Method of deploying carbon dioxide foam flooding in an oil reservoir

Provided are methods of recovering oil from a reservoir using carbon dioxide foam flooding in which a carbon dioxide foam of increased strength is used.

TECHNICAL FIELD

This document relates to a method of oil recovery from a reservoir using carbon dioxide foam flooding, particularly carbon dioxide foam with increased foam strength.

BACKGROUND

Carbon dioxide (CO2) flooding can be effective at recovering remaining oil from a reservoir, even one that has already undergone primary depletion and subsequent waterflooding. However, it is common for the CO2not to sweep the entire target volume, due to channeling (injectant preferentially sweeping the higher-permeability layers) and gravity override (buoyant CO2preferentially sweeps the reservoir's uppermost layers), harming vertical sweep, areal sweep, or both.

A primary cause of the unfavorable sweep is a condition where the viscosity of the injectant is lower than that of the oil that it is intended to displace. To address this problem, the apparent viscosity of the CO2is increased, commonly by foaming the CO2in situ. This can be accomplished by alternate injection of slugs of CO2and surfactant-laden brine. The surfactant stabilizes foam lamellae in the CO2within the rock, adding additional resistance to CO2flow. The more stable the lamellae, the more lamellae exist, and the greater the increase in CO2apparent viscosity.

A problem in this usage lies in the inherent weakness of CO2foam. The water solubility of CO2is much greater than most other enhanced oil recovery (EOR) injectants. This limits the magnitude of the change in pressure (ΔP) that can be maintained across a curved foam lamella, because excessive ΔP leads to diffusion of CO2from one side to the other while the lamella stays stationary. This relieves the ΔP, thus providing a mechanism for limiting the magnitude of ΔP across a lamella, which in turn limits the degree of increase in apparent CO2viscosity by foam. This weakness of CO2foam results in limited success when used to increase CO2sweep. Thus, an improved method is needed for CO2foam flooding that results in increased foam strength.

SUMMARY

Provided in this disclosure is a method for recovering oil from a reservoir. In some embodiments, the method involves: providing a first foam comprising carbon dioxide (CO2) to the reservoir; providing a second foam to the reservoir, wherein the second foam is produced by alternately injecting into the reservoir: a gas-mixture comprising CO2and nitrogen (N2) or CO2and methane (CH4) or CO2and N2and CH4; and a solution comprising brine and a surfactant; and recovering oil from the reservoir.

In some embodiments of the method, the gas-mixture comprises CO2and N2. In some embodiments, the gas-mixture comprises about 1 mol % to about 99 mol % N2. In some embodiments, the gas-mixture comprises CO2and CH4. In some embodiments, the gas-mixture comprises about 1 mol % to about 99 mol % CH4. In some embodiments, the gas-mixture is provided as a slug.

In some embodiments of the method, the surfactant in the solution comprising brine and a surfactant is selected from a nonionic surfactant, an anionic surfactant, a zwitterionic surfactant, and combinations thereof. In some embodiments, the surfactant comprises about 1% to about 15% of the solution. In some embodiments, the solution comprising brine and a surfactant is provided as a slug.

In some embodiments of the method, providing the second foam to the reservoir is repeated until the recovery of the oil from the reservoir reaches an economic limit. In some embodiments, providing the second foam to the reservoir is performed one time or more than one time.

In some embodiments of the method, the first foam is formed in situ in the reservoir. In some embodiments, the first foam is formed by alternately injecting a solution comprising CO2and a solution comprising brine and a surfactant. In some embodiments, the solution comprising CO2and the solution comprising brine and a surfactant are each provided as slugs. In some embodiments, the alternate injection of the solution comprising CO2and the solution comprising brine and a surfactant is repeated until the recovery of the oil from the reservoir reaches an economic limit. In some embodiments, the alternate injection of the solution comprising CO2and the solution comprising brine and a surfactant is performed one time or more than one time.

