|Title of Invention||
METHODS FOR THE RECOVERY OF HYDROCARBONS FROM HYDRATES
|Abstract||A method for recovering hydrocarbon trapped in a hydrate formation, comprising the steps of (a) contacting the hydrate formation with an aqueous solution comprising from 10 % to 75 % by weight of a salt such as potassium formate or acetate salt to liberate hydrocarbon from the hydrate formation and producing a mixture of hydrocarbon and water vapour; (b) transporting the hydrocarbon/water vapour mixture and the aqueous solution to a separator, whereby the said aqueous solution absorbs water vapour from the mixture during the transportation step, to form a more dilute aqueous solution of the alkali metal salt, thereby inhibiting formation of hydrocarbon hydrates; (c) separating hydrocarbon from said dilute aqueous solution; (d) regenerating the aqueous solution of step (a) by heating said dilute aqueous solution to remove absorbed water vapour; and (f) recycling the regenerated aqueous solution to step (a).|
|Full Text||METHOD FOR THE RECOVERY OF HYDROCARBONS FROM HYDRATES
lliepreseirt invasion relates to method hydrate formations, sad in particular to methcKk for the recovery of natural gas.
Hydrate formations ate an example from a class of chemical compounds knows as clathtates. A clafluste is a fonnofcon^undin\vhich one component is enclosed withm the structure of another. In a hydrate fonnado Hydrate formations can be found in vast quantities in the earth's crust, principally in seabed —ijjj"1*!1** and in permafrost regions of the world. Conventional natural gas ncamwlfttiooa are thought to underlie many of these hydrate formations, the hydrate formations possibly having been formed when rising natural gas met subterranean water deposits, which den froze form hydrates and enclose the gas.
It is estimated mat subterranean hydnte formations, einier sub-sea or on land contain over twice the quantity of natural gas, as all the existing provable reserves of conventional accumulations of natural gas. Accordingly, there is a huge potential source of energy if the trapped natural gas can be recovered from these hydrate formations.
Although sub-sea hydrate formations can be located relatively easily be methods such as seismic profiling of the seabed, it has proved difficult or at least very expensive to extract the gas from hydrate formations. Various methods have been developed to address this problem, which employ heat, solution, and reduction of pressure or a combination of the three processes.
Heat can be used to disassociate natural gas from hydrate formations by the introduction of heated solutions into the hydrate formation. Such methods are often somewhat impractical however, doe to the heat loss associated wim transferring a heat-transfer in order to heat a formation which is kxa^ hundreds ofmetres under the permafrost, or thousands of metres under sea level. US, 4,424,866 discloses a method of recovering natural gas from, gas hydrate fbimatkms by pumping a hot supasaturated
solution of calcium chloride or calcium bromide under pressure into the hydrate
formation so as to fracture the formation hydrostatically, dissolve the solid hydrate and
release the gas. U m alternate heating method disclosed in US 5,713,416, an acidic
Uquid and a basic Hqind arc combined and reac*^
solution which is injected into a gas hydras
hydrate and release the gas. Bcihofmese methods are uneconomical.
An alternative solution is discussed in US 4,007,787 involves the introduction into fee gas hydrate formation of freezing point depressants such as methanol, in order to aUsassociate the hydrate and &ee the natural gas. However, this method is also expcinave, due to ^ cost ofmefreezmgpcdnt depressant It is also inefficient, since the depressants suggested do not tower Ac freezing point erf the hydrate by an extent sufficieat to Kberate sufficient natural gas.
US 4,007,787 disclose a pressure reduc^on method mwmch me hydrate is c%associated by reduci^ the ambient pressure. The advaatage of mis me&od is mat me structure ©f #» hydrate can be brdcen down vdtt08t the need to increase the temperature. However, depressurisaticmmethc
weU-Kirilh^ and oU and gas recovery operations. The compositions can comprise ionic
salts. The presence ofthe alcohol m me aqueous solitfc^aUows control of t^
ternperature,wimottf substantial variation m This reference does
no* address me problem of the recovery of hydrc WO-A-9726311 discloses the use of various mixed salt systems for use as
various fluids cmcen^wim drilling operation, such as hydrm^ This
reference is not concerned wiii the particular problems which arise in the recovery of hydrocarbons from hydrate deposits.
