Technical Field

The present invention relates to a gas-lift system, and in particular to a system for gas lift and gas injection for the production of oil from a reservoir with a gas cap.

Background

Gas lift is used to facilitate production of liquid hydrocarbons, e.g. heavy oil, from a reservoir. A wellbore extends through a formation to the reservoir, and the wellbore typically contains production tubing for conveying hydrocarbons from the reservoir, and at least one other tubular, which may be a casing lining the wellbore. In a typical gas lift procedure, high-pressure gas is injected into liquid hydrocarbons contained in the production tubing. The gas is typically injected down an annulus between the production tubing and a casing, and the gas enters the production tubing via a valve. The injected gas aerates the liquid hydrocarbons, reducing the overall density of the liquid and thereby reducing the hydrostatic pressure imposed by the column of liquid hydrocarbons in the production tubing. This reduction in hydrostatic pressure allows the formation pressure, i.e. the pressure in the reservoir, to more easily ‘push’ liquid hydrocarbons from the reservoir into and through the production tubing, thereby increasing the rate of hydrocarbon production. The gas used for the gas lift is typically from an external source, e.g. a different field, and must therefore be transported to the field containing the reservoir.

The liquid hydrocarbons in the reservoir typically contain dissolved gases, and further high-pressure gas may be added to the liquid hydrocarbons for gas lift as described above. In addition to gases, produced liquid hydrocarbons may contain other foreign substances such as water. Produced liquid hydrocarbons must be processed to remove gas and water, to thereby provide oil that is suitable for transport and final consumption. Such processing typically includes gas-liquid separation performed using a gas-liquid separator, a process which may produce large amounts of separated gas. The separated gas must either be used locally or transported to another location for use or further processing. US5794697 relates to a method for increasing oil production from an oil well producing a mixture of oil and gas.

Okenwa, J.C. and Statoil, MSc thesis, Department of Earth Science and Engineering, Centre of Petroleum Studies, September 2011, relates to the modelling of a gas cap gas lift system.

Summary

It is an object of the present invention to overcome or at least mitigate the problems identified above. In particular, it is an object of the invention to allow fields which are remote from gas processing facilities, or fields for which the transport and/or processing of gas may be expensive, to be exploited.

In accordance with a first aspect of the present invention there is provided a gas-lift system for producing oil from a reservoir having a gas cap, wherein a production wellbore and a gas wellbore extend separately into a formation containing the reservoir and the production wellbore contains a tubular in fluid communication with the reservoir for conveying produced reservoir fluids. The gas-lift system comprises a gas-liquid separator for coupling to said tubular for separating produced reservoir fluids into separate liquid and gas phases; a first conduit for extracting gas from the gas cap via the gas wellbore; a gas compressor for coupling to the gas-liquid separator and to the first conduit to selectively receive extracted gas from the first conduit and/or separated gas from the gas-liquid separator, and to compress the received gas to produce compressed gas; a second conduit for injecting compressed gas into the tubular for gas-lift; and a third conduit for injecting compressed gas into the gas cap via the gas wellbore. The gas compressor is configurable to deliver compressed gas to one or both of the second and third conduits.

According to a second aspect of the present invention there is provided a method of producing oil from a reservoir having a gas cap, wherein a production wellbore and a gas wellbore extend separately into a formation containing the reservoir, and the production wellbore contains a tubular in fluid communication with the reservoir for conveying produced reservoir fluids. The method comprises: extracting gas from the gas cap; compressing the extracted gas; using the compressed extracted gas for gas- lift of produced reservoir fluids; separating the produced reservoir fluids into separate liquid and gas phases; compressing the separated gas to produce separated compressed gas; using the separated compressed gas for gas-lift of produced reservoir fluids; and injecting any excess separated compressed gas into the gas cap.

The step of compressing the separated gas to produce separated compressed gas may comprise additionally compressing further extracted gas from the gas cap, and the step of using the separated compressed gas for gas-lift of produced reservoir fluids may comprise using the separated and additional extracted compressed gas for said gas-lift.

The gas cap may be created by: producing reservoir fluids without using gas-lift; separating the produced non-gas-lift reservoir fluids into separate non-gas-lift liquid and non-gas-lift gas phases; compressing the separated non-gas-lift gas to produce compressed non-gas-lift gas; and injecting the compressed non-gas-lift gas into the reservoir.

The method may be performed using an apparatus in accordance with the first aspect of the present invention.

