Alkylsulfonic acid compositions

Abstract

Method of minimizing pitting corrosion at temperatures above 130° C., wherein said method comprises: providing an acidic fluid comprising an alkylsulfonic acid; providing at least one corrosion inhibitor compound comprising an organic compound comprising at least two aldehyde functional groups; combining said acidic fluid with said at least one corrosion inhibitor compound; exposing said mixed fluid to a metallic surface at a temperature of at least 130° C.; and allowing said mixed fluid sufficient time of exposure to a metal to accomplish a pre-determined function.

Claims

1. An alkylsulfonic acid composition comprising an alkylsulfonic acid and a corrosion inhibitor composition, said corrosion inhibitor composition comprising: an organic compound comprising at least two aldehyde functional groups; optionally, a propargyl alcohol; an anionic surfactant which is a dicarboxylic surfactant; at least one amphoteric surfactant; and a solvent; wherein the alkylsulfonic acid is present in an amount ranging from 1 wt % to 50 wt % of the sulfonic acid composition in water.

2. The alkylsulfonic acid composition as claimed in claim 1, wherein the alkylsulfonic acid is methanesulfonic acid.

3. The alkylsulfonic acid composition as claimed in claim 1, wherein the at least one amphoteric surfactant is selected from the group consisting of: a sultaine surfactant; a betaine surfactant; and combinations thereof.

4. The alkylsulfonic acid composition as claimed in claim 1, wherein at least one amphoteric surfactant is a sultaine surfactant and betaine surfactant and are selected from the group consisting of: an amido betaine surfactant; an amido sultaine surfactant; and combinations thereof.

5. The alkylsulfonic acid composition as claimed in claim 1, wherein the at least one amphoteric surfactant is an amido betaine surfactant and is selected from the group consisting of: an amido betaine comprising a hydrophobic tail from C8 to C16.

6. The alkylsulfonic acid composition as claimed in claim 1, wherein the at least one amphoteric surfactant is cocamidobetaine.

7. The alkylsulfonic acid composition as claimed in claim 1, further comprising an anionic surfactant which is a carboxylic surfactant.

8. The alkylsulfonic acid composition as claimed in claim 1, wherein said anionic surfactant is a dicarboxylic surfactant comprising a hydrophobic tail ranging from C8 to C16.

9. The alkylsulfonic acid composition as claimed in claim 1, wherein said anionic surfactant is an iminodicarboxylate including sodium lauriminodipropionate.

10. The alkylsulfonic acid composition as claimed in claim 1, wherein the surfactant is selected from the group consisting of: cocamidopropyl betaine; β-Alanine, N-(2-carboxyethyl)-N-dodecyl-, sodium salt (1:1); and a combination thereof.

11. The alkylsulfonic acid composition as claimed in claim 1, wherein the propargyl alcohol is present in an amount ranging from 20% to 55% by volume of the total weight of the corrosion inhibitor composition.

12. The alkylsulfonic acid composition as claimed in claim 1, wherein the surfactant is present in an amount ranging from 2% to 20% by volume of the total weight of the corrosion inhibitor composition.

13. The alkylsulfonic acid composition as claimed in claim 1, wherein the solvent is present in an amount ranging from 10% to 45% by volume of the total weight of the corrosion inhibitor composition.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) The invention may be more completely understood in consideration of the following description of various embodiments of the invention in connection with the accompanying figures, in which:

(3) FIG. 1 is a schematic depiction of the various type of damage generated by pitting corrosion;

(4) FIG. 2 contains a picture of the surface of 6 metal coupons (identified as B900, A745, A929, A744, A933, and B883) after exposure to acidic fluids as described in the description;

(5) FIG. 3 contains a picture of the surface of 6 metal coupons (identified as B889, B890, A829, A827, A910, and A911) after exposure to acidic fluids as described in the description;

(6) FIG. 4 contains a picture of the surface of 6 metal coupons (identified as A743, A934, B882, A953, A954, and A952) after exposure to acidic fluids as described in the description; and

(7) FIG. 5 contains a picture of the surface of 4 metal coupons (identified as A839, C045, A962 and A963) after exposure to acidic fluids as described in the description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) It will be appreciated that numerous specific details have provided for a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Furthermore, this description is not to be considered so that it may limit the scope of the embodiments described herein in any way, but rather as merely describing the implementation of the various embodiments described herein.