US-A-1866560 is concerned wim a memod for dehydrating gases, using solutions of calcium chloride. Again, ft does not disclc^ any particular problems which arise in fee extraction of hydrocarbons from hydrate formations.
US-A-4979965 is also concerned wMi a memod of dehumidifying gas, in which a salt soluticm is employed that contains salt crystals, so that the c»n^
will not be substantially reduced during absorption. This reference is not concerned with hydrocarbon recovery from hydrate deposits.
composition comprising a glycol and a dissolved'salt The reference is not concerned vMk hydrocarbon recovery feua hydrate doposits.
Bven if gaseous hydrocarbon can be extracted fiom flie hydrate by one of the above processes using heat, solution m depressurisation, further problems can arise m tmig^i^^WbmM^m'bmkU^^^m^ixn&^BmmwdaBagiB. The difficulty is that the Uberatcd gas wm be wet, i.e. it wiU carry with it a proportion of water and water vapour from the hydrate formation, and, at sub-sea temperatures and pressures, die water vapour is likely to freeze, thereby re-forming gas hydrates, which can block the pipeline.
maccowlancemththepresejrtfoveolie^ recovering hydrocarbon trapped in a hydrate feonation, comprising the steps of:
(a) cxmtacting the hydrate formation wita an aqueous solu^
to 75% by weight of a formate salt m an acetate salt of an alkali metal, or a mixture of two ©r more thereof, whereby nil aqueous soistion liberates hydrocarbon from the
(b) %imy^iffg ^* hy#ir*ft«rhi»ifaiffrg wpmir tnhrtme and Hie said aqnenns gnhitinn
to a sepM«tor, whereby Him said aqueoos sohition absorbs water vsa|xj« from tt«mixtee during me transportation step, to form a more dilute aqueous solution of the alkali metal salt, thereby inhibiting formation of hydrocarbon hydrates;
(c) separating hydrocarbon from said dilute aqueous solution;
(d) regeaesrating the aqueous sohition ofstep (a) by heanng said dilute aqueous
solution to remove absorbed water vapour, and
(e) recycUng the regenerated aqueous solution to step (a)
For the avoidance of doubt, the phrase "a mixture of two or more thereof means arnixtureoftwoOTnx^ofanyiwm^tesaM^^
following mixtures: a mixture of different alkali metal formate salts; a mixture of different alkali metal acetate salts; and * mixture of formate and acetate salts.
The total amount ofsaltm said aqueous solution is ^m 10% to 75% by weight, p^braMyittafflt4OI4by weight, and most preferably fiam 4©% to 65% by weight
Although formate or acetate salts of any of the flilf*li metals can be used in the meihod of the piescntinven&on^ is economy
acetate salts ofpc
First, the aqueous soltitkms are "my Relive freezing point depressants. This means
that said aqueous solution does not need to be heated in caste to disassociate the hydrate
aqueous solution can therefore be injected through the permafrost at a low enough temperature into the hydrate formation wiliiout nielting the permafrost and thereby risking a gas leak from the well.
Secottdry, tiie aqueous solutions of the particular salts themselves have very tow freezing points. This means that siiidaqoeoussotation can be used at a very low temperstare, again rninimismg i» risk of a gas leak, wimout risking solidification of said aqueous solution in tiie well bore, m addition, the aqueous solutions will not freeze in storage tanks when used ia me extreme cold such as is found in fee Arctic.
