Field of Invention

The present invention relates to a process for recovering oil and gas from an oil-bearing formation by injecting an enhanced oil recovery formulation into the formation. The process disclosed herein may be particularly suitable for use with an off-shore well.

Background

The listing or discussion of a prior-pubiished document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

In natural mineral oil deposits, mineral oil is present in the cavities of porous formation rocks which tend to be sealed toward the surface of the earth by impermeable top layers. The cavities may be very fine cavities, capillaries, pores or the like.

In oil production, a distinction is drawn between primary and subsequent production such as secondary and/or tertiary production.

In primary production, after commencement of drilling of the deposit, the mineral oil flows of its own accord through the borehole to the surface owing to the autogeneous pressure of the deposit. The autogeneous pressure can be caused, for example, by gases present in the deposit, such as methane, ethane or propane. However, the autogeneous pressure of the deposit generally declines relatively rapidly on extraction of mineral oil. Therefore, it is usually the case that only a limited amount of the mineral oil present in the deposit can be produced in this way. Primary production is no longer feasible if natural reservoir drive diminishes. When this occurs, secondary recovery methods can be applied.

Secondary methods typically rely on the supply of external energy into the reservoir in the form of injecting fluids to increase reservoir pressure. These fluids replace or increase the natural reservoir drive with an artificial drive. Reservoir pressure may be increased through the injection of carbon dioxide, steam and/or water. The injected fluid is typically immiscible, or predominantly immiscible with the in-situ hydrocarbon fluids.

After primary and/or secondary production, enhanced oil recovery can be applied. This process typically involves the injection of an enhanced oil recovery formulation into a production well. As with other oil recovery methods, enhanced oil recovery tends to produce water besides oil. This produced water can come from sources such as aquifers. From an economic and environmental point of view, it would be advantageous if this produced water could be re-injected. Unfortunately, reinjection schemes generally have a tight specification on water to be re-injected, especially on the amount of oil and solids content allows. The residual oil specification for such water can be as low as 10 parts per million weight (ppmw). A higher amount of oil increases the risk of relatively large oil droplets that can block the pores in the formation, resulting in decreased produced water injectivity or even blockage of the reinjection well. Extensive purification generally is required to arrive at the low oil content required by these tight specifications. While it is currently possible to treat the produced water to meet these tight restrictions, this is requires a significant investment in equipment and consumable chemicals to achieve. In addition, the footprint of the equipment makes reinjection of the produced water only feasible at a land-based well (unless the producer is willing to invest in a second oil platform to carry said equipment for sea-based oil production). Thus, there remains issues on how to reduce the costs associated with economically re-injecting produced water on land and, more particularly, how to enable the use of such re-injection techniques off-shore.

Summary of Invention

In a first aspect of the invention, there is provided a process for recovering oil and optionally gas from an oil-bearing formation, which process comprises the steps of:

(i) injecting into the formation via a first well water or an aqueous enhanced oil recovery formulation comprising water and a surfactant;

(ii) producing from the formation via a second well a mixture comprising water, oil, optionally gas and, when an enhanced oil recovery formulation is injected, optionally a surfactant;

(iii) separating oil and optionally gas from the produced mixture to obtain an aqueous mixture comprising at most 1000 parts per million by weight (ppmw) of oil and optionally a surfactant;

(iv) adding surfactant to the aqueous mixture comprising at most 1000 ppmw of oil and mixing it to obtain an oil-in-water emulsion; and

(v) injecting the oil-in-water emulsion as an enhanced oil recovery formulation into the formation via the first well and repeating steps (ii) to (v).

