GB2167467A - A method of supplying offshore oil fields with gases for enhanced oil recovery - Google Patents

Abstract

The invention relates to a method of supplying gases to offshore oil fields for the purpose of enhanced oil recovery, characterised in that the gas [e.g. carbon dioxide or nitrogen] is compressed on shore to such a level that it is delivered to the production facility of the offshore oil field by high pressure pipeline at a pressure higher than the minimum needed for direct injection into the reservoir to produce a miscible flood. The method may employ a pipeline made up of pipe lengths and mechanical couplings of the type and material coded by API for oil field tubing and casing. The pipeline may have an external diameter of up to about 133 DIVIDED 8" [0.34 m].

Description

SPECIFICATION A method of supplying offshore oil fields with gases for enhanced oil recovery This invention relates to a method of supplying carbon dioxide and other gases to offshore oil fields for the purpose of enhanced oil recovery.

The production of crude oil from an oil field is usually by means of the use of secondary recovery techniques whereby the natural reservoir pressure is maintained by the injection of water or hydrocarbon gas. Under the influence of pressure and of the physical presence of the injected gas andl or water the oil is swept to the production wells.

Some 3545% of the original oil in place can be recovered by an oil production method which includes secondary recovery. The balance of the oil remaining in the formation is held in rock pores by capillary action or is left undisturbed in unswept regions of the reservoir.

Techniques used to displace and produce further oil are referred to as tertiary methods and the general description of such activities is known as "enhanced oil recovery" (EOR).

One tertiary recovery method which has proved effective for onshore oilfields is to inject carbon dioxide gas (CO2) into the reservoir at a pressure higher than the minimum miscibility pressure (MMP). The MMP is the pressure at which the oil and CO2 become miscible to a degree and is determined in standard laboratory scale tests. The MMP depends on the type of crude oil, being lower for lighter oil than for heavy oils. The MMP is also temperature dependant. For a given grade of crude oil the hotter the reservoir, the higher the MMP. The reported MMPs for different reservoirs are in the range 1000 to 5000 psi (6.5-32.5 MPa). Under the most favourable reservoir conditions and operating a CO2 miscible flood at a pressure well above the MMP, a recovery of 95% of the original oil in place could be achieved.

Large volumes of CO2 are required for a CO2 miscible flood, in the range tens to hundreds of millions of standard cubit feet per day (or millions to tens of millions of cubic metres per day). For onshore oil fields where the source of CO2 is remote, the gas may be transported by welded steel pipeline to the field area where it is compressed and injected into the reservoir. The compressor units are major items of machinery and are major consumers of motive energy.

Although these methods have been used for the carbon dioxide miscible flooding of onshore oil reservoirs no such method exists for offshore oil fields.

High pressure nitrogen, above its MMP, has also been used successfully in a tertiary recovery method for onshore oil fields although the experience and applicability of nitrogen miscible floods is not as extensive as that of carbon dioxide.

No method exists by which a nitrogen miscible flood could be used for tertiary production from offshore oil fields.

According to the present invention there is provided a method by which offshore oil fields can be supplied with gas in the quantities and at the conditions needed for enhanced oil recovery by means of a tertiary miscible gas flood.

The method is to compress the gas at a coastal compressor station to a level such that after pipeline transmission and control pressure losses, the gas will be available at the field location at a pressure higher than the MMP; thus the gas is capable of being injected into the reservoirwithoutfurther compression.

The subsea pipeline carrying the gas from the onshore compressor to the offshore field will be of a smaller diameter and will be operating a much higher pressure than would an equivalent onshore pipe line transmitting the same quantity of gas.

She offshore line will be made up from lengths of high tensile strength steel pipe joined together by mechanical coupling of the "premium joint" type.

The pipe and couplings and the pipe and the couplings material will be of the API coded type used for oil well tubing and casing. A pipeline of this type can be made from API coded elements up to 138" O.D. (0.34 m).

The line may have a continuous internal and/or external coating of thermosetting polymeric material. The pipeline can be further protected against external mechanical damage and it can be weighted ortrenched in accordance with established subsea pipeline practices.

The method is particularly suitable for use to transmit carbon dioxide or nitrogen to offshore oil fields for the purpose of enhanced oil recovery by means of a miscible gas flood.

The method may also be used if the preferred method of EOR is to alternate slugs of CO2 with slugs of nitrogen. In such a case the carbon dioxide is the miscible agent, operating above its MMP, and the nitrogen slug pushes the CO2 slug through the reservoir formation. A single pipeline could be used to successively transmit CO2 then nitrogen from the onshore compressors to the offshore field production facility. Alternatively separate high pressure pipelines could be used for each gas.

High pressure gases other than nitrogen could be used for the slug pushing duty, for example flue gas.

If instead of a gas, water slugs are used to push the CO2 miscible slug through the formation then the water injection facilities provided for secondary recovery could be used. Similarly if hydrocarbon condensate was to be added to the CO2 injection it could typically be done using condensate injection pumps on the offshore production facility. If this were not possible then hydrocarbon condensate could be injected into the high pressure CO2 stream immediately after the last onshore compression stage.

