This application claims the benefit of U. S. Provisional Application No. 62/295,608 filed 16 February 2016, which is incorporated herein by reference.

Field of the Invention

The present disclosure relates to methods and systems for producing oil from an underground formation.

Background of the Invention

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. As well as mineral oil, including fractions of natural gas, a deposit generally also comprises water with a greater or lesser salt content.

In mineral 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. The autogeneous pressure of the deposit, however, generally declines relatively rapidly on extraction of mineral oil, such that usually only a limited amount of the mineral oil present in the deposit can be produced in this way. Thereafter, the autogeneous pressure is no longer sufficient to produce mineral oil. In those instances, artificial lifting can be applied to produce further oil and/or gas from the formation.

Artificial lifting also can be applied if the pressure difference between the underground oil-bearing formation and the surface facilities is reduced to extend the field life under primary production.

Artificial lift methods may include gas lifting which injects a gas such as air, carbon dioxide, methane or other alkane which is gaseous at ambient conditions, or mixtures thereof into the bottom of an active well, thereby reducing the overall density of fluid in the wellbore.

Primary production is no longer feasible if natural reservoir drive diminishes. In these instances, 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, hence replacing or increasing the natural reservoir drive with an artificial drive. Reservoir pressure may be increased through the injection of water and/or natural gas. The injected fluid is typically immiscible, or predominantly immiscible with the in-situ hydrocarbon fluids. Secondary production may also be supported and extended by the use of artificial lift methods. The secondary recovery stage is typically accepted to have reached its limit when the injected fluid is produced in considerable amounts from the production wells and the production is no longer economically viable.

After primary and/or secondary production, enhanced oil recovery can be applied. As described in EP2239415, a known enhanced oil recovery method utilizes carbon dioxide to increase the amount of oil recovered from the formation. Carbon dioxide is injected into an oil-bearing formation to increase recovery of oil from the formation, either after primary recovery or after a secondary recovery water flood.

We now have found a method and system which make it possible to carry out enhanced oil recovery more efficiently.

Summary of the Invention

The present invention relates to a method for producing oil and optionally gas comprising injecting an enhanced oil formulation into a formation from an injection well and producing oil and optionally gas from the formation from a recovery well wherein the enhanced oil recovery formulation comprises ethane.

A further embodiment of the invention comprises a system for producing oil from an underground oil-bearing formation which system comprises an injection well above the formation, a mechanism to inject an enhanced oil recovery formulation into the formation and a recovery well comprising a mechanism to produce oil from the formation wherein the enhanced oil recovery formulation comprises ethane.

Brief Description of the Drawing

Figure 1 is a diagram of a system and method for enhanced oil recovery with the help of an enhanced oil recovery formulation comprising ethane.

Figure 2 and Figure 3 are diagrams showing different ways in which lift gas can be delivered to the recovery well of an enhanced oil recovery process. Detailed Description of the Invention

Many formations contain gas besides oil. If a formation contains gas besides oil, the oil containing fluid which is recovered from the formation can additionally contain gas. Therefore, the recovery of oil can include recovery of gas from the formation, more specifically hydrocarbonaceous gas.

The method and system of the present invention are especially suitable for use with an underground formation which is beneath a body of water.

Recovery of oil from an underground formation is obtained with the help of an injection well for injecting enhanced oil recovery formulation and a recovery well for recovering oil containing fluid. The exact arrangement of the wells will depend on the formation and other local circumstances. Any array can be used which is known to be suitable to a person skilled in the art.

Injecting the enhanced oil recovery formulation into the formation may be accomplished by any known method. One suitable method is injecting the enhanced oil recovery formulation into a single conduit in a single well, allowing enhanced oil recovery formulation to soak, and then recovering at least a portion of the mixture of oil and enhanced oil recovery formulation from the recovery well. Another suitable method is injecting the enhanced oil recovery formulation into an injection well, and pumping out at least a portion of the enhanced oil recovery formulation with gas and/or liquids through a recovery well. The enhanced oil recovery formulation generally will be pumped into a formation at a pressure up to the fracture pressure of the formation.