(9) The description that follows, and the embodiments described therein, are provided by way of illustration of an example, or examples, of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not limitation, of those principles and of the invention.

(10) According to a preferred embodiment of the present invention, there is provided an acidic composition comprising an alkylsulfonic acid and a corrosion inhibition package, said package comprising:

(11) an organic compound comprising at least two aldehyde functional groups

(12) a propargyl alcohol or derivative thereof;

(13) at least one amphoteric surfactant; and

(14) a solvent.

(15) Preferably, the organic compound comprising at least two aldehyde functional groups is selected from the group consisting of: C.sub.1-C.sub.16 linear hydrocarbon comprising an aldehyde functional group at either extremity of the hydrocarbon. Preferably, the C.sub.1-C.sub.16 linear hydrocarbon comprising an aldehyde functional group at either extremity of the hydrocarbon is a C.sub.5-C.sub.12 linear hydrocarbon. Preferably also, the C.sub.5-C.sub.12 linear hydrocarbon comprising an aldehyde functional group at either extremity of the hydrocarbon is selected from the group consisting of: glutaraldehyde; succinaldehyde; malondialdehyde; adipaldehyde; heptanedial; nonanedial; undecanedial; and dodecanedial.

(16) According to a preferred embodiment, the organic compound comprising at least two aldehyde functional groups is selected from the group consisting of: saturated dialdehyde-containing hydrocarbons and C.sub.5-C.sub.16 branched hydrocarbon dialdehyde.

(17) Preferably, the at least one amphoteric surfactant is selected from the group consisting of: a sultaine surfactant; a betaine surfactant; and combinations thereof. More preferably, the sultaine surfactant and betaine surfactant are selected from the group consisting of: an amido betaine surfactant; an amido sultaine surfactant; and combinations thereof. Yet even more preferably, the amido betaine surfactant and is selected from the group consisting of: an amido betaine comprising a hydrophobic tail from C.sub.8-C.sub.16. Most preferably, the amido betaine comprising a hydrophobic tail from C.sub.8-C.sub.16 is cocamidobetaine.

(18) Preferably also, the corrosion inhibition package further comprises an anionic surfactant. Preferably, the anionic surfactant is a carboxylic surfactant or a sulfonic surfactant. More preferably, the carboxylic surfactant is a dicarboxylic surfactant. Even more preferably, the dicarboxylic surfactant comprises a hydrophobic tail ranging from C.sub.8-C.sub.16. Most preferably, the dicarboxylic surfactant is sodium lauriminodipropionate.

(19) Preferably, the surfactant is selected from the group consisting of: cocamidopropyl betaine; β-Alanine, N-(2-carboxyethyl)-N-dodecyl-, sodium salt (1:1); and a combination thereof.

(20) Preferably, the solvent is selected from the group consisting of: methanol; ethanol; isopropanol; ethylene glycol; Di-n-hexyl-ether; and 2-Butoxyethanol; and combinations thereof.

(21) Preferably, the organic compound comprising at least two aldehyde functional groups is present in an amount ranging from 2% to 25% by weight of the total weight of the corrosion inhibition package. Preferably also, the propargyl alcohol or derivative thereof is present in an amount ranging from 10% to 55% by volume of the total weight of the corrosion inhibition package. Preferably also, the at least one surfactant is present in an amount ranging from 2% to 20% by volume of the total weight of the corrosion inhibition package. Preferably also, the solvent is present in an amount ranging from 10% to 45% by volume of the total weight of the corrosion inhibition package.

(22) According to another aspect of the present invention, there is provided an acidic composition comprising:

(23) an acid;

(24) a corrosion package comprising:

(25) the organic compound comprising at least two aldehyde functional groups

(26) a propargyl alcohol or derivative thereof;

(27) at least one surfactant; and

(28) a solvent;

(29) wherein the volume % of the corrosion package in the acidic composition ranges from 0.1 to 10%. Preferably, the acidic composition further comprises a metal iodide or iodate, Preferably the weight/volume % of the metal iodide or iodate in the acidic composition ranges from 0.1 to 1.5%. More preferably, the wt/vol. % of the metal iodide or iodate in the acidic composition ranges from 0.25 to 1.25%. Even more preferably, the wt/vol. % of the metal iodide or iodate in the acidic composition is approximately 1%. Preferably, the metal iodide or iodate selected from the group consisting of: cuprous iodide; potassium iodide; sodium iodide; lithium iodide and combinations thereof. More preferably, the metal iodide is potassium iodide.

