AU6846887A - Process and installation for integrated separation and storing of oil and gas in mined rock caverns below the ground water level - Google Patents

Description

A process and installation for integrated separation and storing of oil and gas in mined rock caverns below the ground water level.

The present invention relates to a process of combined separation of oil from gas rich oil and storing of the separated phases in mined rock caverns arranged below the ground water level, the process being characterized by the fact that the separation being effected in the very mined rock cavern being utilized as a rock storage cavern.

The invention relates also to an installtion for the carrying out of the said process, the installation being characterized by the fact that a separation unit for separating gas from gas rich oil is arranged in the very cavern and over the highest oil level therein. Preferably the separation unit is in the form of at least a gallery or a tunnel wherein the separation takes place.

It is previously known to store oil products in mined rock caverns, but this previously known technigue is based on three conditions or premises, i.e. firstly that the oil products have a lower density than water, secondly that the product cannot be dissolved in water, and thirdly that the mined rock cavern is situated under and below a stable ground water level so that walls, ceiling and bottom of the rock cavern have a pore water pressure which is higher than the oil pressure against the rock surfaces and the internal gas pressure. Storage in mined rock caverns has previously been used for crude oil having been stabilized for transport by tankers, i.e. having a gas pressure somewhat under 1 bar abs. To increase the safety, the space over the oil level in the rock cavern has been filled with a non-combustible atmosphere. The previously known method embodies that it over the oil in a completely sealed and non-vented space is maintained a non- cumbustible gaseous atmosphere at a pressure varying between 0 and 4 bars abs. depending on the degree of oil filling. This method necessitates a mined rock cavern which is larger than necessary for the very storing of the oil, as the compressed gas cushion usually will constitute about 25 % of the total volume. The great overpressure for a filled storage will also necessitate a particularly thick rock cover as well as control of the pore water pressure to prevent leakage of gas into the rock and to the tunnel system leading to/from the rock chambe .

A further method (known from Norwegian patent application 83.0874) is based on the fact that the gas atmosphere over the crude oil is maintained at a nearly constant pressure of about 1 bar abs. thereby that it is pressure balanced against the surroundings and is maintained non-combustible by supplying inert gas as for example exhaust gas from a combustion process when oil is drawn off.

A purose of the present invention is that neither extra volume for a sealed gas cushoin (as in the first instance) nor supply of inert gas/fuel gas according to need (as in the second instance) will be necessary.

In connection with production of oil and associated gas from an offshore oil- field from installations under the sea level or installations onshore, also heavier gas fractions called condensate will occur.

For pipeline transport from the field to consumer network or shipment terminal it is today known that this takes place in separate pipelines for oil and gas after separation at the field. The condensate can then either be carried along with the gas in the gas pipeline or dissolved at a small over¬ pressure in the oil in the oil pipeline.

For transport of the condensate together with the gas in the gas pipeline either as a two phase system or in supercritical state, the condensate must always be separated before the gas is fed into a gas pipeline network for consumption. For the alternative method where the condensate is transported together with the oil in the oil pipeline, the separation at the field can be terminated at a somewhat higher pressure than otherwise and this makes advantageous reduction both in space and weight of equipment and structures at the field possible. However, the condensate must be separated at the terminal end to give the oil an acceptable low vapour pressure (somewhat under 1 bar abs.) to be enabled to be stored in large volumes for transport by ship. It is previously known that this is effected in large free-standing vacuum separation plants made of steel before passing the stabilized crude oil to storage.

One of the purposes of the present invention is that such vacuum separation plants can be eliminated thereby that their function is integrated into the storage function, the invention thereby providing a process and an installation for combined final separation/stabilizing of gas rich oil as well as storage of the stabilized oil whereby economic safety and environmental hazards of the previously known methods and installations for the two partial functions, i.e. separation and storage, have been overcome.