Thirdly, the aqueous solutions of the said salts have the additional advantage of having vapour absorbent properties. Wbfin me vapour which is absorbed is water vapour, said aqueous solution can bethought of as aBtingasadehydialingageirt. The iwalw vapour which is mixed wffli the liberated natural gas is therefore absorbed by said aqueous solution, forming a dilute aqueous solution, and thereby drying the natural gas. This has *» ©fleet tel the re-fbrmation of gas hydrates in the pipelines leading back to the rig is suppressed. Additionally, the absorption of water vapour by said aqueous solution is accompanied by an increase in tea|>eHtoce of s»i aqueous sotetion,wMeh itself acts to inhibit formation of gas hydrates.
Lastly, the solutions in question have a very low corrosive tendency and very low environmental toxicity.
Typically, carbon dioxide is present m the liberated natural gas, and this dissolves in said aqueous solution and lowers its pH, thereby increasing its corrosive tendency. TMs problem can be addressed either tiy admixing an alkali (such as potassium or sodium hydroxide, or potassium carbosate), preferably to said aqueous solution before it is recycled, to irMa«ase the pH to somewhere in the range 8 to 11, or alternatively to add a corrosion inhibitor such as sodium silicate, amonohydric alcohol, a polyhydric alcohol, a triazole compound, an alkali metal molybdate, or a mixture of two or more thereof.
Said aqueous solution may fee injected into the hydrate formation through an infftrfqifcg jacket, the jacket having a layer of cold insolation UnM comprising an aqiierais
solution ©f a formate salt of any alkali metal of an acetate salt ©f aay alkali metal, or a mixture of any two or more thereof. This provides an addra'onal means of ensuring that the permafrost does not melt, and means that, if necessary, said aqueous solution can be heated prior to injection into ft® hydrate formation.
Aa electrical submersible $®m§ can be placed at the bottom of the well in order to reduce pressure on me hydrate formation, m to stimulate higher levels of gas production.
The arrangement may be such that said aqueous solution is circulated down tubing suspended k a cased w«B in order to displace said dilute tqieoiis solution from fl» hfiKrte formations, thus reducing me need iar pumping either into or ©at of the weH. The natural fas liberated toot me hydrate may assist in carrying said diute aqueous solution fiom tfee hydrate and into the pipeline, thereby reducing ifc pumping costs.
wlien the mixture of gas and said dilute aqueous solution reaches Ae rig, it is separate^ and water vapour is boUedofffrom said dilute aqueous solution in order to regenerate said aqueous solution from me recycling and reuse.
Steam generated from regenerating said aqueous solution can be used for heating said aqueous solution lor re-injection, in order to mc*ease#»iiatecfdisassociati
In a preferred embodiment, the salt (e.g. potassium formate) solution is pumped down a single well drilled into a natural gas hydrate and the gas and water formed by the disassociation of the hydrate produced from the single wdl through concentric pipe. The hole drilled from the surface may intersect the hydrate formation vertically or be drilled irwlir^orhorizciniallytofbUcwtheir^^
Iaa second prefaced embodmienttwo or more wells may be drilled into a
hyrtrate Hqpoait fmm the surface and c«nm»ctfrins r0H^#t- frftWWB titffiW in tfef hyfeltP
formation The connecting holes nmy be drilled
mis manner Ae salt solution may be pumped down caie well and gas and water recovered from fta§ omer(s). H» cormecdng MB mfoaghme hydrate formation might also require ti^ support of a slotted pip© or screen if me surrounding formation becomes unconsolidated following me dissociation of the hydrate.
In a further embodiment of both of the methods above under certain favourable geomechanical conditions a horizontally orientated hydrauUcrrac*urermgjht be made in order to eo^end the contact area to the fracturing fluid within the hydrate formation and thereby facilitate the liberation oftrappedhydrcxarbori,wherem the fracturing fluid comprises an aqueous solution of a formate salt of am alkali metal or an acetate salt of any alkaUns^,OT a mixture c^tvro or more mereof. Pi^erably, said fracturing fluid Ins flie same compositkjn as said aqueous solution.