In some embodiments, the enhanced oil recovery formulation may be mixed with oil in a formation to form a mixture which may be recovered from a well. In some embodiments, a quantity of the enhanced oil recovery formulation may be injected into a well, followed by another component to force the formulation across the formation. Such subsequent flood is referred to as a chasing flood. Preferred components to force the enhanced oil recovery formulation across the formation are selected from the group consisting of methane, nitrogen, carbon dioxide and mixtures thereof. In addition or alternatively, the injection of enhanced oil recovery formulation containing ethane can be alternated with the injection of water. Preferably, ethane is injected in slugs alternating with periods of water injection. In this way, less ethane is being used while advantageous oil recovery is still achievable. At the end of alternatingly injecting ethane and water and preferably following a further period of injecting alternatingly water and a chasing compound, the last injection into the formation may be injecting water to recover mobile gas. It is preferred to recover such mobile gas in order to inject it into other formations to continue producing incremental oil. The chasing flood applied alternatingly with the water flood preferably contains a compound selected from the group consisting of methane, nitrogen carbon dioxide and mixtures thereof.

During the use of ethane in recovering oil from the formation, a substantial amount of ethane generally will be recovered in combination with the oil. Generally, the ethane will be recovered in combination with other gaseous compounds present in the formation such as methane. The gaseous compounds separated from the fluid which is recovered, preferably again are injected into the formation. In these instances, surface facilities handle large volumes of gas and recompress these to high enough pressures to re-inject in the reservoir. The volume of gas increases in time as more ethane will be recovered if ethane injection has taken place during a longer time period.

In the recovery of oil, gas lifting can be used to produce oil from wells that have insufficient reservoir pressure. The present system further relates to a system additionally comprising a mechanism to inject lift gas containing ethane. Injected gas aerates the fluid to reduce its density and the formation pressure is then able to lift the oil column and forces the fluid out of the wellbore. Gas may be injected continuously or intermittently, depending on the producing characteristics of the well and the arrangement of the gas-lift equipment. The use of the system involves injecting gas through the recovery well, most commonly through a conduit formed by the annulus between the main casing of the recovery well and a conduit for recovering oil from the recovery well. Alternatively, it can be preferred in the present process to inject gas through a conduit located inside the tube for recovering oil. This set-up is especially preferred if the lift gas is corrosive such as when the formulation comprises carbon dioxide besides ethane. The separate conduit inside the tube for recovering oil ensures that the lift gas is not in contact with the casing which tends not to be made for corrosive operating conditions.

Alternatively, lift gas can be introduced via a conduit outside but parallel to the conduit for recovering oil while both the conduit for injecting lift gas and the conduit for recovering oil are located inside the well casing. In this instance, the lift gas conduit and recovery conduit are parallel tubes situated within the casing of the recovery well. The conduits for introducing lift gas and recovering fluid tend to be separated by packers. The latter set-up is referred to as dual completion and has the advantage that the lift gas can be introduced independent from the rate with which fluid is recovered.

The amount of gas to be injected to maximize oil production varies with well conditions and geometries. Generally, the optimal amount of injected gas is determined by well tests. The present invention has the advantage that the gas for lifting the fluid from the well can be similar to the fluid used for enhanced oil recovery. This makes that the fluid recovered from the well contains less contaminants that degrade oil recovery performance. The present invention makes that gas which is separated off can be suitable for immediate reinjection for enhanced oil recovery and/or is easier to clean. The fluid recovered most notably tends to contain less methane than if conventional gas lift would be applied.

Therefore, the system of the present invention preferably comprises a mechanism to inject lift gas containing ethane and the method of the present invention preferably comprises injecting recovery lift gas having the same composition as the enhanced oil recovery formulation.

As injected ethane progressively breaks through at the recovery wells together with an increase in recovery of oil, the lift performance of the well tends to improve as the column density is reduced. This is mainly due to the expanding ethane in the production tubing when it travels up from the bottom of the well to the surface. Eventually the recovery well reaches a point where artificial lift is no longer needed. During the time when ethane is being recovered from the formation, significant fluctuations in gas rate can occur depending on the properties of the formation. This makes that the well may experience periods of auto-lift followed by periods when artificial lift is required to maximize offtake rates and project economics.