In some embodiments, the method comprises injecting a solution comprising N2and a solution comprising brine and a surfactant after providing the first foam to the reservoir. In some embodiments, the solution comprising N2and a solution comprising brine and a surfactant are injected simultaneously.

In some embodiments, the method results in a greater apparent viscosity of the CO2as compared to a method that does not include the step of providing the second foam to the reservoir. In some embodiments, the method increases the total sweep efficiency of the reservoir as compared to a method that does not include the step of providing the second foam to the reservoir. In some embodiments, the method provides increased oil recovery as compared to a method that does not include the step of providing the second foam to the reservoir.

Also provided in the present disclosure is a method for flooding a reservoir with CO2foam, comprising providing a foam to the reservoir, wherein the foam is produced by alternately injecting into the reservoir: a gas-mixture comprising CO2and N2or CO2and CH4or CO2and N2and CH4; and a solution comprising brine and a surfactant.

In some embodiments, the gas-mixture comprises CO2and N2. In some embodiments, the gas-mixture comprises about 1 mol % to about 99 mol % N2. In some embodiments, the gas-mixture comprises CO2and CH4. In some embodiments, the gas-mixture comprises about 1 mol % to about 90 mol % CH4. In some embodiments, the gas-mixture is provided as a slug.

In some embodiments, the surfactant in the solution comprising brine and a surfactant is selected from a nonionic surfactant, an anionic surfactant, a zwitterionic surfactant, and combinations thereof. In some embodiments, the surfactant comprises about 1% to about 15% of the solution. In some embodiments, the solution is provided as a slug.

In some embodiments of the method, the alternate injections of the gas-mixture and the solution are performed one time or more than one time.

In some embodiments, the method further comprises injecting into the reservoir a solution comprising N2and a solution comprising brine. In some embodiments, the solution comprising N2and a solution comprising brine and a surfactant are injected simultaneously. In some embodiments, the injection of a solution comprising N2and a solution comprising brine occurs before the foam is provided to the reservoir. In some embodiments, the injection of a solution comprising N2and a solution comprising brine occurs after the foam is provided to the reservoir.

DETAILED DESCRIPTION

The present disclosure provides methods for recovering oil from a reservoir using carbon dioxide (CO2) foam flooding. The methods of the present disclosure utilize a CO2foam that has been made stronger by dilution with nitrogen (N2) or methane (CH4). Methane is less water-soluble than CO2, and N2even less so. Therefore, in some embodiments, CO2foam strength increases from a weak foam to a stronger foam by adding CH4or N2. The methods of the present disclosure using the stronger CO2foam result in an improved volumetric sweep.

Additionally, CO2has the greatest local displacement efficiency (lowest residual oil saturation (Sor)), followed by CH4, with N2leaving behind the greatest Sor. Thus, in some embodiments, the methods of the present disclosure maximize oil recovery by first allowing CO2to sweep as much oil as it can by CO2flooding followed by CO2foam flooding using the stronger CO2foam. In some embodiments, subsequent injection of N2along with CO2foam enables sweeping of some of the remaining unswept regions, albeit at a higher Sor. Thus, the methods of the present disclosure maximize total sweep efficiency of the target reservoir volume. In some embodiments, as the N2-laden CO2front moves through the reservoir, some of the CO2partitions from the vapor into the in situ liquid oleic phase. In some embodiments, this results in a leading-edge of the advancing CO2-front that is enriched in N2, which improves sweep even more due to increased foam strength at the propagating foam front. In some embodiments, this method is used in regions of the reservoir unswept by CO2, where the oil is not yet saturated with CO2.

Also provided are methods for improving CO2foam processes though generation of a stronger CO2foam. In some embodiments, the methods increase CO2sweep. In some embodiments, the methods result in greater oil recovery as compared to methods where a CO2foam is used that does not contain N2or CH4.

The methods of the present disclosure can be used in any type of reservoir or underground formation, such as sandstone or carbonate or porous or fractured rock formation.

Reference will now be made in detail to certain embodiments of the disclosed subject matter. While the disclosed subject matter will be described in conjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter.

