

In one embodiment the emulsion is one produced by a surfactant-polymer enhanced oil recovery flood. In this case the emulsion contains at least water, crude oil, a water-soluble polymer and at least one surfactant (e.g. an anionic surfactant).

Other components which may be present in the oil / water emulsion include components which may be present in compositions used to aid in the recovery of crude oil from an oil reservoir. This includes, for example, alkaline agents. Carboxylate soaps are formed when crude oil (which contains acidic components) reacts with hydroxide ions in an alkaline solution. These soaps (known as "petroleum soaps") are capable of adsorbing at the oil-water interface and further lowering the oil-water interfacial tension.

In one embodiment the emulsion is one produced by an alkali-surfactant-polymer enhanced oil recovery flood. In this case the emulsion contains at least water, crude oil, a water-soluble polymer, at least one alkaline agent and at least one surfactant (e.g. an anionic surfactant).

Alkaline agents which may be present include salts of alkali and alkaline earth metals, e.g. alkali metal carbonate salts, alkali metal bicarbonate salts, and alkali metal hydroxide salts. In these salts the alkali metal ion may be lithium, sodium, potassium or cesium. Typically it will be sodium or potassium. Examples of suitable alkaline agents which may be present therefore include sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium carbonate, potassium bicarbonate and potassium hydroxide.