According to the invention, the gas enriched oil is intro¬ duced and pressure relieved directly into the storage hall, advantageously into its upper entrance tunnel, or a particu¬ larly arranged gallery or a groove means arranged over the highest level of the oil. Here the oil will flow or trickle as a free stream preferably over baffle plates or thresholds to establish thin layers and great contact area between liquid and gas so that the gas can easily escape to the gas space over the oil. The size of this section must be sufficiently large to give the oil the necessary residence time to achieve full equilibrium with the atmosphere in the rock hall. The stabilized oil will flow from this section down into the storage section of the rock cavern. The liberated hydrocarbon gas (the condensate which will now exist in gaseous phase) will make up the whole gaseous atmosphere. This atmosphere is free of oxygen and is therefore no explosion risk.

An installation according to the present invention will preferably comprise a gas compressor plant having the function of drawing produced gas from the storage hall and maintein the pressure of the atmosphere in the storage at the necessary stabilizing pressure for the oil.

The attached drawing shows schematically an installation in accordance with and for carrying out the invention, with a mined rock cavern 1 for storage of crude oil or similar. At the upper level is arranged a sloping gallery 2, which can constitute a part of the very storage rock cavern or the entrance tunnel, with optional means 3 for securing a great contact area in thin layers between liquid/gas and sufficient residence time for the oil. The gas enriched oil is intro¬ duced through the pipe system 4 and is pressure relieved through the choke valve 4a. In the bottom of the rock hall is arranged a thin water bed 5 which catches ground water which has leaked in. The oil is pumped from the storage with pump and pipe 6. Water which has leaked in is pumped from the installation with pump and pipe 7. Through the same water- filled shaft 11 where 6 and 7 are situated, the off-gas tube 8 is arranged and leads to a suction compressor unit 9. Over the liquid level in the rock cavern is maintained an atmosphere 10 consisting of hydrocarbon gas liberated by the separation in the rock cavern. The drawing shows a concrete plug 12 with a water filling arranged in the pipe conduit shaft 11.

Claims (13)

CLAIMS:

1. A process for combined separation of oil from gas rich oil and storing of the separated phases in mined rock caverns (1) arranged below ground water level, characterized by that the separation is carried out in the very mined rock cavern (1) being used as storage rock cavern, and/or in one or more smaller rock caverns or passages connected to the said rock cavern and arranged over the highest oil level.

2. A process according to claim 1, characterized by that the very rock hall, advantageously its upper entrance tunnel, or a particularly arranged gallery or a groove means (2), is used as rock cavern.

3. A process according to claim 1 or 2, characterized by that an atmosphere of pure hydrocarbon gas which represents no explosion risk and which is liberated during the separation process is maintained in the rock caverns.

4. A process according to any of claims 1 - 3, characterized by maintaining the gaseous pressure in the rock caverns below 1 bar abs. for security against gas leakage to the atomosphere.

5. A process according to any of claims 1 - 4, characterized by that the gas pressure in the rock chambers is maintained at most the stabilizing pressure for oil ready for transport.

6. An installation for carrying out the process according to any of claims 1 - 5, characterized by that it in or in connection with the very rock cavern (1) and over the highest oil level therein is arranged a separation means (3) for separation of gas from gas rich oil.

7. An installation according to claim 6, characterized by that the separation means (3) is arranged in the very storage hall, advantageously in its upper entrance tunnel, or in a particularly arranged gallery or a groove means (2) .

8. An installation according to claims 6 or 7, characterized by that the separation means (3) being constituted of contact means to provide great contact are oil/gas in thin layers and sufficient residence time for the oil to escape.

9. An installation according to any of claims 6 - 8, characterized by that the contact means consist of stepped thresholds and/or baffle plates.

10. An installation according to any of claims 6 - 9, characterized by that a suction compressor unit (9) with auxiliary equipment and tube connections to the rock cavern is arranged outside the cavern to maintain the gas pressure in the rock cavern at most at the transport stabilization pressure for the oil.

11. An installation according to any of claims 6 - 10, characterized by that the tube connections for drawing off water (7), drawing off oil (6) and gas (8) are passed up from the storage cavern (1) through a water filled shaft (11).

12. An installation according to any of claims 6 - 11, characterized by the shaft (11) being sealed by means of a water filled concrete plug (12).

13. An installation according to any of claims 6 - 12, characterized by comprising a pipe system (4) containing a choker valve (4a) for supply and pressure relief of the gas rich oil.