Definitions

As used herein, “strong foam” means foam that causes a large reduction in gas viscosity, while “weak foam” means foam that causes only a small reduction in gas viscosity. Stronger foam results in greater improvement in sweep efficiency of the injectant.

As used herein, the term “sweep efficiency” refers to a measure of the effectiveness of an enhanced oil recovery process. In some embodiments, sweep efficiency is the percentage of the reservoir volume displaced of oil by an injection fluid at a particular time.

Residual oil saturation (Sor) is defined as the fraction of the reservoir pore volume which does not flow.

Method for Recovering Oil from a Reservoir

Provided in the present disclosure are methods for recovering oil from a reservoir. The methods of the present disclosure are improved methods for flooding a reservoir with CO2, resulting in increased sweep efficiency, as compared to, for example, standard methods of CO2foam flooding. The methods of the present disclosure include: providing a first foam comprising carbon dioxide (CO2) to the reservoir; providing a second foam to the reservoir, where the second foam is produced by alternately injecting into the reservoir a gas-mixture comprising CO2and nitrogen (N2) or CO2and methane (CH4); and a solution comprising brine and a surfactant; and recovering oil from the reservoir.

In some embodiments, the methods of the present disclosure result in a greater apparent viscosity of the CO2in the foam as compared to a method of CO2foam flooding that does not include flooding with a CO2foam diluted with either N2or CH4. In some embodiments, the methods of the present disclosure increase the total sweep efficiency of the reservoir as compared to a method of CO2foam flooding that does not include flooding with a CO2foam diluted with either N2or CH4. In some embodiments, the methods of the present disclosure provide increased oil recovery as compared to a method of CO2foam flooding that does not include flooding with a CO2foam diluted with either N2or CH4.

In some embodiments, the first step of the method includes injecting a first foam containing CO2into a reservoir or well. For example, the first step can be any method of CO2foam flooding used in enhanced oil recovery (EOR) applications known to those skilled in the art. Thus, in some embodiments of the method, the first foam is any CO2-containing foam that is typically used for foam flooding.

The foam can be formed in any one of a number of ways. In some embodiments, the foam is generated at the surface by combining the ingredients in a suitable mixing device, such as a foam generator, then injected into the reservoir. In other embodiments, the foam is generated in situ by injecting the ingredients of the foam either separately or simultaneously into the reservoir. In yet other embodiments, two or more of the components are mixed together at the surface prior to introduction into the well, then the foam forms during its passage down the well and in the reservoir in the vicinity of the well.

In some embodiments of the provided method, the foam is formed in situ in the reservoir. In some embodiments, forming the foam in situ includes alternately injecting a gas-mixture containing CO2and a solution containing brine and a surfactant into the reservoir. In some embodiments, the gas-mixture and the solution are each injected as slugs. In further embodiments, the steps of injecting the carbon dioxide slug and the brine and surfactant slug is repeated in alternating fashion to enhance recovery of oil from a reservoir.

In some embodiments of the method, the alternate injections of the gas-mixture containing CO2and the solution containing brine and a surfactant are each performed one time or more than one time. For example, the gas-mixture and the solution can each be injected one time, two times, three times, four times, five times, or more. The number of times the gas-mixture and the solution are injected can depend on any number of factors. For example, the number of times the gas-mixture and the solution are injected can depend on the process design for a specific reservoir. In some embodiments, the number of times the gas-mixture and the solution are injected depends on the availability of surface injection facilities. In some embodiments, the alternate injections are repeated until a foam comprising CO2has formed. In some embodiments, the alternate injections of the gas-mixture containing CO2and the solution containing brine and a surfactant are repeated until the recovery of the oil from the reservoir reaches an economic limit. Regarding the economic limit of a well (reservoir) or group of wells, there are costs associated with keeping a well on production, including, but not limited to, artificial lift, surface fluid processing and transport, production testing and monitoring, and well and equipment maintenance. The well (reservoir) or group of wells must not only produce enough valuable fluids, such as oil, to cover all such costs, but also provide an adequate profit for the company. For example, where CO2is injected, often a large fraction of the injected CO2is produced at the production wells. There is a cost associated with handling these sometimes very large quantities of gas. In other examples, the same reasoning applies to having to handle large amounts of produced water in some wells. The economic limit is the flow rate below which the oil and gas production rates have insufficient value to cover the associated costs and the required profit margin.

