GB2400109A

GB2400109A - Drilling fluid and methods of production and use thereof

Info

Publication number: GB2400109A
Application number: GB0404099A
Authority: GB
Inventors: Armen Abazajian; Robert Freerks
Original assignee: Syntroleum Corp
Current assignee: Syntroleum Corp
Priority date: 2003-02-24
Filing date: 2004-02-24
Publication date: 2004-10-06
Anticipated expiration: 2024-02-24
Also published as: GB0404099D0; BRPI0400580A; HK1074056A1; GB2400109B; US20040198618A1

Abstract

A drilling fluid including at least about 5wt% olefin, at least about 5wt% paraffins, including n-paraffins and isoparaffins, wherein the isoparaffins are substantially wholly terminal monomethyl branched is provided. A method for producing the drilling fluid, and a method for drilling a borehole using the drilling fluid are disclosed.

Description

DRILLING FLUID AND METHODS OF PRODUCTION AND USE THEREOF

- CROSS REFERENCE TO RELATED APPLICATIONS

11] Not applicable.

FEDERALLY SPONSORED RESEARCH

[2] Not applicable.

REFERENCE TO MICROFICHE APPENDIX

131 Not applicable.

FIELD OF THE INVENTION

141 The present invention relates to drilling fluids derived from the products of a Fischer s Tropsch synthesis as described. More particularly, the invention relates to base fluids, and drilling fluids produced therefrom, having high stability, lubricity, and biodegradability while also having low toxicity. The invention further relates to processes of producing and using the drilling fluids.

BACKGROUND OF TO INVENTION

[51 Drilling fluids, or drilling muds, are well known for use in oil and gas well drilling operations. In early applications, crude, diesel and mineral oils were used as base fluids in formulating drilling fluids. Such oil based drilling fluids, however, have unacceptable toxicity 2s and persistence, or non-biodegradability. Toxicity and persistence issues are especially critical in offshore drilling operations. Pseudo- oil based drilling fluids are also well known wherein the base fluids are fatty acid esters or synthetic hydrocarbons. Such synthetic hydrocarbons have included polyalphaolefins, linear alphaolefins, internal olefins, and mixtures thereof as well as linear and branched paraffins and mixtures thereof. Synthetic olefinic and parabolic so base fluids produced by oligomerizing low carbon number hydrocarbons are well known but are expensive to produce. Similarly, ester base fluids are expensive to produce. I [6l The cost of drilling fluids may be reduced by use of Fisher-Tropsch synthesis to produce parabolic base fluids, such as the drilling fluids described in U.S. 6,096,690 and U.S. 6,110,874. Paraffnic drilling fluids, however, generally do not have adequate low temperature properties for use in offshore and cold weather drilling operations.

[7] The drilling fluid disclosed in U.S. 6,096,690 approaches the low temperature issue by incorporating multi-methyl branched isoparaffins or isoparaffns with branches of higher carbon number than methyl into the paraffinic base fluid. More specifically, the base fluid disclosed in the '690 patent contains Co-C24 n-paraffms and isoparaffms, with an iso to normal ratio ranging from 0.5:1 to 9:1. Greater than 50% of the total isoparaffin content are mono 0 methyl branched isoparaffns and about 30% are multi-methyl substituted. A similar approach to the low temperature issue is disclosed in U.S. 6, 110,874 which discloses a drilling fluid containing a mixture of n- paraffns and isoparaffms wherein at least 90% of the isoparaffms are mono- or poly-methyl branched isomers. Although the use of Fisher Tropsch synthesis somewhat lowers the cost of such parabolic drilling fluids, the necessity of hydrocracking and hydroisomerization adds to the overall cost of the drilling fluid. Moreover, to sufficiently improve the low temperature properties of the paraffinic base fluids, pour point depressant additives may be required, further increasing the cost of the drilling fluid.

l8] WO 01/02325 discloses a drilling fluid which uses a base fluid of a mixture of linear and branched alphaolefins with the linear to branch ratio ranging from 1:1 to 5:1. This reference also discloses production of the olefins from high temperature Fischer-Tropsch synthesis. High temperature Fischer-Tropsch synthesis, however, results in primarily internal branching of the branched hydrocarbons. Thus, such drilling fluids are not wholly biodegradable resulting in increased persistence. Moreover, because of the reactivity of olefins, drilling fluids composed of olefins are easily oxidizable and, therefore, must be stored under a nitrogen blanket to provide a reasonable shelf life. Even with nitrogen padding, olefinic drilling fluids cannot be stored for periods as long as parabolic drilling fluids.