In accordance with a third aspect of the present invention there is provided a system for producing oil from a reservoir having a gas cap. The system comprises: a production wellbore extending into a formation containing the reservoir; a gas wellbore extending separately into the formation; a tubular contained in the production wellbore and in fluid communication with the reservoir for conveying produced reservoir fluids; a gas-liquid separator coupled to said tubular for separating produced reservoir fluids into separate liquid and gas phases; a first conduit for extracting gas from the gas cap via the gas wellbore; a gas compressor coupled to the gas-liquid separator and to the first conduit, and configured to selectively receive extracted gas from the first conduit and/or separated gas from the gas-liquid separator, and to compress the received gas to produce compressed gas; a second conduit for injecting compressed gas into the tubular for gas-lift; and a third conduit for injecting compressed gas into the gas cap via the gas wellbore. The gas compressor is configured to deliver compressed gas to one or both of the second and third conduits. Brief Description of the Drawings

Figure 1 shows a gas-lift and gas injection system.

Figure 2 shows alternative configurations of the gas well and gas extraction/gas injection conduits.

Figure 3 shows a high-level flow diagram describing a method in accordance with the invention.

Detailed Description

As set out above, typical gas-lift systems are‘open’ systems, in that the gas used for gas-lift is sourced from a different location, i.e. is not sourced from the field to which gas-lift will be applied. This is typically because the focus is on production from the reservoir, rather than the use of gas and the costs associated with transport and processing of gas. In typical systems, the produced fluid, which includes gas injected for gas-lift, is sent to a processing facility for final processing. Following the final processing, the resulting gas, oil, and possibly water will be exported for final consumption.

The existing technology requires the availability of a gas source for the gas used for gas-lift. Such a gas source may be distant from the field, and in that case the gas-lift gas must be transported to the field. Further, the gas resulting from the final processing must be exported or consumed locally. Some of the gas resulting from final processing may be used for gas-lift. Flowever, the amount of gas used by a gas-lift system is typically small relative to the amount of gas resulting from processing. It may therefore not be possible to develop a field, for example an offshore field, if there is no relevant infrastructure nearby to transport gas to or from the field, or to consume or use processed gas. Even for fields which have relevant infrastructure nearby, the handling, processing and transport of gas may be expensive.

The present invention enables the development of an oil field with a gas cap, for example a heavy oil field, where the gas cap holds a moderate amount of gas that cannot be used/consumed locally or exported commercially. In particular, the present invention enables the exploitation of remote oil fields with a lack of local infrastructure for gas transport, and where gas cannot be consumed locally. This is achieved using a closed system that provides the ability to extract gas from a gas cap for gas-lift, i.e. the ability to start up oil production without the requirement for an external gas source, and the ability to inject separated gas into the gas cap, i.e. the ability to use locally the gas produced from processing, without requiring transport. The gas-lift system is for use in a field with a production wellbore and a gas wellbore extending separately into a formation containing a reservoir with a gas cap, where a tubular, e.g. a production pipe, is located within the production wellbore. The tubular is in fluid communication with the reservoir and is for conveying produced reservoir fluids to processing elements of the gas-lift system, wherein the produced reservoir fluids comprise liquid and gas. The gas lift system includes a first conduit configured to be in fluid communication with the gas cap and for extracting gas from the gas cap, wherein the extracted gas is for gas lift of the produced reservoir fluids in the tubular. The processing elements of the gas-lift system include a gas-liquid separator for separating the produced reservoir fluids into separated liquid and separated gas, and a gas compressor configured to selectively receive extracted gas from the first conduit and separated gas from the gas-liquid separator, and to compress the received extracted gas and separated gas to produce compressed gas. The gas-lift system also includes a second conduit for injecting compressed gas into the tubular for gas-lift, and a third conduit for injecting compressed gas into the gas cap. The gas-lift system provides flexible handling of gas, and has the advantage that a single gas well can be used as a gas production and injection well. Use of the present gas-lift system has the potential to significantly reduce the development costs for oil wells, and in particular heavy oil wells, with a gas cap.