The amount of the carbon dioxide injected into the reservoir will vary for different reservoirs, and will be dependent upon total reservoir pore volume, hydrocarbon pore volume, and other unique reservoir characteristics. In some embodiments, the amount of carbon dioxide injected depends on the effective pore volume, which is the portion of the reservoir which is expected to be contacted by the carbon dioxide injected. Effective pore volume can be determined by conventional laboratory and field techniques known to those of skill in the art.

The surfactant included in the brine solution can be any surfactant capable of forming a foam with CO2. In some embodiments, the surfactant is selected from a nonionic surfactant, an anionic surfactant, a zwitterionic surfactant, and combinations thereof.

In some embodiments, the method includes providing a foam to the reservoir that contains CO2diluted with either nitrogen (N2) or methane (CH4), or both N2and CH4. Without wishing to be bound by any particular theory, it is believed that CO2foam can be made stronger by dilution with small concentrations of N2or CH4. Methane is less water-soluble than CO2, and N2even less so. Therefore, CO2foam can increase in strength by adding CH4or N2. Stronger foam can lead to improvement in volumetric sweep.

In some embodiments of the method, the diluted foam containing CO2and either N2or CH4or both N2and CH4is provided to the reservoir after providing a first foam containing CO2to the reservoir. In some embodiments of the method, the diluted foam containing CO2and either N2or CH4or both N2and CH4is provided to the reservoir after CO2foam flooding has been performed, such as CO2foam flooding used in enhanced oil recovery (EOR) applications known to those skilled in the art.

In some embodiments, the diluted foam containing CO2and either N2or CH4or both N2and CH4is produced by alternately injecting into the reservoir a gas-mixture containing CO2and N2, a gas-mixture containing CO2and CH4, or a gas mixture containing CO2, N2and CH4; and a solution containing brine and a surfactant. In some embodiments, the gas-mixture is provided as a slug. In some embodiments, the solution containing brine and a surfactant is provided as a slug.

In some embodiments, the gas-mixture contains CO2and N2. The gas-mixture can contain an amount of N2that is sufficient to increase the strength of the foam once formed. In some embodiments, the gas-mixture contains about 1 mol % to about 99 mol % N2, for example, about 1 mol % to about 90 mol %, about 10 mol % to about 80 mol %, about 20 mol % to about 70 mol %, about 30 mol % to about 60 mol %, or about 50 mol % N2. In some embodiments, the gas-mixture contains about 1 mol %, about 5 mol %, about 10 mol %, about 15 mol %, about 20 mol %, about 25 mol %, about 30 mol %, about 35 mol %, about 40 mol %, about 45 mol %, about 50 mol %, about 55 mol %, about 60 mol %, about 65 mol %, about 70 mol %, about 75 mol %, about 80 mol %, about 85 mol %, about 90 mol %, about 95 mol %, or about 99 mol % N2.

In some embodiments, the gas-mixture contains CO2and CH4. The gas-mixture can contain an amount of CH4that is sufficient to increase the strength of the foam once formed. In some embodiments, the gas-mixture contains about 1 mol % to about 99 mol % CH4, for example, about 1 mol % to about 90 mol %, about 10 mol % to about 80 mol %, about 20 mol % to about 70 mol %, about 30 mol % to about 60 mol %, or about 50 mol % CH4. In some embodiments, the gas-mixture contains about 1 mol %, about 5 mol %, about 10 mol %, about 15 mol %, about 20 mol %, about 25 mol %, about 30 mol %, about 35 mol %, about 40 mol %, about 45 mol %, about 50 mol %, about 55 mol %, about 60 mol %, about 65 mol %, about 70 mol %, about 75 mol %, about 80 mol %, about 85 mol %, about 90 mol %, about 95 mol %, or about 99 mol % CH4.