In embodiments of this aspect: (a) the water for injection or the water used to form the aqueous enhanced oil recovery formulation has a total dissolved solid (TDS) of at most 200,000 ppm (e.g. the aqueous enhanced oil recovery formulation has a total dissolved solid (TDS) of from 2,000 to 50,000 ppm, such as from 2,500 to 35,000 ppm, such as from 3,000 to 30,000 pm, such as from 3,500 to 20,000 ppm, such as from 4,000 to 15,000 pm, such as from 4,500 to 10,000 ppm);

(b) the surfactant in steps (i) and (iv) of the process comprises an anionic surfactant;

(d) the surfactant is present in an amount of from 0.1 to 20 wt%, such as from 0.2 to 5 wt%, of the aqueous enhanced oil recovery formulation;

(e) the surfactant is present in an amount of from 0.1 to 20 wt%, such as from 0.2 to 5 wt%, of the oil-in-water emulsion;

(f) the enhanced oil recovery formulation is formed by mixing water and a surfactant together, where the water contains less than 4,000 ppm of divalent ions, such as less than 2,000 ppm, such as less than 1 ,000 pm, such as less than 500 ppm, such as less than 100 ppm, such as less than 20 ppm of divalent ions;

(g) the oil-in-water emulsion has a total dissolved solid (TDS) of at most 200,000 ppm (e.g. the oil-in-water emulsion has a total dissolved solid (TDS) of from 2,000 to 50,000 ppm, such as from 2,500 to 35,000 ppm, such as from 3,000 to 30,000 pm, such as from 3,500 to 20,000 ppm, such as from 4,000 to 15,000 pm, such as from 4,500 to 10,000 ppm);

(h) the enhanced oil recovery formulation further comprises a polymer suitable for use in enhanced oil recovery (e.g. the polymer suitable for use in enhanced oil recovery is a polyacrylamide, optionally wherein the polymer is a hydrolysed polyacrylamide, such as a partly- or fully-hydrolysed polyacrylamide);

(i) the produced mixture obtained in step (ii) of the process further comprises solid particles and the solid particles are separated from the produced mixture;

(j) the aqueous mixture comprising oil in step (iii) of Claim 1 comprises from 10 to 1 ,000 ppmw of oil, such as from 50 to 900 ppmw, such as from 100 to 800 ppmw, such as from 200 to 750 ppmw of oil (e.g. the aqueous mixture comprising oil in step (iii) of the process comprises from 300 to 1 ,000 ppmw of oil);

(k) the oil in water emulsion comprises from 10 to 1 ,000 ppmw of oil, such as from 50 to 900 ppmw, such as from 100 to 800 ppmw, such as from 200 to 750 ppmw, such as from 300 to 1 ,000 ppmw of oil;

(L) the oil in water emulsion comprises dispersed oil droplets having a diameter of less than or equal to 10 pm, such as less than or equal to 5 pm, such as less than or equal to 2 pm, such as less than or equal to 1 pm, such as less than or equal to 0.5 pm, such from 0.01 to 10 pm;

(m) the oil in water emulsion comprises dispersed oil droplets having a diameter that is less than a reservoir pore throat pore size of the first well; (n) the mixing of step (iv) is carried out using one or more of the group selected from an in-line static mixer, mixing valve, high shear mixers, and pumps;

(o) the mixing of step (iv) further comprises analysis of droplet size within the oil in water emulsion, optionally wherein:

(a) the droplet analysis is conducted continuously or in sample batches; and/or

(b) the analysis of droplet size is used to modulate the shear rate to produce the desired size of droplet within the oil in water emulsion.

Brief Description of Drawings

Certain embodiments of the present disclosure are described more fully hereinafter with reference to the accompanying drawings.

Figure 1 depicts a schematic representation of the oil recovery process of the current invention.

Figure 2 depicts: (a) another embodiment of the current invention with additional filter/filtration system 30 and an oil-in-water droplet dispersion/mixing control system 33; (b) a close-up view of the oil-in-water droplet dispersion/mixing control system 33.

Description

We have now found an efficient process for re-injecting water produced from a well with the help of a surfactant. This process can be applied in reinjection of produced water for disposal well, waterflood and enhanced oil recovery flood operations. This process enables the reinjection of produced water using relatively little equipment, thereby enabling the process to be suitable for use on a single off-shore platform. Thus disclosed herein is a process for recovering oil and optionally gas from an oil-bearing formation, which process comprises the steps of:

(i) injecting into the formation via a first well water or an aqueous enhanced oil recovery formulation comprising water and a surfactant;

(ii) producing from the formation via a second well a mixture comprising water, oil, optionally gas and, when an enhanced oil recovery formulation is injected, optionally a surfactant;

(iii) separating oil and optionally gas from the produced mixture to obtain an aqueous mixture comprising at most 1000 parts per million by weight (ppmw) of oil and optionally a surfactant; (iv) adding surfactant to the aqueous mixture comprising at most 1000 ppmw of oil and mixing it to obtain an oil-in-water emulsion; and

(v) injecting the oil-in-water emulsion as an enhanced oil recovery formulation into the formation via the first well and repeating steps (ii) to (v).