(30) The use of a corrosion inhibitor package with an acidic composition where the acidic composition comprises an acid selected from the group consisting of: an alkylsulfonic acid, said corrosion inhibitor package comprising:

(31) the organic compound comprising at least two aldehyde functional groups

(32) a propargyl alcohol or derivative thereof;

(33) at least one amphoteric surfactant; and

(34) a solvent.

(35) According to another aspect of the present invention, there is provided a use of a alkylsulfonic acid composition comprising a preferred embodiment of the present invention in the oil and gas industry to perform an activity selected from the group consisting of: stimulating formations; assisting in reducing breakdown pressures during downhole pumping operations; treating wellbore filter cake post drilling operations; assisting in freeing stuck pipe; descaling pipelines and/or production wells; increasing injectivity of injection wells; lowering the pH of a fluid; fracturing wells; performing matrix stimulations; conducting annular and bullhead squeezes & soaks; pickling tubing, pipe and/or coiled tubing; increasing effective permeability of formations; reducing or removing wellbore damage; cleaning perforations, nozzles, ports, jets etc.; solubilizing limestone, dolomite, and calcite; and removing undesirable scale from the group consisting of: equipment, cyclical steam wells, steam flood wells, SAGD (steam assisted gravity drainage) wells, unassisted or natural high formation temperature production wells, injection wells and their related surface and down-hole equipment and facilities at high temperatures of up to 180° C. and above.

(36) Alcohols and derivatives thereof, such as alkyne alcohols and derivatives and more preferably 2-Propyn-1-ol complexed with methyloxirane can be used as corrosion inhibitors. Propargyl alcohol itself is traditionally used as a corrosion inhibitor which works extremely well at low concentrations. It is however a very toxic/flammable chemical to handle as a concentrate, so care must be taken while handling the concentrate. In a composition according to the present invention, 2-Propyn-1-ol complexed with methyloxirane is utilized where the toxic effect does not negatively impact the safety of the composition. There are derivatives of propargyl alcohol available in the industry now that are considered safe to handle, non-regulated, and approved for use in North Sea Offshore Oilfield applications under CEFAS (for UK and NL).

(37) Metal iodides or iodates such as potassium iodide, sodium iodide, cuprous iodide and lithium iodide can potentially be used as corrosion inhibitor intensifier. In fact, potassium iodide is a metal iodide traditionally used as corrosion inhibitor intensifier, however it is expensive, but works extremely well. It is non-regulated, friendly to handle, and listed on the offshore PLONOR (Pose Little Or NO Risk) list as safe chemicals to the environment.

Example 1

(38) Formulation and Process to Prepare an Acidic Composition Comprising a Corrosion Inhibitor Package According to a Preferred Embodiment of the Invention

(39) Start by combining the methanesulphonic acid (42 wt % of the composition) with water (58 wt % of the composition) and mix thoroughly for a few minutes. Add a pre-determined volume of the corrosion inhibitor package according to a preferred embodiment of the present invention described in Table 1 below. Add 0.1 wt % of potassium iodide to the composition. Circulation is maintained until all products have been solubilized. Table 1 lists the components of the acid composition of Example 1, including their weight percentage as compared to the total weight of the composition and the CAS numbers of each component.