Therefore, the cost of using olefinic drilling fluids is increased by both its special storage requirements and loss of oxidized product.

[9l Therefore, there remains a need for a Filcher improved lower cost drilling fluid having oxidative stability, high lubricity, low toxicity, improved low temperature properties, and high biodegradability.

SUMMARY OF THE rNVENTION

[10] The invention which addresses these needs and others provides a base fluid, drilling fluid made therefrom, a method for producing such drilling fluid and a process of using the drilling fluid.

BRIEF DESCRIPTION OF TO DRAWINGS

11] None

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[12] The preferred drilling fluid contains a base fluid having from about s to about 90wt% linear alpha- and internal olefins, from about 5 to about 20wt% isoparaffns, from about 5 to about 90wt% n-paraffns and from about 0 to about 10wt% oxygenates. To fully realize the lower cost potential of the invention, the base fluid for the drilling fluid may be obtained from a Fischer-Tropsch synthesis using synthesis gas as a feed stock. Moreover, production of the Is drilling fluid from a base fluid produced by the Fischer-Tropsch synthesis and subsequent processing as described herein is especially desirable as it results in a product having the desirable olefin and paraffm contents. However, the base fluid composition may be otherwise produced while yet retaining the remaining advantages of the invention.

[13] Three basic techniques may be employed for producing a synthesis gas, or syngas, which is used as the starting material of a Fischer-Tropsch reaction. These include oxidation, reforming and autotherrnal reforming. As an example, a Fischer-Tropsch conversion system for converting hydrocarbon gases to liquid or solid hydrocarbon products using autothermal reforming includes a synthesis gas unit, which includes a synthesis gas reactor in the form of an autothermal refonning reactor (ATR) containing a reforming catalyst, such as a nickel containing catalyst. A stream of light hydrocarbons to be converted, which may include natural gas, is introduced into the reactor along with oxygen (O2) . The oxygen may be provided from compressed air or other compressed oxygen-containing gas, or may be a pure oxygen stream.

The ATR reaction may be adiabatic, with no heat being added or removed from the reactor other than from the feeds and the heat of reaction. The reaction is carried out under sub stoichiometric conditions whereby the oxygen/steam/gas mixture is converted to syngas.

Examples of Fischer Tropsch systems are described in U.S. Patent Nos. 4, 973,453; 5,733,941; 5,861,441; 6,130,259, 6,169,120 and 6,172,124, the disclosures of which Ire herein incorporated by reference.

[14] The Fischer-Tropsch reaction for converting syugas, which is composed,:rimarily of carbon monoxide (CO) add hydrogen gas (H2) may be characterized by the following general reaction: 2nH: + nCO (CH:2)n + 1]H20 (1) [lS1 Non-reactive components, such as nitrogen, may also be included or mixed with the syugas. This may occur in those instances where air or some other non-pure OXy8! en source is lo used during the syugas formation.

[163 The syagas is delivered to a synthesis Unlit, which includes a Fischer-Troptch reactor (FTR) containing a Fischer-Tropsch catalyst. Numerous Fischer-Tropsch catalysts may be used in caring out the reachorL Fess include cobalt, iron, ruthenium as well as over (croup VIIIB transition metals or combinations of such metals, to prepare both saturated and Is unsaturated hydrocarbons. For purposes of this invention, a non-iron catalyst mB,r be used. The F-T catalyst may include a support such as a metaloxide support, including silica, aluniina, silica-alumLna or titanium oxides. For the purposes of this reaction, a Co catalyst on transition alumina with a surface area of approximately 100-200 m2/g is used in the form of spheres of 50-150 1lm in diameter. The Co concentration on the support may also be 15-30%. Certain So catalyst promoters and stabilizers may be used. The stabilizers include Group [IA or (croup Im3 metals, while the promoters may include elements Dom Group VIII or Group Vim. The Fscher-Tropsch catalyst and reaction conditions may be selected to be optional for desired reaction products, such as for hydrocarbons of certain chain lengths or number of carbon atoms.