The rate and/or pressure at which the ethane lift gas fraction is injected into the recovery well may be adjusted on the basis of one or more of the following parameters:

- target and/or fluctuation of oil recovery of the one or more recovery wells;

- fluctuation of gas production of the one or more recovery wells;

- density and/or water content of the fluid recovered from the one or more recovery wells;

- available ethane gas (or produced gas and ethane gas mixture) volume and/or ethane gas (or produced gas and ethane gas mixture) compressor capacity; and

- bottom hole pressure in the recovery well. The method of the present invention preferably further comprises separating ethane from fluid obtained from the recovery well, compressing at least part of the ethane separated off and injecting compressed ethane into the formation from the injection well. As mentioned above, the ethane will generally be separated off in combination with further gaseous compounds such as methane which can be compressed and injected again together with the ethane.

The enhanced oil recovery formulation may comprise other hydrocarbon gas or other components besides ethane to mix with and displace crude oil within the pores of the formation by a miscible or sub-miscible process and flows towards the recovery well.

The enhanced oil recovery formulation preferably comprises of from 50 to 100 % by mole (%mole) of ethane, more preferably of from 70 to 100 %mole, most preferably of from 80 mole% to 100 %mole. Further compounds which can be present are selected from the group consisting of methane, alkanes containing of from 3 to 6 carbon atoms, carbon dioxide, carbon monoxide, nitrogen, hydrocarbon ethers containing of from 2 to 6 carbon atoms, hydrocarbon thiols containing of from 1 to 6 carbon atoms and hydrogen sulphide. If the enhanced oil recovery formulation comprises compounds other than ethane, the amount is preferably at most 5 %mole.

The lift gas preferably comprises of from 50 to 100 % by mole (%mole) of ethane, more preferably of from 70 to 100 %mole, most preferably of from 80 mole% to 100 %mole. Further compounds which can be present are selected from the group consisting of methane, alkanes containing of from 3 to 6 carbon atoms, carbon dioxide, carbon monoxide, nitrogen, hydrocarbon ethers containing of from 2 to 6 carbon atoms, hydrocarbon thiols containing of from 1 to 6 carbon atoms and hydrogen sulphide. If the lift gas comprises compounds other than ethane, the amount is preferably at most 5 %mole. Preferably, the lift gas and the enhanced oil recovery formulation have the same composition.

In some embodiments, the enhanced oil recovery formulation is injected at a pressure from 0 to 37,000 kilopascals above the initial reservoir pressure, measured prior to when ethane injection begins.

These and other features, embodiments and advantages are hereinafter described in relation with the accompanying drawings.

Figure 1 shows a crude oil containing underground formation 1, which is located underneath an overburden 2. Ethane can be injected into the formation 1 via injection well 3. Oil is recovered via recovery well 4 comprising a well casing 5, which is perforated near the bottom of the well to enable influx of crude oil into the well 4 as illustrated by arrows 6.

A volume of ethane gas obtained from a natural or an industrial ethane gas source 7 is distributed to the field well pads or well head platforms through a distribution network 22 and split at a manifold 21 into an enhanced oil recovery formulation 11 to be injected into the formation 1 through perforations 13 in a well casing 14 within the injection well 3 as illustrated by arrows 15, and lift gas 12, which is injected into a lift gas injection conduit which in this case is well annulus 20 outside the conduit for recovering oil in recovery well 17. The lift gas enters the recovery tubing through lift gas injection openings 16. If required, a conventional downhole safety valve 28 can be installed below the lift gas openings 16 and above a production packer 19. The rate of lift gas injection is controlled by a choke 23. The packer 19 is arranged near the bottom of a well annulus 20 between the tubing of the recovery well 17 and well casing 5 to inhibit crude oil and/or lift gas to flow into the well annulus 20.

Produced fluids comprising crude oil, brine (mixture of formation water and injected water), associated hydrocarbon gas, ethane gas back produced from the reservoir and ethane gas injected directly into the producer for gas lift are produced back to a processing facility 24 through a flowline with valve 28. The processing facility comprises facilities 25 to separate crude oil from brine and the produced gas. Oil can be removed via line 31 and brine can be removed via line 32. The produced gas largely comprises ethane with a limited amount of other hydrocarbon gas such as methane. It is possible to remove with the help of extraction some of the hydrocarbon gas other than ethane. Produced gas 27 is compressed with the help of compressor 26, which raises the pressure to the level required for injection. The high pressure gas 30 is combined with the fresh ethane gas imported from the industrial source 7 and routed once more to the wells 3 for injection into the reservoir for enhanced oil recovery and to the producers 4 for gas lift.