Fracturing it a yptt known mcftiod for acossyfag JHIMBHIPBIHI fbrm^nns ntr^ ^"r stimulating tteproduc^onofhydrocwbons from such fornianon^^ Fracturing fluid is pumped £tom me surface and down flic well drilled Into the formation at a sufficiently high pressure to ovweome the natural confining pressures in the formation and to induce a fracture in the formation. The openuig created wiU remain open as long as sufficient fluid is injected at a high enough pressure to overcome tfse effect of fluid seepage from the walls of the induced fracture.
The skilled addressee wHI be able to select suitable additives for mefiwshaing fluid from the common general knowledge in me art
The advantage of using the claimed aqueous solutions as fracturing fluids are mat they remain fa solution in the induced fxacsires at very low temperatures, because they hem much tewrar fteeafag pmit than other known aqueous fracturing fluids.
The method is omyrwssibte however wim^
vAereiMuc If favourable elasto-plastic gootQechanieal conditions exist in the hydrate formation, far instance ifbeneathalarge salt deposit, flow between two wells may be induced by pressurising la fk« injector "wal wWi potassium fonnatewhtte drawing down pressure in the producing well.
ft 'm iiaportaat to take
Tie decompo^tion oi gas hydrate increases the level of gas production from underlying gas reservoirs, Deptesamisstion of tiie hydrates will enhance production as decomposition progresses.
A number of preferred eatnbodunents of tibe invention will now be described, wim reference to fee drawings, inwhich:-
Figure I shows a schematic diagram of a method in accordance wiua the invention, and
Figure 2 is a schematic diagram of an alternative embodiment of the method.
and permafi^st 2 mto hydrate formation 3 overlying natural gas re^ Well bore 8 has a casing 7 having an annulus sealed w$h cement to pteveM fte escape of gas from well bore 8.
In use, mje
hydrate formation 3» thereby cmi|t dissolution zone 5 comprising natural gas, water vapour, and potassftzmfoi&iflteSQtBiloiL These conipoiieirtsaic primped up tubing production strmg 6 by ela^ The concentrated potassium formate solution absorbs water vapour in the pipeline 28, thereby inhibiting the formation of gas hydrates.
The mixture of natural gas and dilute formate solution is separated by separator 9, and most ofme gas is pqjed for shipment or storage along pipeline 10.
The dilute formate solution is piped along pipeline 11 to boiler 12, which is powered by a proportion of the recovered gas. The dilute formate solution is boiled in boUer 12 td regeaeiate conceiiU'ated fornwte solution and water. The water is stored in tanks 17, and tberancentrated formate s^^ tempenifeire) is piped to a heat exchange preheat the iqe
Referring to Figure 2, two we! bores 8a and 8b are drilled vertically through impermeable cap rook 1 and permafrost 2 into hydrate foi»ation 3 ovei^a^ natetl gts reservoir 4. WeU bores 8a and 8b are have respective casings 7a and 7b, each of which has an armuras sealed wSk cement to prevent tie escape of gas from well bores 8ft and 8b respectively. Other parts of the arrangement of Figure 2 operate in the same mariner as the arrangement of Figure 1.
Drilling is continued in an inclined or horizontal orientation in the direction ftom one w«l to fte ©the*; intersecting the hydrate formation. The purpose of this isSo be able to pwiiie fluid circulation d©wn an injector well aodfawfaietkHiuptttesecoiMi interconnected well.
In ofctet to maintain the integrity of tb& horizontal hole it may be necessary to insert a slotted liner or screen assembly throughout its length. Reservoir modelling can be aaaployed to determine the corafefoftd goptttnohanioai and lim'iuodynaniic forces cat thehwdzoo^opeiaagtodctenainB^iedMrorm partiGiilsr application.
Additional multilateral well openings may be drilled from the vertical well to additonal surrounding vertical wdte to increase the area penetrated within the hydrate formation.
It wiU be understood by one of skiUm me art that many modifications of the arrangements specifically described are possible, within tte scope of the appended claims.
