BACKGROUND OF THE INVENTION

The invention relates to a method and system for inhibiting freezing of low salinity water in a subsea low salinity water injection flowline.

It is known from International patent applications WO 2012/051511 , WO 2010/092097 and WO2010/092095 to Enhance Oil Recovery (EOR) by injecting desalinated water into an oil containing formation.

At offshore locations the low salinity water may be obtained by desalinating seawater at an offshore platform and then injecting the desalinated water flux through a subsea water injection flowline to a subsea wellhead of subsea desalinated water injection well.

In arctic and deepwater environments the ambient seawater temperature at the seabed may be -2 degrees Celsius. At this subzero temperature the seawater

surrounding the subsea flowline does not freeze, but the desalinated injection water in the subsea injection flowline, which has a freezing point of about 0 degrees Celsius, may freeze and plug and rupture, in particular if the flux of low salinity water would temporarily stop, for example due to an injection process upset.

Upon melting, the flowline contents would spill into the sea and the flowline would require replacement causing production deferment and unplanned expenditure.

Known solutions to prevent freezing of subsea flowlines involve flowline heating, insulation and/or injection of anti-freeze additives, which are costly both installation-wise and operation-wise and require a active flowline monitoring and/or anti-freeze injection systems that are prone to malfunctioning. A potential platform black-out is one scenario where not only the desalinated water injection would stop, but where the heating and/or anti-freeze injection would stop as well.

There is a need for an improved method and system for inhibiting freezing of subsea desalinated water injection flowlines which are cost effective and less vulnerable to malfunctioning than available flowline heating, insulation and anti-freeze injection systems.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method for inhibiting freezing of low salinity water in a subsea low salinity water injection flowline, the method comprising providing the flowline with a pressure control system which maintains an elevated fluid pressure within the flowline throughout low salinity water injection operations and temporary flowline shut in periods during which the flowline remains filled with substantially stationary low salinity water.

The pressure control system may be configured to maintain the pressure within the flowline above 200 Bar, and optionally above 250 Bar, throughout the injection operations and the shut in periods and may thereto comprise :

- a pump that injects low salinity water at the elevated pressure into the flowline during low salinity injection operations ;

- an upstream shut in valve arranged adjacent to an upstream end of the subsea flowline;

- a downstream shut in valve arranged adjacent to a downstream end of the flowline; and

- means for initially closing the downstream valve and subsequently the upstream valve prior to deactivating the pump at the beginning of each shut in period. In accordance with the invention there is

furthermore provided a system for inhibiting freezing of low salinity water in a subsea low salinity water injection flowline, the system comprising a pressure control system which is configured to maintain an elevated fluid pressure within the flowline throughout low salinity water injection operations and temporary flowline shut in periods during which periods the flowline remains filled with substantially stationary low salinity water .

The upstream and downstream shut in valves may be block valves and the pressure control system may

furthermore comprise a pressure sensing gauge which gives an alarm signal when the pressure within the flowline drops below a predetermined pressure.

Furthermore the desalinated water injection flowline may be configured to inject desalinated seawater into a subsea oil containing formation to Enhance Oil Recovery (EOR) from the formation.

These and other features, embodiments and advantages of the method and system according to the invention are described in the accompanying claims, abstract and the following detailed description of non-limiting

embodiments depicted in the accompanying drawing, in which description reference numerals are used which refer to corresponding reference numerals that are depicted in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 shows a subsea desalinated water injection flowline in which freezing of desalinated water is inhibited in accordance with the method and system according to the invention. DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENT

Figure 1 shows an offshore oil production platform 1 for the production of crude oil from a subsea crude oil containing formation 2.

The crude oil is produced via one or more crude oil production wells 3 and to stimulate crude oil production low salinity water (H ₂ 0) is injected into the formation 2 via a low salinity water in ection well 4.

The injected low salinity water (H ₂ 0) is obtained by desalinating seawater in a water desalination plant 5 at the oil production platform 1 and injecting the

desalinated water (H ₂ 0) into the injection well using a pump 6 and a subsea desalinated water injection flowline 7 that extends along the seabed 8 from a foot of the platform 1 to the wellhead 9 of the injection well 4.

The system according to the invention is furthermore equipped with an upstream shut in valve 10 and a

downstream shut in valve 11, which are configure to be closed prior to shutting off the pump 6 at the end of a desalinated water injection cycle.

The closing of the shut in valves 10 and 11 prior to shutting off the pump 6 generates a pressure control system that is able to maintain a high enough pressure in the flow line 7 such that the freezing point of the desalinated water (H ₂ 0) drops to/below the ambient seawater temperature. For -2 degrees Celsius ambient seawater temperature at the seabed, this requires a minimum of - 395.2 * (((273.16 - 2) /273.16) ^Λ 9 - 1) = 25.3 MPa = 253 bar pressure to be maintained in the seabed desalinated water injection flow line 7.

In many cases, this effectively means that during shut-in periods, when injection of desalinated water (H ₂ 0) is interrupted, the pressure of desalinated water (H ₂ 0) within the injection flowline 7 is maintained at or near the pressure at which desalinated water (H ₂ 0) is injected to the subsea wellhead 9.

The shut in valves 10 and 11 may consist of a set of block valves on either end of the flowline 7 that

automatically shut in the pressure once the flux of desalinated water through the flowline 7 is interrupted, for example due to a trip of the injection pump 6.

To avoid freezing of the subsea flowline 7 in case the flow is interrupted for a longer period of time, say more than a day or a week, the flowline 7 may be drained and filled with a fluid that does not freeze at the ambient subzero seabed temperature. This fluid may

comprise seawater or desalinated water to which an

antifreeze additive, such as methanol and/or Mono Ethylene Glycol (MEG) , is added.