GB2196665A - Steam foam process - Google Patents

Description

GB2196665A 1 SPECIFICATION foam i.e. gas-liquid dispersion which (a) is

capable of both reducing the effective mobil Steam foam process ity, or ease with which such a foam or disper sion will flow within a permeable porous me The invention relates to a steam foam process 70 dium and (b) has steam in the gas phase for producing oil from, or displacing oil within, thereof. -Mobility- or - permeability- refers to a subterranean reservoir. an effective mobility or ease of flow of a In this process steam is injected into, and foam within a permeable porous medium. A fluid is produced from, horizontally spaced lo- 11 permeability reductionor---mobilityreduc cations within a portion of an oil reservoir in 75 tion- refers to reducing the ease of such a which the disposition of a steam flow path is foam flow due to an increase in the effective determined by gravity and/or oil distribution. viscosity of the fluid and/or a decrease in the After a steam channel has been formed the effective permeability of the porous medium.

composition of the fluid being injected is A reduction in such a mobility or permeability changed from steam to a steam-foam-forming 80 can be detected and/or determined by mea mixture by addition of surfactant components. suring differences in internal pressures within The composition of the mixture is correlated a column of permeable porous material during with the properties of the rocks and the fluids a steady state flow of fluid through a column in the reservoir so that the pressure required of such material. -Steam quality- as used re to inject the mixture and to move it through 85 garding any steam- containing fluid refers to the steam channel exceeds that required for the weight percent of the water in that fluid steam alone but is less than the reservoir frac- which is in the vapour phase of the fluid at turing pressure. The composition and rate of the boiling temperature of that water at the injecting the mixture is subsequently adjusted pres sure of the fluid. For example: in a mono to the extent required to maintain a flow of 90 component steam- containing fluid which con steam foam within the channel at a relatively sists entirely of water and has a steam quality high pressure gradient at which the oil-displac- of 50%, one-half of the weight of the water is ing and channel-expanding effects are signifi- in the vapour phase; and, in a multicomponent cantly greater than those provided by the steam-containing fluid which contains nitrogen steam alone. Oil is recovered from the fluid 95 in the vapour phase and dissolved or dis produced from the reservoir. persed surfactant and electrolyte in the liquid The present invention also relates to an im- phase and has a steam quality of 50%, one- provement in an oil recovery process in which half the weight of the weight of the water in steam is cyclically injected into and fluid is the multi-component steam- containing fluid is backflowed from a heavy oil reservoir which is 100 in the vapour phase. Thus, the steam quality susceptible to a gravity override that causes of a steam-containing fluid can be calculated an oil layer to become adjacent to a gas or as, for example, 100 times the mass (or mass vapour-containing substantially oil-desaturated flow rate) of the water vapour in.that fluid zone in which there is an undesirable intake divided by the sum of the mass (or mass flow and retention of the injected fluid within the 105 rate) of both the water vapour and the liquid desaturated zone. In such a process, the fluid water in that fluid. --- Steam-foarn-forming mix being injected is changed from steam to a ture- (or composition) refers to a mixture of steam-foam-forming mixture by addition of steam and aqueous liquid solution (or disper surfactant components arranged to form. a sion) of surfactant, with some or all, of the steam foam within the reservoir having physi- 110 steam being present in the gas phase of a cal and chemical properties such that it (a) is steam foam. The gas phase may include non capable of being injected into the reservoir condensable gas(es) such as nitrogen.

without plugging any portion of the reservoir Object of the invention is an improved pro- at a.pressure which exceeds that required for cess for displacing oil within an oil-containing injecting steam but is less than the reservoir 115 subterranean reservoir by flowing a steam fracturing pressure and (b) is chemically weak- containing fluid in conjunction with a surfac ened by contact with the reservoir oil so that tant component through a relatively steam it is more mobile in sand containing that oil permeable zone within said reservoir.

than in sand which is substantially free of that According to the invention the surfactant oil. The surfactant-containing steam is injected 120 component comprises in substantial part at into the reservoir at a rate slow enough to be least one sulphonate of the formula IRSOX in conducive to displacing a front of the steam which R is di-alkylaryl including aryl is phenyl, foam along the oil-containing edge portions of toluyl or xylyl having attached thereto two lin the oil-desaturated zone than along the central ear alkyl groups with equal or different chain portion of that zone. And, fluid is backflowed 125 length, but each containing 11-20 carbon from the reservoir at a time at which part or atoms in the alkyl chain and X is sodium, all of the steam is condensed within the potassium, lithium or ammonium. Such sulpho steam foam in the reservoir. nates will be further on denoted as C11-C20- As used herein the following terms have the dialkylarenesulphonates.