(40) TABLE-US-00001 TABLE 1 Composition of a corrosion inhibitor used in a composition according to a preferred embodiment of the present invention Component CI-D1 CI-D2 2-Propyn-1-ol, compd. with Vol % 45 45 methyloxirane .beta.-Alanine, N-(2-carboxyethyl)- Vol % 11.6 11.6 N-dodecyl-, sodium salt (1:1) Cocamidopropyl betaine Vol % 11.6 11.6 Nonane-1,9-dial (NL) and 2- Vol % 7 0 methyloctane-1,8-dial (MOL) Glutaric Dialdehyde Vol % 7 Isopropanol Vol % 24.8 24.8 Total Vol % 100 100

(41) The chemical formula for Nonane-1,9-dial (NL) and 2-methyloctane-1,8-dial (MOL) is as follows:

(42) ##STR00001##

(43) The chemical formula for Glutaric Dialdehyde is as follows:

(44) ##STR00002##

(45) The resulting composition of Example 1 is a clear, liquid with a strong odour having shelf-life of greater than 1 year. It has a freezing point temperature of approximately minus 30° C. and a boiling point temperature of approximately 100° C. It has a specific gravity of 1.21±0.02. It is completely soluble in water and its pH is less than 1.

(46) The composition is readily biodegradable, non-fuming and has no volatile organic compounds nor does it have any BTEX levels above the drinking water quality levels. BTEX refers to the chemicals benzene, toluene, ethylbenzene and xylene. Surrogate toxicity testing carried out on rats shows the LD50 to be not less than 1100 mg/kg.

(47) Corrosion Testing

(48) The compositions according to the present invention were exposed to corrosion testing. Various steel grades were exposed to various novel alkylsulfonic acid solutions for periods of time ranging up to 6 hours at temperatures of up to 180° C.

(49) The following corrosion testing outlined in Tables 2 to 9 (below) for acid compositions with known corrosion inhibition packages, for acid compositions with proprietary corrosion inhibition packages and for compositions according to the present invention (diluted at 50% of the stock solution, i.e. example 1) at various temperatures for various durations of exposure. A desirable result was one where the lb/ft.sup.2 corrosion number is at or below 0.05. More preferably, that number is at or below 0.02. Also desirable is the control of pitting corrosion as pitting weakens locally a metal, it is desirable to minimize or even completely eliminate pitting. Where coupons are identified, FIGS. 2, 3, 4 and 5 provide a photograph of the surface of the coupon post-corrosion testing. Pitting was noted in some coupons after exposure.

(50) The predominant cause of corrosion of metals by MSA is known to be pitting corrosion, the below testing allows to determine the effectiveness of the corrosion inhibition packages against this very serious type of corrosion. FIG. 1 provides a schematic view of various types of pitting corrosion and methods of identifying each groups of pitting corrosion. In a first case, the pitting corrosion can be identified by simple visual inspection. In a second case, the pitting corrosion can be identified with the use of special inspection tools. And in a third case, the pitting corrosion can be identified by microscopic examination.

(51) TABLE-US-00002 TABLE 2 Corrosion testing performed at 150° C. with MSA (21%) for a duration of 6 hours where the steel density is 7.86 g/cc Surface Steel Corrosion Loss area Mm/ Lb/ type inhibitor (g) (cm.sup.2) Mils/yr year ft2 J55 B900 2.0% CI- 0.1046 28.992 263.8452 6.7017 0.007 5, 1.5% CI-1A N80 A745 2.0% CI- 0.1997 28.0774 520.136 13.2115 0.015 5, 1.5% CI-1A L80 A929 2.0% CI- 0.1897 28.0774 494.0901 12.5499 0.014 5, 1.5% CI-1A CI-1A is a 10% potassium iodide solution. Thus, the total KI present is 0.15% CI-5 refers to a proprietary corrosion inhibitor package comprising a terpene; a cinnamaldehyde or a derivative thereof; at least one amphoteric surfactant; and a solvent.

(52) TABLE-US-00003 TABLE 3 Corrosion testing performed at 150° C. with MSA (21%) where the steel density is 7.86 g/cc Duration Steel Corrosion Loss Surface area of exposure type inhibitor (g) (cm.sup.2) (hr) Mils/yr Mm/year Lb/ft2 N80 A744 2.0% CI- 2.1114 28.0774 4 8248.987 209.5243 0.154 2, 1.5% CI-1A L80 A933 2.0% CI- 2.7938 28.0774 6 7276.695 184.8280 0.204 2, 1.5% CI-1A J55 B883 2.0% CI- 0.8099 28.992 6 2042.909 51.8899 0.057 2, 1.5% CI-1A CI-1A is a 10% potassium iodide solution. CI-2 refers to a commercially available corrosion inhibitor package.