Any of the follonog reactor configurations may be employed for FischerTrot: sch synthesis: fixed bed, slurry reactaó, ebullating bed, fluidizing bed, or continuously stirred tanlc reactor (CSIX). Fofthe purposes of this reaction, a slurry bed reactor is used. The FTR may be operated at a pressure of 689 kPa to 3.45 mPa (100 to 500 psia) and a temperature of 190 C (375 F) to 500 C. The reactor gas hourly space velocity ("GHS\/') may be from 1000 to 8000 hr'. Syngas useful in producing a Fischer-Tropsch product useful in the invention may contain gaseous hydrocarbons, hydrogen, carbon monoxide and nitrogen with Hz/CO ratios from about 1.8 to about 2.4. l

The hydrocarbon products derived Tom the Fischr-Tropsch reaction may range Dom methane (Car) to high molecular weight paraffinic waxes containing more than 100 carbon atoms. An overhead product stream is recovered Dom the FTR on a once-through basis and may be separated into tail gas and light Fischer-Trolsch liquid ("Lulls") products in a cold separator. The LFTL stream may then be distilled to yield a hydrocarbon product of primarily C'3-C22 olefins and paraffins.

[171 The C3-C2: distillation cut may be used directly as the base fluid. Low le rels of Ct3.

hydrocarbons may be present in the C3-C2: cut provided such levels do not cause the dulling fluid to exceed current regulatory limits. C3 hydrocarbons may be present in amounts 0 between about 0.01 and 1Owt%. The presence of such small amounts of Ct3 hydrocarbons permits incorporation of a wider distribution of Cx3' hydrocarbons, particularly Coca hydrocarbons, for improved lubricity. The hydrocarbon number distribution is such as to result in a drilling fluid pour point below about 10 C. 1181 The base fluid contains Tom about 5 to about 90wt% linear alpha- and incenal olefins.

The olefin content may provide the mixture With lower pour-point, better surface activity, better lubricity and better adherence to metal. When the base fluid is produced Dom the Fischer-Tropsch synthesis with the appropriate Fischer-Tropsch catalyst and operating conditions' the Fischer-Tropsch product will have approximately 5% alpha and eternal olefin content. Depending upon the reaction conditions of the FTR and catalyst used In the Fischer Tropach reaction, it my be necessary to concentrate the olefin content to acid:ve the higher percentages of olefins in the base fluid. Concentration of olefins may be undertaken, for example, by one or more ofthe following known techniques (1) molecular siev separation of olefins and paragons, such as UOP's olex process, and (2) distillation of paraffins away from individual C# cuts.

[19] The base fluid may contain behveen about 5 to about 95wt% paraffin;. Of the total paraffin content from about 3 to about 20wt% are isoparaffins. Substantially all of the isoparaffns are terminal monomethyl species. For the purposes of this invent) ant the terminal species are 2and 3 methyl branched. The presence of monomethyl isoparafEnis improves low [11 temperature properties, such as pour point, as well as lutncib and viscosit,'. Moreover, because the isoparaffins are predominately textually branched, the paraffin conte at of Me base fluid is substantially wholly biodegradable. Using the Fischer-Tropsch synthesis descner herem, about Swt% temn1 methyl branched paraffins are produced the, LENTIL.

121 Concentration of isoparaDins may be increased by one or more of t he follow): ig techniques.

(1) molecular sieve separadon of linear and branched paraffins, such as UOP's Rolex process, and (2) isonaenc distillation of isoparaflin as described in co-pending comnonly owned U.S. Application entitled "Hydrocarbon Products and Methods of Prepping Hydrocarbon Products By Skeletal Isomenzation of Olefin/Paraffin M]xbres" listing A:men Abazajim Is inventor.

[31 The oxygenates are principally primary alcohols. Aldebydes, letories, carbo;ylic acids and esters and all-esters of carboxylic acids are present 1 amounts. Oxygenate. content the Cal - C22 cut of He Fischer- Xropsch reaction product ranges from between abo at 0.5 to about 5.0w1:%. Low levels of oxygenates the drilling fluid Dom between about 0 ar about 10vvt% Is provides improved lubricity. Moreover, oxygenates may assist in emulsification in invert drilling muds.