In some embodiments, the gas-mixture contains CO2and both N2and CH4. The gas-mixture can contain a total amount of N2and CH4that is sufficient to increase the strength of the foam once formed. In some embodiments, the gas-mixture contains about 1 mol % to about 99 mol % N2and CH4, for example, about 1 mol % to about 90 mol %, about 10 mol % to about 80 mol %, about 20 mol % to about 70 mol %, about 30 mol % to about 60 mol %, or about 50 mol % N2and CH4. In some embodiments, the gas-mixture contains about 1 mol %, about 5 mol %, about 10 mol %, about 15 mol %, about 20 mol %, about 25 mol %, about 30 mol %, about 35 mol %, about 40 mol %, about 45 mol %, about 50 mol %, about 55 mol %, about 60 mol %, about 65 mol %, about 70 mol %, about 75 mol %, about 80 mol %, about 85 mol %, about 90 mol %, about 95 mol %, or about 99 mol % N2and CH4.

The amount of the gas-mixture containing carbon dioxide and either N2or CH4or both N2and CH4injected into the reservoir will vary for different reservoirs, and will be dependent upon total reservoir pore volume, hydrocarbon pore volume, reservoir fluid composition and other unique reservoir characteristics. In some embodiments, the amount of carbon dioxide injected depends on the effective pore volume, which is the portion of the reservoir which is expected to be contacted by the carbon dioxide injected. Effective pore volume can be determined by conventional laboratory and field techniques known to those of skill in the art.

The surfactant included in the brine solution can be any surfactant capable of forming a foam with CO2. In some embodiments, the surfactant is selected from a nonionic surfactant, an anionic surfactant, a zwitterionic surfactant, and combinations thereof. Suitable surfactants include, but are not limited to a cocamidopropyl betaine surfactant (for example, Amphosol®, sold by Stepan Company, Northfield, Ill., USA), sodium dodecyl sulfonate, and selected anionic, cationic, zwitterionic or optimized proprietary blends of many surfactants.

In some embodiments, the solution contains about 0.1% to about 5% of the surfactant, such as about 0.1% to about 4%, about 0.5% to about 3%, about 1% to about 2%, or about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1%, about 1.5%, about 2%, about 2.5%, about 3%, about 3.5%, about 4%, about 4.5%, or about 5% surfactant.

In some embodiments of the method, the alternate injections of the gas-mixture containing CO2and either N2or CH4or both N2and CH4and the solution containing brine and a surfactant are each performed one time or more than one time. For example, the gas-mixture and the solution can each be injected one time, two times, three times, four times, five times, or more. In some embodiments, the alternate injections are repeated until a foam has formed. In some embodiments, the alternate injections of the gas-mixture and the solution are repeated until the recovery of the oil from the reservoir reaches an economic limit.

In some embodiments of the method, the method further includes a step of injecting an N2foam into the reservoir. In some embodiments, the N2foam is formed by injecting a solution containing N2and a solution containing brine and a surfactant into the reservoir. In some embodiments, the solution containing N2and the solution containing brine and a surfactant are injected simultaneously. In some embodiments, the solution containing N2and the solution containing brine and a surfactant are injected sequentially. In some embodiments, the solution containing N2is injected first, followed by injection of the solution containing brine and a surfactant. In some embodiments, the N2foam is provided as a slug.

In some embodiments, the N2foam is provided to the reservoir prior to providing the diluted foam containing CO2and either N2or CH4or both N2and CH4. In some embodiments of the method, the N2foam is provided to the reservoir after providing a first foam containing CO2to the reservoir. In some embodiments of the method, the N2foam is provided to the reservoir after CO2foam flooding has been performed, such as CO2foam flooding used in enhanced oil recovery (EOR) applications known to those skilled in the art. Without wishing to be bound by any theory, it is believed that because CO2has a higher local displacement efficiency (lowest residual oil saturation (Sor)) than N2, allowing CO2to first sweep as much oil from the reservoir by CO2foam flooding, the subsequent injection of the N2foam enables improved volumetric sweeping of the remaining unswept regions, albeit at a higher Sor, thus maximizing total sweep efficiency of the target reservoir volume.