(53) TABLE-US-00004 TABLE 4 Corrosion testing performed at 180° C. with MSA (21%) where the steel density is 7.86 g/cc Duration Steel Corrosion Loss Surface area of exposure type inhibitor (g) (cm.sup.2) (hr) Mils/yr Mm/year Lb/ft2 J55 B889 3.0% CI-5 0.1291 28.992 4 488.4668 12.4071 0.009 2.5% CI-1A 0.2% NE-1 J55 B890 3.0% CI-5 0.4215 28.992 6 1063.2 27.0053 0.030 2.5% CI-1A 0.2% NE-1 N80 A829 2.25% CI-5 0.2113 28.0774 4 825.5238 20.9683 0.015 2.0% CI-1A 0.2% NE-1 N80 A827 3.0% CI-5 0.4842 28.0774 6 1261.141 32.0330 0.035 2.5% CI-1A 0.2% NE-1 L80 A910 3.0% CI-5 0.1661 28.0774 4 648.9328 16.4829 0.012 2.5% CI-1A 0.2% NE-1 L80 A911 2.25% CI-5 0.2693 28.0774 4 1052.123 26.7239 0.020 2.0% CI-1A 0.2% NE-1 CI-1A refers to a 10% solution of potassium iodide; CI-5 refers to a proprietary corrosion inhibitor package comprising a terpene; a cinnamaldehyde or a derivative thereof; at least one amphoteric surfactant; and a solvent. NE-1 is a non-emulsifier.

(54) TABLE-US-00005 TABLE 5 Corrosion testing performed at 180° C. with MSA (21%) where the steel density is 7.86 g/cc for a duration of exposure of 4 hours Steel Coupon Corrosion Loss Surface Mils/yr Mm/year Lb/ft2 N80 A743 2.25% CI-2, 3.652 28.0774 14267.93 362.4053 0.267 L80 A934 2.25% CI-2 1.3818 28.0774 5398.527 137.1226 0.101 J55 B882 2.25% CI-2 0.3347 28.992 1266.381 32.1661 0.024 CI-1A is a 10% potassium iodide solution.

(55) TABLE-US-00006 TABLE 6 Corrosion testing performed at 150° C. with MSA (21%) where on L80 steel coupons where the steel density is 7.86 g/cc (coupon surface area 28.0774 cm.sup.2) Duration Corrosion Loss of Exposure inhibitor Coupon # (g) (hours) Mils/yr Mm/year Lb/ft2 2.0% CI-DA1, A953 0.0612 4 239.1010683 6.0732 0.004 1.5% CI-1A 2.0% CI-DA1, A954 0.1155 6 300.8297755 7.6411 0.008 1.5% CI-1A CI-1A is a 10% potassium iodide solution.

(56) TABLE-US-00007 TABLE 7 Corrosion testing performed at 180° C. with MSA (21%) where the steel density is 7.86 g/cc for a duration of exposure of 4 hours Steel Corrosion Weight loss Surface area type Coupon # inhibitor (g) (cm2) Mils/yr Mm/year Lb/ft2 L80 A952 2.25% CI- 0.1379 28.0774 538.7587798 13.6845 0.010 DA1, 2.0% CI-1A N80 A839 2.25% CI- 0.1483 28.0774 579.3903339 14.7165 0.011 DA1, 2.0% CI-1A J55 C045 2.25% CI- 0.1493 28.992 564.8961506 14.3484 0.011 DA1, 2.0% CI-1A CI-1A is a 10% potassium iodide solution.

(57) TABLE-US-00008 TABLE 8 Corrosion testing performed at 150° C. with MSA (21%) where on L80 steel coupons where the steel density is 7.86 g/cc (coupon surface area 28.0774 cm.sup.2) Corrosion Weight loss Exposure Coupon # inhibitor (g) time Mils/yr Mm/year Lb/ft2 A962 2.0% CI-DA2, 0.2033 4 794.268745 20.1744 0.015 1.5% CI-1A A963 2.0% CI-DA2, 0.169 6 440.1751694 11.1804 0.012 1.5% CI-1A CI-1A is a 10% potassium iodide solution.

