GB2179081A - Offshore hydrocarbon recovery system - Google Patents

Abstract

An offshore hydrocarbon recovery system comprises a plurality of wells each terminated by a subsea structure, a subsea flowline network interconnecting the subsea structures, and a riser connected to the flowline network for delivering hydrocarbons from the flowline network to a surface production unit. Gas is injected at the subsea structures into the associated wells, and the gas injection is controlled such that the production pressure for each of the wells connected to the flowline system is substantially the same. All the wells can thus be produced at a similar pressure and the wells can be connected via a subsea co- mingling manifold using fixed production chokes.

Description

SPECIFICATION An offshore hydrocarbon recovery system and a method for operating such a system The present invention relates to an offshore hydrocarbon recovery system and to a method for operating such a system.

Offshore oil-field developments are known in which a fixed or floating production platform processes hydrocarbons delivered to it from a number of subsea wells. The production platform is connected to the wells via a subsea flowline system and a riser extending from the seabed to the production platform.

Individual subsea wells are often separated by a considerable distance from the riser. It is therefore convenient to connect more than one well to a single flowline system and a single flowline extending up the riser. A considerable effort has been made to design subsea units which are capable of separating the oil and gas normally present in crude hydrocarbon materials before they are transferred from the subsea units to the production platform. In most of the known systems the subsea units are designed to work at low pressure and to provide three separate flow phases, that is oil, water and gas.It is advantageous from the point of view of minimising the amount of seabed equipment to use comingling manifolds but where such manifolds are used the production wells must produce hydrocarbon product at similar pressures if a steady flow of product from each of the wells is to be achieved. This means that the pressure at which hydrocarbon product is delivered to the flowline system by any one well must not exceed significantly the pressure of the product delivered to the same system by any other well. Accordingly the rate of production from a number of interconnected wells is controlled by the rate of production of the lowest pressure well. Furthermore the pressure of the product delivered to the flowline system from each well must be controlled by a variable valve known as a choke.The known chokes are subject to wear and require regular replacement when they are operating on the crude hydrocarbon product which can include corrosive and abrasive elements.

As hydrocarbons are produced from a series of interconnected wells during the life of the oil-field the reservoir pressure is reduced and an increasing percentage of the production becomes water. The effect of the water is to suppress the release of gas in the flowline system and in the separation equipment used on the production platform. Further the gas is at low pressure and there is a considerable process plant and power requirement required to adequately treat the gas and oil to meet the required specification for hydrocarbon product to be delivered from the production platform to an export pipeline or a tanker.

It is an object of the present invention to obviate or mitigate the above problems.

According to the present invention there is provided an offshore hydrocarbon recovery system comprising a plurality of wells each terminated by a subsea structure, a subsea flowline network interconnecting the subsea structures, and a riser connected to the flowline network for delivering hydrocarbons from the flowline network to a surface production unit, comprising means at the subsea structures for injecting gas into the associated wells, and means for controlling the gas injection such that the production pressure for each of the wells connected to the flowline system is substantially the same.

The present invention also provides a method for enhancing the production of an offshore hydrocarbon recovery system comprising a plurality of wells each terminated by a subsea structure, a subsea flowline network interconnecting the subsea structures, and a riser connected to the flowline network for delivering hydrocarbons from the flowline network to a surface production unit, wherein gas is injected into the wells from the subsea structures such that the production pressure of each of the wells connected to the flowline system is substantially the same.

Thus in accordance with the present invention high pressure gas is injected into the annulus provided in the subsea structure associated with each production well. This is similar in some respects to the conventional "gas iift" operations in which gas is supplied to a production well to force hydrocarbon product from a reservoir to which the well is connected. In contrast with the conventional gas lift operations however the injected gas supply is controlled using a variable valve or choke which allows a varying flow rate at constant pressure. It is thus possible to produce all the wells of an interconnected system at a similar pressure and to connect those wells via a subsea co-mingling manifold using fixed production chokes.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a template and manifold subsea structure for use in a system in accordance with the present invention; Fig. 2 is a typical section through the manifold and template structure of Fig. 1; Fig. 3 illustrates in greater detail the connection of one well to the various components of the manifold and template structure illustrated in Figs. 1 and 2; and Fig. 4 is a schematic illustration of the relationship between the subsea structures described with reference to Figs. 1 to 3 and a surface structure from which control of the subsea structures is effected.