141 Oxygenate control may be used on He C3-C22 cut of the Pischer-Tropsch product stream.

:Ea orate embodiment of the mrention, a base fluid is produced by vapounz product stream and parsing He vaporized product over an activated adumina catalysts to dehydrate alcohols to corresponding olefins. Ibe covemon of He alcohol content of the product steam occurs according to the followgreacdo: . , CH3-(CH2)X-CH:-CH2OH (H3-(CHz)x-CH=CF+HzO (:) [5] For example, the LFTL, C,3-C22 or other product stream may be vaporized at a temperature from about 204 C (400 F) to about 427 C (800 F) and then passed over one or more 2s dehydration beds containing activated treated alumina or silica- alumina. In some embodiments, the dehydration beds are packed beds. Essentially all of the primary and internal alcohols present in the vaporized stream are dehydrated to their corresponding olefins, with conversion rates of at least 95%.

[6] Dehydration reaction temperature may range from between about 204 C and 427 C (400 and 800 F). The vaporized feed for the dehydration unit may be superheated prior to being fed into the dehydration beds or alternatively, may be heated within packed beds. The LHSV of packed beds may range from about 0.10 hr' to about 2.0 hr'. Reaction pressure may be maintained by the pressure of the accumulator and must be such to vaporize all of the dehydration feed.

Typically, the pressure may range from between about O kPa (0 psia) to about 689 kPa (100 psig).

(71 IN an altemative embodiment, a moving bed of mama or silica-alna catalyst may be used. Fluidized beds, slurry beds or ebullating beds may be used with Continuous batch or se=-batch catalyst removal and regeneratior The catalyst may be removed b one of these methods and regenerated by passing a mixture of nitrogen and oxygen or far at elevated temperatures over the catalyst.

[81 Depending upon the alumina used, some of the oldies present or produced the 1 dehydration beds may also be isomenzed to intem olefins. Alna catalyst; useful for the dehydration of alcohols are known and include, for example, gamma-alumina, theta-al,mina, pacified alders, and activated alumina. High surface area aluas are particularly used in the invention and include those aluminas having a surface area of about 100 m7/gm or greater.

Commercially available alumna useful in the integrated Fischer-Tropsch pros ess include, for example, S-400, which has a surface area of about 335 m21gm, and D:C>-4;O, which has a surface area of about 375 m2/gm. S400 ad DD70 are alumina catalysts made and shld by Alcoa. Alumina catalysts for use in. the integrated Pischer-Tropsch process gel Orally cotton at least about 90wt% Allot, oxides of silicon and iron present in amounts of less than about 0.1wt%, arid oxides of sodium present in an amount of less than about 1 ONTO. The alumina catalysts are generally supplied as substantially spbencal particles hang dia; aster Tom about 1/8 to about l/4 inch.

[9] :En another embodiment of the invention, molecular sieve or zeolitic molecular sieve forms Of the aiming or siIica-alumina catalysts may be used For example, silico a undo phosphate ("SAPO") molecular sieves may be used in the packed beds. SAPO molecular sieves contain a 2s 3-dimensional microporous crystal suture having 8, 10, or I2 membered nag structures. Ibe rug structures can slave an average pore size ranging Tom between about 3,5 angstroms to about 15 angstroms. Other silica- containing zeolitic molecular sieve catalysts, such as Z$M-5, may be used in the dehydration bed.

p0] Following dehydration, the aqueous and organic phases may be separate! Sum dehydration process may further be used to increase the olefin content of product stream to be used to produce the base fluid.

(11] Other methods of oxygenate control include, for example, reaction of the alcohol content of the Ci3-C22 cut of a Fischer-Tropsch product stream with maleic or suCCiDiC anhydide or win a carboxylic acid, such as formic acid, acetic acid, or odor acids. The carboxylic acid esters may be retained the stream as they ale excellent lubricants which are also highly biodegradable. :130th the lubricity and the biodegradability of the bare fluid may be improved by converting at least a portion of the alcoholic oxygenate content to arboxylic acid esters.