following meanings: -steam foam- refers to a 130 The dialkylarenesulphonate-containing 2 GB2196665A steam-foam-forming mixture suitably includes plugging any portion of the reservoir, in re an aqueous solution of electrolyte and advan- sponse to a pressure which exceeds that re tageously further also includes a substantially quired for displacing steam through the reser nonconclensable gas, with each of the surfac- voir but is less than the fracturing pressure of tant, electrolyte and gas components being 70 the reservoir, and (b) can be weakened by present in proportions effective for steam- contact with the reservoir oil to an extent foam-formation in the presence of the reser- such that the weakened foam is significantly voir oil. The invention also relates to the dimore mobile in reservoir oil-containing pores alkylarenesulphonate-containing steam-foam- of a porous medium than in oil-free pores of forming mixtures which are described herein. 75 that medium; The invention is useful where it is desirable injecting the steam-foam- forming mixture at to remove oil from, or displace oil within, a a rate equivalent to one which is slow enough subterranean reservoir. For example, the inven- to cause the foam formed by that mixture to tion can be used to move oil or an emulsion advance more rapidly through the pores of a of oil and water away from a well borehole in 80 reservoir oil-containing permeable medium than a well-cleaning type of operation, and/or to through the pores of a substantially oil-free displace oil into a producing location in an oil- permeable medium; and recovery operation. backflowing fluid from the reservoir after a In particular, the present invention relates to steam soak time sufficient to condense part a process for recovering oil from a subterra- 85 or all of the steam in the injected steam-foam nean reservoir, comprising: forming mixture. The steam-foam-forming mix- injecting steam and producing fluid at hori- ture preferably comprises steam, a noncon- zontally spaced locations within a portion of clensable gas, a linear C,,- C,,-dialkylarenesul an oil reservoir in which the disposition of a phonate surfactant and an electrolyte.

steam flow path is determined by the effect 90 The invention provides unobvious and be- of gravity and/or oil distribution, rather than neficial advantages in oil displacement proce being substantially confined within at least the dures by the use of the cli-alkylarenesulpho one most permeable layer of reservoir rocks; nate surfactant in the steamfoam-forming advantageously maintaining rates of steam compositions. For example, where a steam- injecting and fluid production such that a 95 foam-forming mixture contains such a surfac steam channel has been extended from the tant and an electrolyte in proportions near op injection location; timum for foam formation, the present surfac- changing the composition of the fluid being tant components provide exceptionally strong injected from steam to a steam-foam-forming steam foams having mobilities many times mixture by addition of a linear C,,-C,O-di-alkyless than those of steam foams using other larenesulphonate-containing compound, whilst surfactants. In addition, significant reductions continuing to produce fluid from the reservoir; are reached in the mobilities of the steam correlating the composition of the steam- foams at concentrations which are much less foam-forming mixture with the properties of than those required for equal mobility reduc the rocks and fluids in the reservoir so that 105 tions by the surfactants which were previously the pressure required to inject the mixture and considered to be the best available for such a the foam it forms or comprises into and purpose. The use of the present di-alkylarene through the steam channel exceeds that re- sulphonate surfactant components involves no quired for steam alone but is less than the problems with respect to thermal and hydro reservoir fracturing pressure; and 110 lytic stability. No chemical or physical deterior- adjusting the composition of the fluid being ation has been detectable in the present alky- injected into the steam channel to the extent larenesulphonate surfactants that were reco required to maintain a flow of both steam and vered along with the fluids produced during foam within the channel in response to a rela- productions of oil from subterranean reser tively high pressure gradient at which the oil115 voirs. In each of those types of sulphonate displacing and channel-expanding effects are surfactants the sulphur atoms of the sulpho significantly greater than those provided by nate groups are bonded directly to carbon steam alone, without plugging the channel. atoms. The surfactants which were recovered The invention also relates to an oil recovery and tested during the production of oil had process in which steam is cyclically injected 120 travelled through the reservoirs at steam tem into and fluid is backflowed from a subterra- peratures for significant times and distances.

nean heavy oil reservoir which is susceptible The present Cll-C20-dialkylarenesulphonate- to gravity override and tends to intake and containing steam foams have been found to retain undesirably large proportions of the in- represent a substantial improvement in mobil jected fluid. This process comprises: injecting 125 ity reduction over foams based on the mono steam mixed with a linear C,,-C,,-c1i-aIkyIarenealkylaryl sulphonates e. g., cloclecylbenzene sul sulphonate-containing steam-foam-forming phonates. The foams to be used according to compound which is arranged for forming a the present invention represent also substan steam foam which (a) can be displaced tial improvement over the Cl,-C, alpha-olefin through the pores of the reservoir, without 130 sulphonate-containing foams.