Referring to Figs. 1 and 2, the illustrated manifold and template has four template feet 1 arranged at the corners of a rectangle, the sides of the rectangle being typically of the order of twenty metres and ten metres in length. The template feet support a manifold structure 2 on which three flowlines are supported, that is a six inch gas lift flowline 3, a six inch production and pigging flowline 4, and a six inch water injection and production pigging flowline 5. Guide posts 6 are provided for flowline pull-in units of conventional type used when making connections to the flowlines 3, 4 and 5. Identical structures are provided at each end of the manifold assembly to provide for the interconnection of more than one template and manifold structure.

The illustrated structure is intended to handle up to twelve wells each capped by a respective "tree". To avoid over complication of the drawings the tree completions are only illustrated for two of the twelve locations in Fig. 1 and details of similar tree completions are illustrated in Fig. 2. Also for reasons of clarity the following features are not illustrated: 1) Duplicate tree connections to the water injection flowline 5; 2) Well kill/test lines; 3) A control bundle incorporating electrical and hydraulic control lines; 4) A stab receptical guided compliance system.

The above features are essentially conventional however and will not be described in any detail herein.

The manifold structure 2 is mounted on guide posts 7 supported on the template structure 8 which in turn is mounted on the template feet. Control pods 9 incorporate the necessary control and monitoring equipment.

Again for reasons of simplicity control connections between the control pod and control devices on the manifold assembly are not shown.

The main flowlines 3, 4 and 5 are connected to the individual trees by two inch water flowlines 10, four inch production flowlines 11 and two inch gas flowlines 12. The interconnections between the flowlines 3, 4 and 5 and 10, 11 and 12 are controlled by valves 13, 14 and 15 and the positioning of the equivalent valves for the other trees is shown schematically in Fig. 1. Six inch controls valve 16 are also provided for isolating the entire manifold structure.

Thus the template and manifold structure of Figs. 1 and 2 enables a plurality of wells to be interconnected to water, gas and production flowlines. Fig. 3 illustrates the interconnection of these flowlines to a single tree.

Referring to Fig. 3, the gas, production and water flowlines 3, 4 and 5 and the control pod 9 of Figs. 1 and 2 are identified by the same reference numerals in Fig. 3. Manual isolating valves 17 are provided to isolate the tree from the three flowlines. All the other valves shown in the drawing are remote control valves.

The tree assembly and the manual and remote control valves may be of the type available from a wide variety of suppliers, for examples Vetco or Cameron. A gas flow regulator valve 18 may be a Willis variable orifice choke.

The three flowlines 3, 4 and 5 are connected to the tree structure through a multiway stab connector 19 of the pull-in type. A conventional guide tree connector 20 is provided for making connections to down hole safety valves 21, direct hydraulic control lines 22, a production master valve 23, and an annulus gas lift master valve 24. Production swab valve 25 and annular swab valve 26 are provided for use in a conventional manner.

The production output from the tree is controllable by production wing valve 27 and the gas injection is controllable by gas lift wing valve 28.

The control pod 9 is connected to the surface by a multiplex electrical control cable 29 which extends up an associated riser structure. In accordance with the invention the pressure delivered from the flow line 4 is controlled by adjustment of the gas flow regulator choke 18 so that each of a series of subsea wells produce at the same pressure. The gas flow regulator valve 18 is controlled from the control pod 9 as will be described hereafter with reference to Fig. 4. The control pod 9 may be of a generally conventional type available from Marconi Avionics.

Referring now to Fig. 4, the control pod 9 and control cable 29 are connected to an above surface control system via a flexible riser connector 30. The control pod 9 is connected to an associated well via a control system indicated by box 31, the control system incorporating a plurality of valves and pressure transducers enabling the production pressure of the well to be monitored. Data indicative of this production pressure is transmitted to the surface from the control pod. The cable 29 is in the form of a control and services umbilical and incorporates a hydraulic supply line 32, a hydraulic return line 33 and an electrical supply cable 34. The hydraulic return line 33 is connected to accumulators 35.