[121 The drilling fluid may optionally include one or more surfactants (e g., emulsifiers, wetting agents), viscosifiers, weighting agents, fluid loss control agents, and proppants.

lo Because the drilling fluid should be non-toxic' these optional ingredients, like, the base fluid, are preferably also non-toric. Acceptable emulsifiers include, but are not Limited to, fate acids, and fate acid derivatives including amido-amnes, polyamides, polrammes, esters, midaxiolines, and alcohols. Typical wetting agents Include, but are not limited to, lecithin fatty acids, crude trill oil, oxidized crude tall oaf, organic phosphate sters, modified imidazolines, modified amidoamines, allcyl aromatic sulfates, allyl aromatic: sulfonates, and organic esters of polyhyic alcohols. Exenplarf weighting agents include, bitt are not limited to, bante, iron oxide, gelana, sideTite, calcium oxide, arid calcium carbonate. Acceptable proppants include sand, gravel, and nut shells. Exemplary viscofieTs inc. Ode, but are riot limited to, organophilic clays, non-organop}ilic clays, oil soluble polymers, polyamide resins, and polycarboxylic acids and soaps. Where additives are used in the dills: fig fluid, the base fluid constitutes from about 25 to about 85 volume percent of the total drilling fluid. Dlusative fluid loss control agents include, but are not limited to, asphaltics (e.g. asphaltenes and sulfonated asphaltenes), modified ligates, and polymers, such an polystyrene, polybutadiene, polyethylene, polypropylene, polybutylene, polyisoprer.e, natural rubber, and Dactyl rubber.

Is L13l The following examples illustrate, but are not intended to limit, Me invention.

Example 1

[14] A pilot installation consisting of two distillation colas was Deal to produce C6 lo naphtha, Cog:3 light kerosene, and Cal 20+ drilling fluid feedstock steams. Idle columns were fed approximately 3400 g/hr of light Fischer Tropsch liquid (:LFTL). I.le LFTL feed had approximately tl1e composition shown in Table l:

TABLE I

Carbon # 'b by wt.

_4 <0.1 0.01 6_ 0-3_ 7 1.0 _ 8 5.9 8.1 11 9.2 12 _ 9.5 13 g.2 14 _ 8.4 7. 9 16 7.1 17 6.2 18 5.4 19_ 4.6 3.7 21 3.0 22 2.3 23 _ 1.7 24 1.2 25+ 2.6 Total 1 00.00 [15] The LFTL feed was introduced into a first packed distillation column and C'3 and lighter materials were distilled overhead. The first distillation column was operated at the following conditions: 68.9 kPa (1 Opsig), 249 C (480 F) feed preheat temperature 208 C (407 F) overhead temperature, 306 C (582 F) bottoms temperature. The first distillation column had approximately 2.5 m (98 inches) of Sulzer Mellapack 750Y packing. The bottoms of the first distillation column constituted a C'3- C20+ hydrocarbon fraction, the composition of which in shown in Table 2. The overhead stream of the first distillation column was fed into a second packed distillation column operated at 82.7 kPa (12 psig), 187 C (370 F) overhead temperature and 225 C (437 F) bottoms temperature. The second distillation column had about 0.71 m (28 inches) of Sulzer EX packing. The bottoms of the second column constituted a C10 3 light koresene stream.

TABLE 2

Total no awns, isoparaffns, olcfins and alcohols ^ 11-: Masso/o 0.97 12: Mass% 1.77 13: Mass% 11.43 14: Mass% 13.68 15: Mass /O 12.35 216: Mass /e 10.96 C17 Mass /O 9.06 ClS: Mass% 7 84 Cl9: Mass% 6.79 C20: Mass% 7.04 C21: Masso/o 5.66 C22: Mass% 4.63 C23+: . mass% 7.83 100. 00

Example 2

[16] The C,3 20 and stream from Example 1 was fed via a syringe pump and mixed with cc/min of nitrogen. The gas/liquid mixture was introduced upflow into a vessel packed with stainless steel mesh saddles, where the liquid was vaporized and superheated to reaction temperature of 357 C (675 F). The vaporized feed as fed upflow into a reactor packed with 1/8 alcoa S-400 alumina catalyst and suspended in a heated sandbath. The sandbath was maintained at the reaction temperature and ebulated by air. Reactor LHSV was maintained at about 0.26 hr-. The outlet pressure was maintained at about 34.5 kPa (5 psig). The reaction product composition is shown in Table 3.