As current state-of the-art processes require that the oil in reinjected water is less than 10 ppmw, there is a need for the use of additional water-polishing equipment, which results in significantly increased costs (both in capital expenditure and in operating costs) and the need for a large space (footprint) to house said equipment. This makes it difficult to conduct such reinjection operations offshore, as it would generally require that a second offshore platform is provided to house the water-polishing equipment. In contrast to state-of-the-art processes, the water used for reinjection in the currently disclosed process may contain up to 1000 ppmw of oil. This relaxation in the amount of oil allowed in the water used for reinjection means that only a limited amount of purification of the recovered water is needed before it is reinjected, thereby significantly reducing the amount of equipment needed, which in turn reduces the costs associated with the process and the amount of space required for the equipment. This reduction in footprint required by the currently disclosed process also makes it possible for the currently disclosed process to be conducted offshore using a single offshore platform.

In embodiments herein, the word“comprising” may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word“comprising” may also relate to the situation where only the components/features listed are intended to be present (e.g. the word“comprising” may be replaced by the phrases“consists of or“consists essentially of). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word“comprising” and synonyms thereof may be replaced by the phrase“consisting of or the phrase“consists essentially of” or synonyms thereof and vice versa.

When used herein “aqueous enhanced oil recovery formulation” refers to a water-based solution that includes at least one suitable surfactant. Any suitable amount of surfactant(s) may be present in the formulation. For example, the formulation may contain from 0.1 to 20 % by weight of surfactant, such as from 0.2 to 5 % by weight. As will be appreciated, the“aqueous enhanced oil recovery formulation” mentioned herein refers to the composition that is to be injected into the well. This formulation may be prepared by the addition of a concentrate to water in a suitable quantity to arrive at the desired weight percentage of surfactant in the formulation for injection. For example, the concentrated formulation may not contain any water whatsoever, or only a minor amount. For example, the concentrated formulation may be one that comprises from 75 to 99 wt% surfactant, with the balance comprising water and/or other components (e.g. polymers as described below).

When used herein, the first well will generally be referred to as the injection well while the second well will generally be referred to as a producer well.

Without wishing to be bound by theory, it is believed that the surfactants used herein (whether as part of an initial aqueous enhanced oil recovery formulation or added to the recovered water reduce the interfacial tension in an oil/water mixture, which may allow for the formation of suitably small oil droplets in the water to enable the current process to function with an increased amount of oil compared to other processes. The surfactants that may be used in the current process (whether in an initial aqueous enhanced oil recovery formulation or added to the recovered water) can be any surfactant known to be suitable by a person skilled in the art. Generally, the surfactant will be a nonionic or an anionic surfactant or a mixture thereof. Examples of suitable surfactants include, but are not limited to, alkyl benzene sulfonates, internal olefin sulfonates, alpha olefin sulfonates, alkyl propoxy sulfates, alkyl ethoxy sulfates, alkyl pro poxy/ethoxy sulfates, alkyl propoxy carboxylates, alkyl ethoxy carboxylates, alkyl propoxy/ethoxy carboxylates and alkyl alcoxy glycidyl sulfonates.

In certain embodiments of the invention that may be disclosed herein, the surfactant may be an anionic surfactant. Examples of suitable anionic surfactants that may be mentioned herein include, but are not limited to alpha olefin sulfonate compounds, internal olefin sulfonate compounds, branched alkyl benzene sulfonate compounds, propylene oxide sulfate compounds, ethylene oxide sulfate compounds, propylene oxide-ethylene oxide sulfate compounds, and mixtures of two or more of said compounds. In particular embodiments of the invention the surfactant (e.g. non-ionic and/or anionic surfactants) may contain from 10 to 30 carbon atoms.