Figure 1 shows gas-lift system 100 for extracting gas from gas cap 106 of reservoir 102 in formation 150, and for injecting gas into the gas cap 106. The gas-lift system 100 is located at the seabed. In an alternative embodiment the field containing the reservoir 102 is on land, and the gas-lift system 100 is located at the surface. The reservoir 102 contains an oil layer 104 underneath the gas cap 106. A production wellbore 108 extends into the formation 150, and a tubular 1 10, referred to here as production tubing 1 10, is contained within the production wellbore 108. The production tubing extends from processing elements 1 12 of the gas-lift system and is in fluid communication with the oil layer 104 of the reservoir 102. A gas wellbore 140 extends through the formation 150 to the gas cap 106 of the reservoir 102, which is located above a layer of liquid hydrocarbons 104 in the reservoir 102. A first conduit 142 is in fluid communication with the gas cap 106. The first conduit 142 is for extracting gas from the gas cap. A third conduit 144 is in fluid communication with the gas cap 106 (a second conduit is described below). The third conduit 144 is for injecting gas into the gas cap 106. The first conduit may be provided by e.g. a pipe or tubing, extending through the gas well 140. As set out below, the third conduit may be provided by the same pipe or tubing that provides the first conduit, or the third conduit may alternatively be provided by a different pipe or tubing extending through the gas wellbore. The production wellbore 108 and the gas wellbore 140 are connected by the processing elements 1 12 of the gas-lift system 100, forming a closed system whose effective endpoints are the liquid hydrocarbon layer 104 of the reservoir 102 and the gas cap 106 of the reservoir 102. The processing elements 1 12 of the gas-lift system include a gas-liquid separator 1 14 and a gas compressor 1 16. The gas-liquid separator 1 14 is configured to receive produced reservoir fluids from the production tubing 1 10, to separate the produced reservoir fluids into separated liquid and separated gas, and to output separated liquid, via liquid outlet 1 15, and separated gas. The gas compressor 1 16 is configured to selectively receive separated gas from the gas-liquid separator 1 14 and gas extracted from the gas cap 106 via the first conduit 142, and to output compressed gas. A second conduit 160 extends into the production wellbore and is in fluid communication with the interior of the production tubing 1 10. Compressed gas produced by the gas compressor is injected into the production tubing 1 10 for gas-lift via the second conduit 160. In an embodiment, the second conduit 160 is an annulus between the production tubing 1 10 and a casing surrounding the production tubing, and the compressed gas enters the production tubing 1 10 via a valve. Compressed gas produced by the gas compressor can be injected into the gas cap via the third conduit 144. Wellhead equipment 1 18 for the production wellbore includes valves, spools and fittings for routing produced reservoir fluids and compressed gas, and potentially other fluids. Wellhead equipment 1 18 may be in the form of, or may include, a Christmas tree. Wellhead equipment 120 for the gas wellbore includes valves, spools and fittings for routing gas, and may be in the form of, or may include, a Christmas tree. A fluid path 130 from gas wellbore wellhead equipment 120 to gas compressor 1 16 includes a first valve 134 to control the flow of extracted gas to the gas compressor. A fluid path 128 from the gas compressor 1 16 to gas wellbore wellhead equipment 120 includes a second valve 132 to control the flow of compressed gas to wellhead equipment 120, and hence to the gas cap 106 via third conduit 144. A fluid path 126 from the gas compressor to production wellbore wellhead equipment 1 18 includes a third valve 136 to control the flow of compressed gas to wellhead equipment 1 18, and hence the flow of compressed gas for gas-lift of the produced reservoir fluids in the production tubing 1 10.

In an embodiment the first conduit 142, which is for extracting gas from the gas cap, and the third conduit 144, which is for injecting gas into the gas cap, are effectively the same conduit, provided by the same e.g. pipe or tubing extending through the gas wellbore 140. In this embodiment gas extraction via the first conduit and gas injection via the third conduit cannot be carried out simultaneously, and must be carried out alternately or at different times. When gas extraction is to be carried out, the first valve 134 is opened and the second valve 132 is closed. When gas injection is to be carried out, the first valve 134 is closed and the second valve 132 is opened. In this way the flow of gas (extracted or injected) is controlled using the first valve 134 and second valve 132. Alternative valve arrangements are possible, for example a single valve that switches between fluid path 128 and fluid path 130, or blocks both fluid paths. Using a single tubular (e.g. pipe or tubing) for extraction and injection reduces the cost and complexity of the required gas extraction and injection apparatus.