An advantage of the integrated system is that the lift gas 12 can be used for ethane gas lift without significant additional capital expenditure using the basic system of surface facilities infrastructure (22, 28 and 24) required for ethane gas enhanced oil recovery. Over the lifetime of a crude oil production project at most a small increase in the compression capacity can accommodate all the ethane gas lift gas requirements without requiring additional equipment. Since the gas has to be compressed to inject into the formation 1, there is always sufficient pressure to operate a gas lift system without the need for conventional gas lift equipment. By allocating the volume of lift gas using chokes 23, the artificial lift capacity can be progressively adjusted to match the wells potential and can respond to short term changes in gas production rate from the reservoir.

Figure 2 and Figure 3 are diagrams showing different ways in which lift gas can be delivered to the recovery well.

In Figure 2, oil is recovered via recovery well 204 comprising a well casing 205 and tubing 217 which is the conduit for recovering fluid from the well. Well 204 is perforated near the bottom of the well to enable influx of crude oil into well 204 as illustrated by arrows 206. Lift gas is injected via conduit 233 and the flow of lift gas is controlled by a choke 223. Lift gas mixes with oil and flows into the conduit for recovering fluid as shown by arrows 207.

In Figure 3, fluid comprising oil is recovered from recovery well 304 via tube 317 located inside well 304 comprising a well casing 305. Well 304 is perforated near the bottom of the well to enable influx of crude oil into the well 304 as illustrated by arrows 306. Lift gas is injected via conduit 333 and the flow of lift gas is controlled by a choke 323. Lift gas mixes with oil and flows into the conduit for recovering fluid as shown by arrows 307.

A further embodiment of the invention comprises a method as described above further comprising artificial lift with the help of a gas compressor and one or more valves located downhole in an injection well. In this method lift gas is pressurized in the compressor and subsequently introduced via one or more valves. It can be preferred that the lift gas consists of ethane. Alternatively, it can be preferred that the lift gas comprises ethane in admixture with one or more compounds selected from the group consisting of methane, alkanes containing of from 3 to 6 carbon atoms, carbon dioxide, carbon monoxide, nitrogen, hydrocarbon ethers containing of from 2 to 6 carbon atoms, hydrocarbon thiols containing of from 1 to 6 carbon atoms and hydrogen sulphide.

The method according to the invention provides the option to benefit from synergies between the produced fluids processing facilities and ethane gas lift in a fully integrated system using conventional annular lift gas injection in combination with gas lift valves annular gas lift, or any other configuration that protects the integrity of the well, including but not restricted to the use of concentric lift strings or a separate tubing within the annulus to convey ethane gas to a deep injection point in the production tubing, to reduce capital expenditure and operational complexity compared to artificial lift schemes based on electric submersible pumps or any other artificial lift method requiring a separate supporting surface system.

The present invention allows lift gas to be made available without requiring additional equipment and accompanying utilities such as electric submersible pumps. This is especially advantageous for offshore operations.

A further advantage is that the process offers increased flexibility. Well rates are uncertain and if ethane gas is from a fixed capacity source, sufficient wells must be operating to take available ethane gas at all times to maximize project returns. In a low productivity realization more wells are needed. With an ethane gas-lift system, the lift gas can easily be reallocated to a larger number of wells (each of which requires a lower lift gas rate).

It tends to be uncertain how fast the enhanced oil recovery formulation is produced and thereby the overall recycling requirement. The gas lift system intrinsically manages this. In a downside outcome with earlier gas breakthrough, more recycling of ethane gas is needed, but the extra gas handling is partly offset by the reduced requirement for gas lift as wells move to autolift sooner. Conversely in an upside outcome of reduced gas cycling, more gas lift is needed which exploits the consequent usage in compression capacity.

The ethane gas-lift rate can be constantly adjusted to match the back produced gas and production target rate, responding rapidly to fluctuations in produced gas. The ethane gas lift system reduces the molecular weight variation in the recycle gas stream which is beneficial for the centrifugal gas compressors through project life.

The present disclosure is not limited to the embodiments as described above and the appended claims. Many modifications are conceivable and features of respective embodiments may be combined.