In embodiments that make use of and enhanced oil recovery formulation, said formulation may also comprise a polymer suitable for use in enhanced oil recovery. A polymer class that is suitable for use in enhanced oil recovery are the polyacrylamides, more specifically hydrolysed polyacrylamides. A hydrolysed polyacrylamide can be either partly or fully hydrolysed. Examples of partially hydrolysed polyacrylamides include those that exhibit 1 % or more hydrolysis, but less than 100% hydrolysis.

As noted above, the process may use water in step (i) or it may be used to make an aqueous enhanced oil recovery formulation. The water, whether used alone or to make an enhanced recovery formulation, will generally have a total dissolved solids (TDS) content of from 2000 to 200,000 ppm as measured by ASTM D5907-13. More specifically, the water may have a TDS of at most 50,000 ppm, more specifically at most 35,000 ppm, more specifically at most 30,000 ppm, more specifically at most 20,000 ppm, more specifically at most 15,000 ppm, most specifically at most 10,000 ppm.

It is especially advantageous if the water used for preparing the formulation contains a limited amount of divalent ions such as less than 4000 ppm, more specifically less than 2000 ppm, more specifically less than 1000 ppm, more specifically at most 500 ppm, more specifically at most 100 ppm, most specifically at most 20 ppm of divalent ions based on total amount of water. More specifically, these amounts relate to the calcium and/or magnesium containing salts. Without wishing to be bound by theory, it is believed that the presence of divalent ions will induce inorganic scale formation, especially when the produced water is mixed with injection water/ seawater.

The water or enhanced oil recovery formulation is injected into the oil-bearing formation via a first well. The water or enhanced oil recovery formulation is then pushed through the formation either by the continual injection of the formulation or by introducing a different composition such as water, miscible or immiscible gas, steam or a mixture of one or more of these into the formation.

If water is used in initial step (i) of the process, then water may be simply continually pumped into the formation or it may be moved through the formation through the use a miscible or immiscible gas, steam or a mixture of one or more of these. Oil and optionally gas (i.e. if gas is present in the formation) are recovered from the formation via the second well. As will be appreciated, if only water is used initially, no polymer or surfactant will be present in the initially produced/recovered mixture (which will comprise water, oil and optionally gas if present in the formation). However, when surfactant and optionally polymer is added to the recovered water to provide an enhanced oil recovery formulation for re-injection into the first well in step (iv) of the process described above, then the mixture subsequently recovered from the second well will tend to comprise water, surfactant, polymer, alkali, oil and optionally gas.

If an enhanced oil recovery formulation is used in step (i), then when the enhanced oil recovery formulation has reached the second well, the enhanced oil recovery formulation will be recovered and produced in combination with oil and optionally gas. From then onwards, the mixture which is produced from the second well tends to comprise water, surfactant, polymer, alkali, oil and optionally gas. The mixture produced from the second well may also contain solids such as sand and particles originating from the formation. These particles can be asphaltenes, wax from oil or scale particles from the water. Furthermore, additional particles can occur due to corrosion or interaction of production chemicals. Solids can be separated from the recovered mixture by any method known to a person skilled in the art. A suitable method can be filtration or by allowing the solids to settle at the bottom of a vessel to form a sediment or slurry at the vessel base or by hydrocyclone separation. It will be appreciated that the separation of the solids can be done at any stage after step (ii) and before step (v) of the process disclosed herein.

Oil and optionally gas are recovered from the mixture obtained from the second well. Recovery can be carried out in any way known to be suitable to somebody skilled in the art. For example, the mixture produced via the second well can be separated into gaseous and liquid components in a suitable separator. The separator can be an in-line gas-liquid separator or a large vessel designed to separate production fluids into their constituent components of oil, gas and water. The separator can be operated at increased pressure, at ambient pressure or at decreased pressure (i.e. relative to atmospheric pressure). Separators generally operate at a pressure of from 1 to 100 x 10 ^s N/m ² . The separator is generally situated at or near the wellhead and function by using the different densities of the components, thereby allowing the components to stratify when moving slowly.