Considering the application of the invented method to offshore oil fields within the UK continental shelf, carbon dioxide sources could be selected so as to minimise the length of the onshore and offshore pipelines and could include the following coastal sources:- CO2 from ammonia plants on Teeside; CO2 extracted from effluent stack gases from refineries at Seal Sands or Grangemouth; CO2 extracted from stack gases of power stations capable of burning low sulphur liquid fuels such as Peterhead power station; CO2 extracted from the stack gases from coal fired power stations after the stack gases had first been treated to remove sulphur dioxide, e.g. stations such as Blyth power station or Cockenzie power station.

Nitrogen could conveniently be available from modified existing oxygen air separation plants associated with steel works and presently underutilised, for example air separation plants in the Teeside area.

The oil fields which could benefit from the introduction of tertiary recovery using a miscible CO2 flood include those where secondary recovery has almost drained the field (such as the Montrose field). CO2 tertiary recovery could benefit early producing fields whose production rate has passed the peak and is in decline notwithstanding the use of secondary recovery methods (such as the Piper field or the Forties field).

In cases where the oil is heavy, tertiary production by CO2 injection could be introduced before the production peak is passed in order to improve the production rate as well as enhancing the ultimate recovery (for example the Beatrice field or the Claymore field).

A nitrogen miscible flood would be preferred to a CO2 flood where the oil is very light and the oil field formation is such that the oil can be displaced downwards to the producing wells. An early candidate for a nitrogen miscible flood might be the Auk field followed at a later data by the Fulmar field.

EXAMPLE A method according to the invention will now be described in detail, by way of example only, with reference to the accompanying block diagram Fig. 1.

Stage 1 Compression and Drying The CO2 is delivered to the onshore coastal compression unit at a low pressure, say 5 psig (34.5 kPa), by a short pipeline from an adjacent source such as an ammonia plant or a unit recovering CO2 from combustion gases. Alternatively the CO2 can be delivered from a similar but distant source at a pressure economically appropriate to onshore transmission by a welded pipeline, e.g. about 1000 psi or 6.5 MPa.

The CO2 is compressed in stages. It is dried at an intermediate pressure stage, for example at about 2000 psi or 13 MPa, in a glycol drying unit, to remove moisture. The final stage of compression raises the pressure to the required sub sea pipeline inlet pressure. In the example shown this is 6500 psi (44.95 MPa).

Stage 2 Pipeline Transmission The compressed gas is transmitted from the onshore compression station to the offshore oil field by a subsea pipeline.

The pipeline is be made up of lengths of API grade high strength oilwell tube or casing joined with mechanical couplings.

Atypical line with maximum working pressure of about 8000 psi (55.2 MPa), suitable for the conditions shown in the example, is made up of pipe having 65-" (0.175 m) OD, a 0.475" (12 mm) wall thickness in API 5AC steel grade 95C connected by a VAMATAC coupling in the same material.

Stage 3 Gas Reception and Injection The subsea pipeline delivers the carbon dioxide into a riser on an offshore oil field production facility, typically a platform supported by a steel jacket.

In the example shown the transmission pressure loss in the subsea line is about 2000 psi (13 MPa).

The carbon dioxide is available at about 4500 psi (30 MPa) for direct injection into the reservoir. This pressure is comfortably above the MMP shown in the example for a typical north sea field containing light oil of gravity 40 API at a temperature of 220"F (104.5"C). Thus even after further control and downhole transmission pressure losses, the actual injection pressure into the reservoir formation will be sufficiently high to ensure that the pressure does not fall below the MMP anywhere in the reservoir.

This gives the possibility of an ultimate recovery approaching 95% of the original oil in place in the reservoir.

The field operator may alternatively choose to recover some of the CO2 from the oil which has been produced by a miscible CO2 flood and then to recompress it and reinject it into the reservoir along with the main supply of CO2 being delivered by the high pressure,subsea pipeline from the shore.

Claims (6)

1. A method of supplying gases to offshore oil fields for the purpose of enhanced oil recovery, characterised in that the gas (e.g. carbon dioxide or nitrogen) is compressed onshore to such a level that it is delivered to the production facility of the offshore oilfield by high pressure pipeline at a pressure higher than the minimum needed for direct injection into the reservoirto produce a miscible flood.

2. A method according to claim 1, wherein a gas which is to follow a slug of miscible gas is also compressed on shore and transmitted in serial in one pipeline or in parallel in a second pipeline.

3. A method according to claim 1 or 2, which employs a pipeline made up of pipe lengths and mechanical couplings of the type and material coded by API for oil field tubing and casing.

4. A method according to any preceding claim utilising a pipeline having an external diameter of up to about 138 inches (0.34 m).

5. A method according to any preceding claim utilising a pipeline coated internally with a thermosetting polymeric coating.

6. A method substantially as hereinbefore described with reference to Figure 1.