TABLE 4

iample Reference Number | FEED B | PRODUCT D

TOTAL

4-PARAFFIN mass % 2.46 82.87 KLPHA OLEFIN mass % 2.26 3.48 INTERNAL OLEFIN mass 96 2.75 3.68 BRANCHED PARAFFIN mass % 10.10 9.97 (LCOHOL mass % 2.45 0.00 30. 100.00 100.00 1171 An LFTL feed, substantially having the composition shown in Table 1, was hydrotreated at reactor conditions of 55.2 MPa (800 psig) and 280 C to 31 0 C (550 to 590 F). The resulting hydrotreated stream was distilled under conditions as described in Example 1 forming a hydrotreated analog of the C'o C'3 light kerosene product. This hydrotreated light kerosene material was analyzed on a Hewlett Packard Series II gas chromatograph with 60 m RTX 1 column with 0.32 mm diameter and 3 micron film thickness. The isomer content of this product is shown in Table 4.

TABLE 5 l

Component Wt. % nC9 - 0.02 2- and 3-monomethyl C10 0.20 4- and higher monomethyl C10 0.03 nC10. 22.22 2- and 3-monomethyl Cll 1.19 i- and higher - monomethyl Cl 1 0.42 nC11 27. 93 2- and 3-monomethyl C12 1.09 4and higher - monornethyl C12 O.SO nC12 24.96 2- and 3-monomethyl C13 0.92

_

4- and higher- monomethyl C13 0.48 nC13 18.99 2- and 3-monomethyl C14 0. 11 4- and higher - monomethyl Cli 0.13 nC14 0.41 rotal normal hydrocarbon 94. S4 [total 2- and 3- monomethyl 3.51 rotal 4- and higher hydrocarbons 1.55 Total monomethyl hydrocarbons 99.61.

Others 0.39 [181 While the foregoing describes preferred embodiments of the invention, it is apparent that a number of changes and variations are within the scope of the invention.

Claims

CLAIMS: 1. A base fluid comprising: at least about 5wt% olefins; at least

about 5wt% e-paraffins; and between about
2 and 50wt% branched paraffins wherein substantially all of the branch groups are monomethyl and wherein the ratio of terminal monomethyl branching to internal monomethyl branching is at least about 1:1.5.

to 2. The base fluid of claim 1 wherein the ratio of terminal monomethyl branching to internal monomethyl branching is at least about 1:1.
3. The base fluid of claim 1 wherein the e-paraffins are present in an amount of at least about 20wt% and wherein the ratio of terminal monomethyl branching to internal monomethyl branching is at least about 1. 5:1.
4. The base fluid of claim 1 wherein the e-paraffins are present in an amount of at least about 40wt% and wherein the ratio of terminal monomethyl branching to internal monomethyl is at least about 2:1.
5. The base fluid of any preceding claim wherein the base fluid is a product of a Fischer-Tropsch reaction.
6. The base fluid of claim 5 wherein the Fischer-Tropsch reaction incorporates feed syngas having 10-60% N2.
7. A drilling fluid comprising: the base fluid of any preceding claim.