In one embodiment the gas-lift system is used in a field, e.g. a remote field, containing a reservoir with oil that cannot be produced without gas-lift, where the reservoir has a gas cap containing an amount of gas that is not commercially exploitable. Gas-lift may be required because the oil is heavy oil, and/or because the reservoir pressure is low. Where the field does not have the relevant infrastructure necessary for importing gas for gas-lift or for exporting produced gas, or where the field has such infrastructure but it is not commercially viable to import/export gas, it may not be possible to develop the field using existing techniques. Production from such a field can be started up using the gas-lift system 100 shown in Figure 1 , without the need to import gas for gas-lift from another source. In particular, gas is extracted from the gas cap using the first conduit and is compressed using the gas compressor. The compressed gas is then injected into the produced reservoir fluids in the production tubing for gas-lift via the second conduit, and production is thereby stimulated. In the state in which they enter the production tubing from the reservoir, the produced reservoir fluids contain oil, i.e. liquid hydrocarbons, and will typically contain some dissolved gases. The produced reservoir fluids may contain other substances such as water. Following injection of the gas-lift gas, the produced reservoir fluids will contain more dissolved gas. It is necessary to separate the produced reservoir fluids into liquid hydrocarbon and gas phases, and potentially to remove water, to produce liquid hydrocarbons that are suitable for transport and final consumption. The produced reservoir fluids are therefore routed to the gas-liquid separator, which produces separated liquid and gas phases. The separated liquid is transported away, e.g. by a pipeline. The separated gas is routed to the gas compressor, which compresses the separated gas to produce compressed gas. The resulting compressed gas can then be routed back to the production wellbore to be used for gas-lift of the produced reservoir fluids, which will subsequently be separated to produce more separated gas. In this circular fashion, eventually enough gas-lift gas will be produced via separation of the produced reservoir fluids that no extracted gas, i.e. gas extracted from the gas cap of the reservoir, is required for gas lift. Once stable gas circulation is established in this way, the flow of gas out of the gas cap can be stopped, and gas-lift can be sustained using only the gas produced by separating the produced reservoir fluids. It may be the case that the volume of gas produced by separating the produced reservoir fluids is so large that it is not possible to use all of the separated (and subsequently compressed) gas for gas-lift. Excess gas may also be produced due to depressurization in the well path. The excess compressed gas, i.e. the compressed gas that cannot be used for gas-lift, is injected into the gas cap of the reservoir via the third conduit. This means that it is not necessary to transport the excess gas away from the field. The injection of the excess compressed gas into the gas cap may have a further beneficial effect in that the pressure in the reservoir may be increased, which may increase the rate of production of reservoir fluids.

In another embodiment the gas-lift system is used with a reservoir with no gas cap. This is possible if the oil in the reservoir contains a sufficient amount of gas and has a bubble point pressure close to the reservoir pressure, and if a natural production flow can be achieved at an early stage of production. In this case, gas-lift is not used in the early stages of production. The separation and compression stages are carried out as set out above. The compressed gas produced in the early stages is re-injected into the reservoir to form a gas cap, and once the gas cap is formed the situation is similar to that set out above, i.e. where production can be started up using gas extracted from the gas cap for gas-lift if necessary. Figure 2A shows an alternative configuration of the first and third conduits (i.e. the gas extraction and gas injection conduits). Gas wellbore 240 extends into the gas cap 206 of reservoir 202, and the gas cap 206 lies above a layer of oil 204. In one embodiment the first conduit 242 for extracting gas from the gas cap 206 is an annulus between the third conduit 244 and the gas wellbore 240, where the third conduit 244 is for injecting compressed gas into the gas cap 206. The third conduit 244 is a tubular, for example a pipe or tubing, located concentrically within and extending through the gas wellbore 240. In the case that that wellbore is cased, the first conduit 242 is an annulus between the third conduit 244 and the casing.

Figure 2B shows a cross-sectional view of a further alternative embodiment in which the first conduit 242b and third conduit 244b are separate tubulars extending individually through gas wellbore 240b. In particular, the first conduit 242b and third conduit 244b are adjacent tubulars, with neither tubular being located within the other.

Figure 2C shows a cross-sectional view of a still further alternative embodiment in which the first conduit 242c and third conduit 244c are two halves of a tubular extending through gas wellbore 240c. In particular, the tubular has a dividing wall bisecting its internal volume, such that one half of the internal volume of the tubular forms the first conduit 242c, and the other half forms the third conduit 244c.

The gas extraction and injection directions are indicated in Figure 2A by arrows. Of course, the configuration of the extraction and injection conduits can be reversed, such that the first conduit for extracting gas from the gas cap in Figure 2A is the tubular located concentrically within the gas wellbore and the third conduit for injecting gas into the gas cap is the annulus between the first conduit and the wellbore wall or a casing. The configuration of the first and third conduits is similarly interchangeable in the embodiments shown in Figures 2B and 2C.

Figure 3 shows a high-level flow diagram describing a method in accordance with the invention. A production wellbore and a gas wellbore extend separately into a formation containing the reservoir, which has a gas cap, and the production wellbore contains a tubular in fluid communication with the reservoir for conveying produced reservoir fluids. In step 302, gas is extracted from the gas cap, e.g. via a first conduit. In step 304, the extracted gas is then compressed, e.g. using a gas compressor, to produce compressed extracted gas, and in step 306 the compressed extracted gas is used for gas-lift of produced reservoir fluids. In step 308, the produced reservoir fluids, which are produced as a result of the gas-lift, are separated into separate liquid and gas phases, e.g. using a gas-liquid separator coupled to the tubular. In step 310, separated gas from the gas-liquid separator and/or extracted gas is compressed to produce separated compressed gas. In step 312, the separated compressed gas is used for gas-lift of produced reservoir fluids, e.g. using a second conduit, and in step 314 any excess compressed gas is injected into the gas cap, e.g. using a third conduit. It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention.