Method for storing carbon dioxide compositions in subterranean geological formations and an arrangement for use in such methods

Abstract

A method and arrangement are proposed for injecting CO.sub.2 into a subterranean aquifer for storage therein. In order to reduce the effects of water evaporation from brine in the aquifer when dry CO.sub.2 is injected into the aquifer, the CO.sub.2 is supplied mixed with a salt-lean fluid, i.e. a fluid that contains a low concentration of ions that can precipitate out as salts. The mixing may take place at the wellhead, with the CO.sub.2 salt-lean fluids supplied via separate low-grade material pipelines. The proportion of CO.sub.2 and salt-lean fluid in the mixture is such as to obtain a CO.sub.2 composition that is saturated with salt-lean fluid at the site of injection into the aquifer. By injecting saturated or wet CO.sub.2, less water is evaporated from the brine and the salt precipitation is greatly reduced, so keeping the pore spaces clear and providing an increased accessible pore volume for CO.sub.2 storage.

Claims

1. A method of introducing a CO.sub.2 and salt-lean fluid mixture into a subterranean aquifer for storage of CO.sub.2 therein, said method comprising the steps of: providing a supply of a fluid-saturated or wet CO.sub.2 mixture of a CO.sub.2 composition and a salt-lean fluid, wherein the mixture is compressed to assume a liquid or supercritical state at a site of injection, and passing said mixture downwards via a shaft and injecting said mixture from said shaft into said aquifer at the site of injection, wherein proportions of said CO.sub.2 composition and said salt-lean fluid in said mixture is such as to obtain the CO.sub.2 and salt-lean fluid mixture in which the CO.sub.2 composition is between 50% oversaturated and 50% under-saturated with said salt-lean fluid at the site of injection of said mixture into said aquifer.

2. The method as claimed in claim 1, wherein the step of providing said supply of said mixture includes: providing a supply of the salt-lean fluid and a separate supply of the CO.sub.2 composition, wherein the rate of supply of each of said salt-lean fluid and said CO.sub.2 composition is such as to obtain the CO.sub.2 and salt-lean fluid composition.

3. The method as claimed in claim 2, further comprising the step of providing said supply of the salt-lean fluid and said separate supply of the CO.sub.2 composition to a static mixer located at or close to said shaft.

4. The method of claim 3, wherein said step of providing said supplies of the salt-lean fluid and the CO.sub.2 composition includes providing two separate pipelines for each supply, said pipelines terminating at said mixer.

5. The method of claim 1, wherein the proportions of said CO.sub.2 composition and said salt-lean fluid in said mixture is such as to obtain the CO.sub.2 and salt-lean fluid composition that is between 50% oversaturated and 50% under-saturated in which the CO.sub.2 composition is between the 10% oversaturated and 10% under saturated, with said salt-lean fluid at the site of injection of said mixture into said aquifer.

6. The method according to claim 1, wherein said salt-lean fluid has a salt concentration that is less than 50% of the salt concentration of formation water present in said aquifer at the site of injection.

7. The method according to claim 1, wherein the step of providing a supply of the salt-lean fluid and providing a supply of the CO.sub.2 composition includes obtaining said salt-lean fluid and CO.sub.2 composition as side products from a processing plant.

8. An arrangement for introducing a CO.sub.2 and salt-lean fluid mixture into an aquifer, said arrangement comprising: a well including a shaft having an injection port which injects a fluid-saturated or wet CO.sub.2 mixture into the aquifer at a site of injection at a rate of flow, a first conduit for supplying a CO.sub.2 composition which is compressed to assume a liquid or supercritical state at the site of injection, said first conduit being connected to a wellhead portion of said shaft, a second conduit for supplying a salt-lean fluid, said second conduit being connected to the wellhead portion of said shaft, wherein the rate of flow of said CO.sub.2 composition and said salt-lean fluid is such as to form said mixture in which the CO.sub.2 composition is between 50% oversaturated and 50% under-saturated with said salt-lean fluid at the site of injection of said mixture into said aquifer.

9. The arrangement as claimed in claim 8, further comprising a mixer arranged at the wellhead portion of said shaft for mixing said CO.sub.2 composition and said salt-lean fluid to form said mixture.

10. The arrangement as claimed in claim 9, wherein said mixer is a static mixer.

11. The arrangement as claimed in claim 8, wherein the proportions of said CO.sub.2 composition and said salt-lean fluid in said mixture is such as to obtain a CO.sub.2 and salt-lean fluid composition in which the CO.sub.2 composition is between 10% oversaturated and 10% under-saturated with said salt-lean fluid at the site of injection of said mixture into said aquifer.

12. The arrangement as claimed in claim 8, further comprising a processing plant connected to said first and second conduits for providing a source of a substantially dry CO.sub.2 composition and the salt-lean fluid.

13. The arrangement as claimed in claim 8, wherein said salt-lean fluid comprises less than 5% wt of salts.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows an arrangement for introducing CO.sub.2 into a subterranean reservoir in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) An aquifer, within the context of the present invention shall be understood as being an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay). An aquifer, within the context of the present invention, may also be referred to as a reservoir.

(3) A site of injection, within the context of the present invention, shall be understood as being a position adjacent an opening of an injection port, through which opening CO.sub.2 is injected into an aquifer; said position being outside an outer surface of the conduit or well.

(4) The present invention relates to methods for storing CO.sub.2 in subterranean geological formations, in particular, in subterranean aquifers.

(5) The CO.sub.2 injected is preferably a CO.sub.2 composition compressed to assume a liquid or supercritical state, also referred to as dense phase, at the site of injection, i.e., at reservoir conditions. The compressed gas may include CO.sub.2 and additional compounds or impurities, such as lower alkanes, nitrogen and oxygen. These impurities preferably amount to less than 50% wt, 40% wt, 30% wt, 20% wt, 10% wt, 5% wt, 2% wt, most preferably to less than 1% wt, based on total compressed gas weight. The terms CO.sub.2 composition and CO.sub.2, according to the invention, and depending on the context, may relate to the above described mixtures of CO.sub.2.

(6) The invention shall now be explained with reference to the appended figure.

(7) FIG. 1 shows an arrangement for introducing CO.sub.2 into an aquifer according to the present invention. A shaft 2 is provided to transport CO.sub.2 from a level substantially above surface into a reservoir 6 within a subterranean formation. The shaft 2 may be in form of a tube disposed within the casing of a well. Alternatively, the casing of the well itself may constitute the shaft 2. The distal end of the shaft 2 terminates within the reservoir 6 in an injection port 4. CO.sub.2 is injected via the shaft 2 into the reservoir 6 under controlled pressure and temperature and is a liquid or supercritical fluid at the injection port 4. In the illustrated embodiment, the well is a vertical well, however, it will be understood by those skilled in the art that it could alternatively be an inclined or deviated well or have a substantially vertical, or inclined upper (proximal) portion and a substantially horizontal distal portion. The well shaft 2 is connected at a proximal or wellhead end via a pipeline 8 to a processing plant 10 which produces CO.sub.2 as a side or secondary product. For example, the processing plant may be a natural gas processing plant. In such a plant, the CO.sub.2 is separated from the natural gas then conventionally compressed and dehydrated and before being transported via pipeline 8 to the well-head. The CO.sub.2 obtained in this way may be pure CO.sub.2 or may alternatively contain specific levels of contaminants, including the lower alkanes, methane, ethane, propane and butane. The pipeline 8 supplying the CO.sub.2 is conventionally of mild steel (carbon steel) and would be subject to corrosion if CO.sub.2 were not dehydrated first. This CO.sub.2 dehydration thus avoids the need for a stainless steel pipeline over distances that can number many hundreds of kilometers.

(8) In accordance with the present invention, a second pipeline 12 joins the shaft 2 at the wellhead and provides a source of salt-lean fluid. A salt-lean fluid in the context of the invention is a fluid containing low concentrations of ions that can precipitate. The salt concentration in the salt-lean fluid is preferably defined with reference to the salt concentration or salinity of the formation water, i.e. the water or brine already present at the injection site. Preferably the salt concentration of such a fluid is less than 50% of the salt concentration of the formation water and most preferably less than 25% of the salt concentration of the formation water. Examples of suitable salt-lean fluids include an aqueous fluid, such as water with a salt concentration of less than 1% wt. Another possibility is propylene glycol, also known as methyl ethylene glycol or MEG. Processing plants that produce CO.sub.2 as a side product commonly have salt-lean fluid streams for various types of treatments. Examples of such streams are the knock-off water from the CO.sub.2 compression train, wash water and steam condensate.

(9) This salt-lean fluid is mixed with the CO.sub.2 in the wellhead, preferably with a static mixer 14 provided there. The wellhead, shaft 2 and mixer 14 are typically made of high-grade stainless steel and thus are not subject to corrosion by the fluid-CO.sub.2 mixture or wet CO.sub.2 composition. In addition to the mixer 14, the shaft 2 may be provided with a compressor (not shown) upstream or downstream of the mixer 14 for adjusting the pressure of the CO.sub.2-fluid mixture.

(10) The proportion of salt-lean fluid to CO.sub.2 mixed together is such as to provide salt-lean fluid saturated CO.sub.2 at the injection site. In other words, the mixture is such that the CO.sub.2 is around the saturation point or substantially saturated with the salt-lean fluid at the temperature and pressure prevailing in the reservoir at the point of injection.

(11) The exact proportions of CO.sub.2 and salt-lean fluid will thus depend on the conditions prevailing in each reservoir. For example, in the Sleipner project in which CO.sub.2 is stored at a depth of between 800 and 1000 below sea level, the pressure and temperature in the reservoir are around 29 C. and 74 bar. In deeper reservoirs, such as at the Snhvit project located in the Barents Sea offshore Norway at a depth of 2600 m below sea level, the prevailing pressure and temperature are considerably higher. Clearly the proportion of salt-lean fluid to CO.sub.2 composition to obtain saturation will be higher at these higher temperature and pressures. It is possible to model the prevailing conditions at the injection site of any particular reservoir. Hence it is possible to set the required proportions at the well head.

(12) While the ideal state is saturated CO.sub.2, some margin is possible. Thus the CO.sub.2 may be between 10% oversaturated and 10% under-saturated with the salt-lean fluid, preferably 10% oversaturated and 5% under-saturated with the salt-lean fluid and most preferably 5% oversaturated and 2% under-saturated with the salt-lean fluid. In any event, the mixture is not a liquid in which CO.sub.2 is dissolved, but rather fluid-saturated or wet CO.sub.2.

(13) Since the CO.sub.2 injected into the aquifer 6 is no longer dry, less water will be evaporated from the brine and consequently less salt will precipitate out. As a result, the pathways in the pore structure will be less obstructed by salt precipitates and the accessible pore volume will be considerably larger than when CO.sub.2 is injected in a dry state. In addition, the quantities of water or other salt-lean fluids required to saturate the CO.sub.2 are such that these fluids can be obtained entirely from the CO.sub.2 source processing plant. When these fluids are subject to costly treatments, such as biological treatment or demineralization, these costs can be offset by the recycling of this fluid at the CO.sub.2 injection well to increase the levels of CO.sub.2 sequestration.

(14) While the above description has centered on the arrangement illustrated in FIG. 1 with two separate pipelines supplying a CO.sub.2 composition and a salt-lean fluid and a mixer at the wellhead to achieve the desired saturation or wetness of the CO.sub.2 composition, it will be understood that the CO.sub.2 composition and salt-lean fluid could be mixed upstream of the wellhead and supplied via a high-grade, corrosion-resistant pipeline to the well. In some cases, it may even be advantageous to pipe a wet CO.sub.2 composition directly from the processing plant, i.e. omit the dehydration step, or modify this process in the plant to obtain the desired proportions of CO.sub.2 composition and salt-lean fluid.