so
8. The drilling fluid of claim 7 further comprising: at least one additive selected from the group of surfactants, viscosifiers, weighting agents, fluid loss control agents and proppants.
9. A drilling fluid comprising a base fluid comprising: s from about 2 to about 90wt% olefins; from about 2 to about 50wt% isoparaffins; wherein the isoparaffins are substantially terminal monomethyl branched; from about 5 to about 90wt% e-paraffins; and from about O to about 1 Owt% oxygenates.
10. The drilling fluid of claim 9 wherein the olefins are present in an amount of from about 7 to about 10wt%.
11. The drilling fluid of claim 9 or claim 10 wherein the isoparaffins are present in an amount of from about 3 to about 15wt%.
12. The drilling fluid of any of claims 9 to 11, wherein the e-paraffins are present in an amount of from about 65 to about 90wt%.
13. The drilling fluid of any of claims 9 to 12, wherein the oxygenates are present in an amount of from about O to about 5wt%.
14. The drilling fluid of any of claims 9 to 13, wherein the base fluid is a product of Fischer-Tropsch reaction on a synthesis gas.
15. The drilling fluid of claim 14 wherein the Fischer-Tropsch reaction incorporates feed syngas having 10-60% N2.
16. The drilling fluid of claim 14 or claim 15 wherein the synthesis gas is so produced by autothermal reformation.
17. The drilling fluid of claim 16 wherein the autothermal reformation occurs in the presence of air.
18. The drilling fluid of claim 16 wherein the autothermal reformation occurs in s the presence of 10-60% N2.
19. The drilling fluid of any of claims 9 to 18 further comprising: at least one additive selected from the group of surfactants, viscosifiers, weighting agents, fluid loss control agents and proppants.
20. A drilling fluid comprising a base fluid comprising: from about 7 to about 10wi% olefins; from about 2 to about 15wt% isoparaffins; wherein the isoparaffins are substantially terminal monomethyl branched; from about 65 to about 90wt% e-paraffins; and from about O to about 5wt% oxygenates.
21. The drilling fluid of claim 21 wherein the drilling fluid is a product of a Fsicher-Tropsch reaction.
22. The drilling fluid of claim 21 further comprising: at least one additive selected from the group of surfactants, viscosifiers, weighting agents, fluid loss control agents and proppants.
23. The drilling fluid of claim 21 or claim 22 wherein the FischerTropsch reaction incorporates feed syngas having 10-60% N2.
24. The drilling fluid of claim 23 wherein the feed syngas is produced by autothermal reformation in the presence of air.
25. The drilling fluid of any of claims 7 to 24 wherein the base fluid comprises from about 25 to about 85 volume % of the drilling fluid.
26. The drilling fluid of claim 25 wherein the base fluid comprises from about 25 s to about 85 volume % of the drilling fluid.
27. A process for producing a drilling fluid comprising the steps of: (a) producing a light Fischer-Tropsch liquid; (b) distilling the light Fischer-Tropsch liquid to obtain a C,3-C20+ product to having C,3-C20+ hydrocarbons and oxygenates; (c) dehydrating all or a part of the alcohols in the C,3-C20+ product by passing the C,3-C20+ product over an activated alumina catalyst to produce a dehydrated product; (d) recovering the dehydrated product; and (e) separating the aqueous and organic phases of the dehydrated product.
28. The process of claim 27 further comprising the step of: (f) adding one or more additive selected from the group of surfactants, viscosifiers, weighting agents, fluid loss control agents and proppants to the organic phase of the dehydrated product.
29. The process of claim 27 or claim 28 further comprising the step of (be) vaporizing the C3-C20+ product before step (c) and after step (b).
30. The process of any of claims 27 to 29 wherein the dehydrated product from step (c) is in the gaseous state and step (d) further includes condensing the dehydrated product.

so
31. The process of claim 30 wherein the heat from condensing the dehydrated product is recycled to at least partially vaporize the C,3-C20+ product in step (be).
32. The process of any of claims 27 to 31 wherein the light FischerTropsch liquid is produced from a feed syngas having 10-60% N2 s
33. The process of any of claims 27 to 31 wherein the feed syngas is produced by autothermal reformation in the presence of air.
34. The process of any of claims 27 to 33 wherein a Cj4-C'8 product is obtained in step (b) and dehydrated in step (c).
35. A method of drilling a borehole in a subterranean formation comprising the to steps of: (a) rotating a drill bit at the bottom of the borehole; (b) introducing a drilling fluid into the borehole wherein the drilling fluid comprises a base fluid comprising: from about 5 to about 90wt% olefins; Is from about 2 to about 50wt% isoparafffins; wherein the isoparaffins are substantially terminal monomethyl branched; from about 5 to about 90wt% e-paraffins; and from about O to about 10wt% oxygenates.

JO
36. The method of claim 35 wherein the drilling fluid comprises: from about 7 to about 1 Owt% olefins; from about 2 to about 1 5wt% isoparaffins; wherein the isoparaffins are substantially terminal monomethyl branched; from about 65 to about 90wt% e-paraffins; and from about O to about 5wt% oxygenates.
37. The method of claim 35 or claim 36 wherein the base fluid is derived from a Fischer-Tropsch reaction.
38. The method of claim 37 wherein the Fischer-Tropsch reaction incorporates feed syngas having 10-60% N2
39. The method of claim 38 wherein the feed syngas is produced by autothermal reformation in the presence of air.
40. A method of producing a base fluid for a drilling fluid, the method being substantially as described hereinabove with reference to any of Examples 1 to 3.

to
41. A base fluid for a drilling fluid, the base fluid having a composition substantially as described hereinabove with reference to Table 4 or Table 5.
42. Use of a drilling fluid as claimed in any of claims 7 to 26 in a method of drilling a borehole in a subterranean formation.