(58) TABLE-US-00009 TABLE 9 Corrosion testing performed at 180° C. with MSA (21%) where the steel density is 7.86 g/cc for a duration of exposure of 4 hours Surface Steel Corrosion Loss area Mm/ Lb/ type inhibitor (g) (cm.sup.2) Mils/yr year ft2 L80 2.25% CI- 0.2058 28.0774 804.0359455 20.4225 0.015 DA2, 2.0% CI-1A N80 2.25% CI- 0.152 28.0774 593.8457906 15.0837 0.011 DA2, 2.0% CI-1A J55 2.25% CI- 0.1871 28.992 707.9174131 17.9811 0.013 DA2, 2.0% CI-1A CI-1A is a 10% potassium iodide solution.

(59) With respect to the corrosion impact of the composition on typical oilfield grade steel, it was established that it was clearly well below the acceptable corrosion limits set by industry for certain applications, such as spearhead applications or downhole scaling. Moreover, it was noted upon visual inspection that the commercially available corrosion inhibitor package did not perform well in the prevention of pitting corrosion (see coupons A744, A933, B883) at temperature of 150° C. and, as expected, was even worse at 180° C. (see coupons A743, A934 and B882) which exhibited very large pits. The proprietary corrosion inhibition package (CI-5) perform well at 150° C. where coupons had few or no pits (see coupons B900, A745 and A929). Where pits were present, these were quite small. At 180° C., pits were more evident (see coupons B889, B890, A829, A827, A910 and A911). Overall, the pits were small but present. For the corrosion inhibition packages according to preferred embodiments of the present invention (CI-DA1 and CI-DA2), there were no visible signs of pitting at 150° C. (see coupons A953, A954, A962 and A963). At 180° C., no pits were seen on coupons A952 and A839 while very minimal and light pitting was observed on coupon C045. Visual analysis of the coupons confirms that the compositions according to a preferred embodiment of the present invention were superior when considering the pitting corrosion predominant in the use of alkylsulfonic acids such as MSA.

(60) Overall, the corrosion rates using a composition according to preferred embodiments of the present invention obtained were up to 3 times less compared to composition using a similar corrosion inhibitor but with an aldehyde containing compound in place of an organic compound comprising at least two aldehyde functional groups. The difference being that CI-DA1 and CI-DA2 comprise a saturated dialdehyde instead of a monoaldehyde. It is hypothesized that the aldehyde reacts with the protonated tertiary amine group in sodium lauriminodipropionate. It appears the organic compound comprising at least two aldehyde functional groups reacts with sodium lauriminodipropionate and forms an aggregate with the ionic groups available from the surfactant and, therefore, is a much more effective film former, which leads to lower corrosion rates.

(61) The uses (or applications) of the compositions according to the present invention are listed in Table 10 (below) and includes (upon dilution thereof ranging from approximately 1 to 75% dilution) but are not limited to: injection/disposal in wells; squeezes and soaks or bullheads; acid fracturing, acid washes or matrix stimulations; fracturing spearheads (breakdowns); pipeline scale treatments; cement breakdowns or perforation cleaning; pH control; and de-scaling applications.

(62) TABLE-US-00010 TABLE 10 Applications of the acid compositions according to preferred embodiments of the present invention Suggested Application Dilution Benefits Injection/Disposal 50% Compatible with mutual solvents and Wells solvent blends, very cost effective. Squeezes & Soaks 33%-50% Ease of storage & handling, cost Bullhead Annular effective compared to conventional acid stimulations. Ability to leave pump equipment in wellbore. Acid Fracs 50%-66% Decreased shipping and storage compared to conventional acid, no blend separation issues, comprehensive spend rate encourages deeper formation penetration. Frac Spearheads 33%-66% Able to adjust concentrations on the (Break-downs) fly. Decreased shipping and storage on location. Cement 50% Higher concentrations recommended Break-downs due to lower temperatures, and reduced solubility of aged cement. pH Control 0.1%-1.0% Used in a variety of applications to adjust pH level of water based systems. Liner De-Scaling, 1%-5% Continuous injection/de-scaling of Heavy Oil slotted liners, typically at very high temperatures.

(63) While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be appreciated by those skilled in the relevant arts, once they have been made familiar with this disclosure that various changes in form and detail can be made without departing from the true scope of the invention in the appended claims.