It will be appreciated that once the mixture comprising water, oil, optionally gas and optionally a surfactant (and optionally a polymer) is obtained from the second well in the initial set-up phase of operation, the recovered water that is separated from the oil and gas may then be reinjected into the well as described in steps (iii) to (v) of the process. Following the initial setup phase of the process, the mixture obtained in step (ii) will be a mixture comprising water, oil, a surfactant and optionally gas.

The aqueous mixture which is used for preparing emulsion for injection in step (v) will contain surfactant and can contain a certain amount of oil. The amount of oil which can be present in the emulsion depends on the specific circumstances, but oil may be present in an amount of up to 1000 ppmw. The amount of oil present in the aqueous mixture may be is at least 10 ppmw, more specifically at least 50 ppmw, more specifically at least 100 ppmw, more specifically at least 200 ppmw, more specifically at least 300 ppmw. For example, the oil present in the aqueous mixture comprising at most 1000 parts per million by weight (ppmw) of oil and optionally a surfactant in step (iii) of the process above may contain from 10 to 1 ,000 ppmw of oil, such as from 50 to 900 ppmw, such as from 100 to 800 ppmw, such as from 200 to 750 ppmw of oil (e.g. the aqueous mixture comprising oil in step (iii) of the process may comprise from 300 to 1 ,000 ppmw of oil). It will be appreciated that the oil in water emulsion produced in step (iv) of the process will also display the same levels of oil.

Additional surfactant is added to the aqueous mixture before it is converted into an emulsion for injection into the formation (i.e. step (iv) of the process). The surfactant and the amount added thereof depends on the composition of the mixture which has been produced from the formation. Due to various factors, the surfactant concentration of the enhanced oil recovery formulation tends to reduce while moving through the formation, such as by adsorption in the reservoir. In addition, the formulation also tends to get diluted by formation water. If a combination of surfactants has been injected, it is possible that the ratio between the surfactants in the mixture produced from the second well will differ from the ratio present in the desired enhanced oil recovery formulation. Furthermore, the amount of surfactant may need to be modulated depending on the amount of residual oil left in the recovered water after the separation process. For example, a higher amount of surfactant will be required if the amount of oil is close to 1000 ppmw, while less surfactant may be required if the residual oil is 15 ppmw.

In embodiments of the invention, the oil-in-water emulsion may contain from 0.1 to 20 % by weight of surfactant, more specifically of from 0.2 to 5 % by weight. The surfactant which is added to the aqueous mixture obtained in step (iii) of the process may be the same surfactant(s) present in the enhanced oil recovery formulation injected via the first well. The additional surfactant and aqueous mixture can be mixed either during the addition of the surfactant or thereafter. The oil-in-water emulsion may have a total dissolved solid (TDS) of at most 200,000 ppm (e.g. the oil-in-water emulsion has a total dissolved solid (TDS) of from 2,000 to 50,000 ppm, such as from 2,500 to 35,000 ppm, such as from 3,000 to 30,000 pm, such as from 3,500 to 20,000 ppm, such as from 4,000 to 15,000 pm, such as from 4,500 to 10,000 ppm).

Water is the continuous phase in the oil-in-water emulsion obtained in step (iv) of the process. The maximum diameter of the oil droplets in the oil-in-water emulsion depends on the diameter of the pores of the formation in question. As will be appreciated, the maximum size of the dispersed oil droplets should be smaller in size than the reservoir pore throat size of the first well. In embodiments of the invention disclosed herein, the droplets may have a diameter of at most 10 micrometer, more specifically at most 5 micrometer, more specifically at most 2 micrometer, more specifically at most 1 micrometer, more specifically at most 0.5 micrometer. The surfactant and aqueous mixture generally have to be contacted intensely (i.e. mixed) in order to ensure that the oil droplets are sufficiently small and are stabilized by the surfactant. Mixing may be carried out by any suitable method, such as with an in-line static mixer, a mixing control valve, high shear mixers, pumps or any other means known to persons skilled in the art. In view of the desired intense contact between the aqueous mixture and the surfactant, the mixing zone may in certain embodiments contain at least 2 successive static mixer elements, such as at least 3 elements, such as from 3 to 10 elements. A single static mixer element is an internal component (e.g. having a suitable shape, such as a screw-shape or having multiple blades) which is suitable per se for mixing fluids. A single element often has a diameter which is of from 50 to 100 % of the inner diameter of the pipe, more preferably of from 50 to 90 %, more preferably of from 60 to 85 % of the inner diameter, while its length tends to be of from 50 to 300% (e.g. 50 to 150 %) of the diameter of the pipe in which it is located.