1. A method for recovering hydrocarbon trapped in a hydrate formation, comprising
the steps of
(a) contacting the hydrate formation with an aqueous solution comprising from 10% to 75% by weight of a formate salt or an acetate salt of an alkali metal, or a mixture of two or more thereof, whereby said aqueous solution liberates hydrocarbon from the hydrate formation, producing a mixture of hydrocarbon and water vapour;
(b) transporting the hydrocarbon/water vapour mixture and the said aqueous
solution to a separator, whereby the said aqueous solution absorbs water vapour
from the mixture during the transportation step, to form a more dilute aqueous
solution of the alkali metal salt, thereby inhibiting formation of hydrocarbon
(c) separating hydrocarbon from said dilute aqueous solution;
(d) regenerating the aqueous solution of step (a) by heating said dilute aqueous solution to remove absorbed water vapour; and
(e) recycling the regenerated aqueous solution to step (a).
2. A method as claimed in Claim 1, including the step of subjecting the hydrate formation to hydraulic fracturing with a fracturing fluid in order to depressurise the hydrate formation and thereby facilitate the liberation of trapped hydrocarbon, wherein the fracturing fluid comprises an aqueous solution of from 10% to 75% by weight of a formate or acetate of an alkali metal, or a mixture of two or more thereof.
3. A method as claimed in Claim 2, wherein said fracturing fluid has the same composition as said aqueous solution of Claim 1.
4. A method as claimed in any one of the preceding claims, wherein said aqueous solution comprises from 40% to 75% by weight of said salt.
5. A method as claimed in any one of the preceding claims, wherein said aqueous solution comprises from 40% to 65%) by weight of said salt.
6. A method as claimed in any one of the preceding claims, wherein said alkali metal is potassium, sodium, rubidium or caesium.
7. A method as claimed in any one of the preceding claims, wherein said salt is potassium formate.
8. A method as claimed in any one of the preceding claims, wherein said aqueous solution is heated prior to contact with the hydrate formation.
9. A method as claimed in any one of the preceding claims, wherein said aqueous solution is injected into the hydrate formation through an insulating jacket, the jacket having a layer of insulation fluid comprising an aqueous solution of from 10% to 75% by weight of a formate salt of any alkali metal or an acetate salt of any alkali metal, or a mixture of any two or more thereof.
10. A method as claimed in any one of the preceding claims, wherein a single well is drilled into a natural gas hydrate deposit, the well having concentric pipes providing at least two passageways, wherein the said salt is pumped down at least one of the said passageways, and gas and water formed by the disassociation of the hydrate are recovered from at least a second of the said passageways.
11. A method as claimed in claim 10, wherein the well is drilled so as to intersect the hydrate deposit vertically or so as to follow the inclination of the hydrate deposit.
12. A method as claimed in any one claims 1 to 9, wherein two or more wells are drilled into a hydrate deposit from the surface and connected by connecting passageways between the said wells in the hydrate formation, and wherein the said salt is pumped down at least one said well, and gas and water formed by the
disassociation of the hydrate are recovered from at least a second said well via the said connecting passageways.
13. A method as claimed in claim 12, wherein the connecting passageways are
inclined, horizontal or multilateral.
14. A method as claimed in claim 12 or claim 13, the connecting passageways are
provided with supports.
15. A method as claimed in any one of claims 12 to 14, wherein pressure is applied to
the said at least one said well, and wherein reduced pressure is applied to the said
at least one second said well.
16. A method for recovering hydrocarbons trapped in a hydrate formation
substantially as herein described with reference to the accompanying drawings.
|Indian Patent Application Number||726/DELNP/2004|
|PG Journal Number||43/2010|
|Date of Filing||22-Mar-2004|
|Name of Patentee||ATKINSON STEPHEN|
|Applicant Address||Zanderijpad 24, NL-2242 GW Wassenaar|
|PCT International Classification Number||E21B 43/26|
|PCT International Application Number||PCT/GB02/04382|
|PCT International Filing date||2002-09-27|