Hydraulic power is supplied from a hydraulic power pack and back up unit 36. Electrical power is derived from an electrical power supply 37 provided with battery back-up 38 and controlled by a Marconi Avionics multiplex subsea control system 39. Water or kill fluid can be delivered via flowline 40. A single well header connection is provided incorporating a movable manual choke 41, an automatic choke 42, and isolating gate valves 43. A test separator is connected to flowline 44 and the main separator is connected to flowline 45 via feed valve 46.

The control system 39 monitors data received from the control pod 9 indicative of the production pressures of each of the wells connected to the system. Assuming an initial condition in which each well produces at the same pressure, the control system 39 monitors this condition and takes no action until the monitored pressure of one of the wells begins to change so as to be significantly different from the production pressures of the other wells. When such a circumstance is detected by the control system 39 a control signal is sent to the control valve 18 (Fig. 3) associated with that one well and adjusts the gas supply pressure to that well so as to return the well production pressure to its former value.Over a predetermined period it will be appropriate in normal circumstances to adjust the common production pressure of all of the wells to take into account changes in the subsea structures from which hydrocarbons are being extracted as a result for example of depletion of reserves. In normal circumstances however the control system simply adjusts the gas supply to each of the wells so as to maintain production pressures at a predetermined common level.

Thus as described above in accordance with the present invention a multiplex data transmission system can be provided to monitor and control the supply of injected gas to individual wells and to monitor the output pressure from each individual well, appropriate feedback techniques being used to provide the desired relationship between the production pressures of each of the interconnected wells.

This allows the production from each well to be enhanced and the assistance that each well is given as a result of the injection of gas fully compensates for the decline in its bottom hole pressure as the reservoir to which the well is connected is gradually depleted over time. The production rate is thus enhanced and also the total recovery from the reservoir. The system gives an overall advantage of considerable economies in the design of the required production platform in terms of the size and weight and number of the components of the production platform, the horsepower installed on the platform and the actual size of the production platform selected for a particular field.

The invention enables gas lift mandrels to be sized accurately. It also enables the use of a comingling manifold system at a higher pressure and avoids gas break-out in the subsea flowlines and riser system.

The process plant can be operated at a constant and higher inlet pressure, thereby enabling the simplification of the process plant design.

A higher operating pressure can be selected for the first stage separator of the production platform and thus treatment can be undertaken in small units with less chance of liquid drop-out in the dehydration and purification processes.

Produced gas can be compressed to meet gas export requirements without requiring directly driven turbine powered compressors and so allowing the production platform to have a central power station facility.

The system requires less subsea maintenance as the injected gas can be treated prior to compression so as to be reasonably free of corrosive and abrasive elements and therefore wear and tear of the gas supply chokes is very much less than is the case with variable production chokes which are rapidly damaged by the corrosive and abrasive elements in crude hydrocarbon product.

Claims (7)

1. An offshore hydrocarbon recovery system comprising a plurality of wells each terminated by a subsea structure, a subsea flowline network interconnecting the subsea structures, and a riser connected to the flowline network for delivering hydrocarbons from the flowline network to a surface production unit, comprising means at the subsea structures for injecting gas into the associated wells, and means for controlling the gas injection such that the production pressure for each of the wells connected to the flowline system is substantially the same.

2. An offshore system according to claim 1, comprising means for measuring the production pressure of each well, means for determining the pressure of gas supplied to each well, means for monitoring the measured production pressures, and means for controlling the gas pressure determining means to maintain the measured production pressures substantially equal to each other.

3. An offshore system according to claim 2, whereas the monitoring means is positioned on the surface and connected to the subsea structures by a control umbilical extending along the riser.

4. An offshore system according to claim 2 or 3, wherein the gas flow determining means comprises a variable valve connected to control the injection of gas into an annulas of the respective well, the variable valve being controllable to deliver gas at a selected constant pressure with a varying flow rate.

5. A method for enhancing the production of an offshore hydrocarbon recovery system comprising a plurality of wells each terminated by a subsea structure, a subsea flowline network interconnecting the subsea structures, and a riser connected to the flowline network for delivering hydrocarbons from the flowline network to a surface production unit, wherein gas is injected into the wells from the subsea structures such that the production pressure of each of the wells connected to the flowline system is substantially the same.

6. An offshore hydrocarbon recovery system substantially as hereinbefore described with reference to the accompanying drawings.

7. A method for enhancing the production of an offshore hydrocarbon recovery system substantially as hereinbefore described with reference to the accompanying drawings.