Method for Flooding a Reservoir

Also provided in the present disclosure are methods for flooding a reservoir with diluted CO2foam, such as the diluted foam provided herein. In some embodiments, the foam is diluted with N2. In some embodiments, the foam is diluted with methane. In some embodiments, the foam is diluted with both N2and methane. The method includes providing a foam to a reservoir, where the foam is produced by alternately injecting into the reservoir a gas-mixture containing CO2and N2or CO2and CH4or CO2and N2and CH4; and a solution comprising brine and a surfactant. In some embodiments, the gas-mixture is provided as a slug. In some embodiments, the solution is provided as a slug.

In some embodiments of the method, the alternate injections of the gas-mixture containing CO2and either N2or CH4or both N2and CH4and the solution containing brine and a surfactant are each performed one time or more than one time. For example, the gas-mixture and the solution can each be injected one time, two times, three times, four times, five times, or more. In some embodiments, the alternate injections are repeated until a foam has formed. In some embodiments, the alternate injections of the gas-mixture and the solution are repeated until the recovery of the oil from the reservoir reaches an economic limit.

In some embodiments of the method, the method further includes a step of injecting an N2foam into the reservoir. In some embodiments, the N2foam is formed by injecting a solution containing N2and a solution containing brine and a surfactant into the reservoir. In some embodiments, the solution containing N2and the solution containing brine and a surfactant are injected simultaneously. In some embodiments, the solution containing N2and the solution containing brine and a surfactant are injected sequentially. In some embodiments, the solution containing N2is injected first, followed by injection of the solution containing brine and a surfactant. In some embodiments, the N2foam is provided as a slug.

In some embodiments, the N2foam is provided to the reservoir prior to providing the diluted foam containing CO2and either N2or CH4or both N2and CH4. In some embodiments, the N2foam is provided to the reservoir after providing the diluted foam containing CO2and either N2or CH4or both N2and CH4.

Thus, also provided are methods for increasing sweep efficiency using the diluted CO2foam containing either N2or CH4or both N2and CH4.

Other Applications

The methods of the present disclosure can be used in any subterranean formation or reservoir where oil recovery is desired. For example, the methods can be used to recover remaining oil from a reservoir that has already undergone primary depletion and subsequent waterflooding. The methods can be used to produce oil from a reservoir after secondary or tertiary production methods have been performed. The methods of the present disclosure can be used in carbonate formations or sandstone formations.

Examples

Laboratory flooding experiments of reservoir core plug samples were conducted to demonstrate the effects of gas composition on foam in porous media. Foams containing different gases (N2, flue gas (a mixture of 20 mol % CO2and 80 mol % N2emitted as exhaust from power plants), CH4, CO2, and a mixture of CO2and CH4(50/50 mol %)) were prepared and the strength of the foam was tested across a pressure gradient, as described in Zeng et al., Langmuir, 32:6239-6245 (2016).FIG.1displays the apparent viscosity of foam of different compositions at low pressures (greater pressure gradient means greater apparent viscosity). At low pressure conditions, CO2was a weaker foam, with CH4twice as strong and N2three times as strong. Mixing CO2with other gases resulted in a foam as strong as the other gases alone. Since CO2becomes more water-soluble at higher pressures, the CO2foam became much weaker. As shown inFIG.2, at 30 bar CO2foam was only 5× weaker than N2, while at 280 bar CO2foam was 100× weaker (note the logarithmic scale on the graph). See also Solbakken et al., Ph.D. dissertation, University of Bergen (2015).

Because many CO2floods occur at the higher reservoir pressures, this shows that diluting the injected CO2with other gases, such as N2or CH4, at reservoir conditions can increase foam strength.

Other Embodiments