In embodiments of the invention that may be mentioned herein, a suitable mixer element may contains blades that are at an angle of at least 30 degrees with respect to the fluid flow, more specifically at least 40 degrees. Suitable mixers are available from Statiflo International. Further examples of suitable mixers are static mixer types SMX and SMX plus which are commercially available from Sulzer Ltd. SMX is a trade mark.

Furthermore, the mixing of step (iv) can include analysis of the droplet diameter more specifically a system comprising a mixing valve coupled with an oil-in-water droplet analysis system. The oil in water analysis measures the oil in water concentration and dispersed oil droplet size. The oil in water analysis system is coupled with a logic controller which adjusts the mixing valve opening and if required the surfactant injection rate into the injection stream, in order to achieve the desired oil in water droplet size prior to the injection into the first well.

An embodiment of the oil recovery process of the current invention is depicted schematically in Figure 1 . This figure is based upon a process that has already been initiated and is providing produced effluent water from a producer well. In this embodiment, the fluid stream 11 , which comprises a mixture of water, oil and optionally gas, is produced from a formation via a producer well 10. The fluid stream 11 is then fed into a separator 12, which is a device suitable for separating oil and (optionally) gas from the produced fluid mixture 11 , to provide a produced water stream 16, oil stream 14, and a gas stream 15 (if present). The produced water stream 16 may still typically contain a concentration of 1000 ppm or less oil in the produced water stream. This stream 16 is then subjected to a revised produced water treatment process 18 which involves: (a) an oil in water removal/treatment system 20 to reduce the amount of oil in the water (i.e. to less than 300 ppm oil in water; and (b) filtration 22 to remove suspended particles (to remove particles typically larger than 2 pm such that the total suspended solids left are 2 pm or smaller in size and the particulate concentration is reduced to less than 10 ppm). A surfactant 23 may also be added to the produced water during treatment 18. The treated produced water is then combined with injection water 24 (which may include a surfactant 23), followed by mixing in a mixer 26 (i.e. an inline mixer) to produce an oil-in-water emulsion 27 having an oil content of less than 300 ppm (with oil droplets having an average size of about 20 pm). The emulsion 27 is then injected into the formation via an injection well 13. The emulsion 27 is also referred to as the“aqueous enhanced oil recovery formulation” hereinbefore.

Figure 2a discloses a further embodiment of the invention, which process may be particularly suited for use offshore. As mentioned above, the produced effluent water 11 is subjected to separation 12 to remove bulk oil and (optionally) gas. The produced water stream 16 is subjected to produced water treatment 18, which includes an oil in water removal/treatment system 20 and filtration 22, as discussed above. Subsequently, the treated water is passed through a self-cleaning filter/filtration system 30, and (optionally) through an additional cartridge filter. This allows the removal of solid particles larger than a certain size, typically 2 pm. The filtration effluent is then passed through an oil-in-water droplet dispersion/mixing control system 33 which comprises an inline mixer 26, a high shear controllable mixer 35, a valve 37 (e.g. a pneumatic valve), a droplet size analyser 38 and injectors 34, 36 that add shear-resistant chemicals (e.g. surfactants) and shear-sensitive chemicals (e.g. polymers) to the treated water upstream (for shear-resistant chemicals) or downstream (for shear-sensitive chemicals) of the high-shear mixer. The resulting mixture is then mixed with injection water to form the final mixture for injection into the injection well.

A close-up view of the water-in-oil droplet dispersion/mixing control system 33 is shown in Figure 2b. The droplet size analyser 38 allows the characterisation of the size of the emulsion droplets before and after passing through the mixing system consisting of either inline mixer 26 and/or high shear mixer 35. The output of the analyser 38 can be used to regulate the high shear mixer 35 in order to achieve the desired emulsion state/ oil droplets size.