3 GB2196665A 3 The present invention further relates to mobility.will now be described with reference compositions containing at least one C11-C20to Figures 1 and 2.

di-alkylarenesulphonate, and steam, optionally Figure 1 shows schematically a sand pack electrolyte, and optionally noncondensable test apparatus which can be made of currently gas, that are suitable for use in oil-displacing 70 available apparatus components. The appara and/or producing pTocesses. Of particular in- tus consists of a cylindrical tube 1 that is 400 terest in this respect are steam-foam-forming mm long and has a cross- sectional area of 8 compositions consisting essentially of (a) CM2. Such a tube is preferably arranged for a water, which is present in the composition, at horizontal flow of fluid from an inlet 2 to-an a temperature substantially equalling its boiling 75 outlet 3. The tube is preferably provided with temperature, at the pressure of the compo- 5 pressure taps 4, 5, 6, 7 and 8. The loca sition, in both a liquid phase and a vapour tion of the first pressure tap 4 is at a distance phase; (b) a surfactant component present in of 150 mm from the inlet 2. The locations of the liquid phase of the composition in an the other taps are chosen so as to divide the amount between 0.01 and 10 percent by 80 part of the tube 1 situated behind tap 4 into weight, calculated on the weight of the liquid equal parts of 50 mm. The tube 1 contains a phase, said surfactant component comprising permeable and porous column of suitable ma in substantial part at least one Cll-C20-di-alky- terial, such as a sand pack, which is capable larenesul phonate; (c) an electrolyte present in of providing an adequately realistic laboratory the liquid phase of the composition in an 85 model of a subterranean reservoir.

amount between 0.001 percent by weight At the inlet end 2, the sand pack or equiva- (calculated on the weight of the liquid phase) lent column of permeable material is arranged and an amount tending to partition the surfac- to receive separate streams of steam, noncon tant into a separate liquid phase; and (d) a densable gas such as nitrogen, and one or noncondensable gas present in the vapour 90 more aqueous liquid solutions or dispersions phase in an amount between about 0.0001 containing a surfactant to be tested and/or a and 0.3 percent by mol, calculated on total dissolved or dispersed electrolyte. Some or all mols in the vapour phase. of those components are injected at constant Illustrative of the di-alkylarenesulphonate sur- mass flow rates proportioned so that steam factants suitably employed in steam-foam 95 of a selected quality, or a selected steam drive processes of enhanced performance, ac- containing fluid or composition, or a steam cording to the invention, are the di-alkylarene- foam-forming mixture of a selected steam sulphonates obtained by reacting a linear C,,- quality can be injected and will be substan C,,-di-alkylbenzene linear C,-C,,,-di-alkyltoluene tially homogeneous substantially as soon as it and/or linear C,,-C,,,-di-alkylxylene with sulphur 100 enters the face of the sand pack.

trioxide followed by neutralization of the sul- In the tests, steam-foamforming mixtures phonic acid. Particularly suitable for purposes are compared with and without surfactant of the invention is a sulphonate derived from components added thereto, by measuring substantially linear C,,-C,O-di-alkyl-benzene. pressure gradients formed within a sand pack Different reservoir materials have different 105 during flows through the pack at the same debilitating effects on the strength of a steam substantially constant mass flow rate.

foam. Tests should therefore be carried out to Numerous tests have been made of different determine the sulphonates or sulphonate-consteam-foam-forming mixtures using sand taining steam-foam-forming compositions that packs composed of a reservoir sand and hav perform optimally in a given reservoir. This is 110 ing a high permeability, such as 10 darcys.

preferably done by testing the influence of The pressures were measured with pressure specific sulphonates on the mobility of a detectors (not shown) (such as piezoelectric steam-containing fluid having the steam quality devices) installed at the inlet 2 and at the taps selected for use in the reservoir in the pres- 4, 5, 6, 7 and 8 of the tube 1. The results of ence of the reservoir material. 115 such tests have proven to be generally com- Such tests are preferably conducted by parable with the results obtained in the field.

flowing steam-containing fluids through a sand In the laboratory tests, the steam-foam- pack. The permeability of the sand pack and forming components were injected at constant foam-debilitating properties of the oil in the mass rates until substantially steady-state sand pack should be at least substantially 120 pressures were obtained at the inlet and at equivalent to those of the reservoir to be the taps. The ratio between the steady-state treated. Comparisons are made of the mobility pressures at the taps during flow of steam of the steam-containing fluid with and without mixed with the foam- forming surfactant com the surfactant component. The mobility is indi- ponent and the steady- state pressure at the cated by the substantially steady-state pres- 125 taps during flow of the steam by itself are sure drop between a pair of points.located indicative for the mobility reduction. The between the inlet and outlet portions of the higher this ratio, the stronger the steam foam sand pack in positions which are substantially and the higher the mobility reduction caused free of end effects on the pressures. by the steam-foam-forming mixture.

Some laboratory tests to determine steam 130 - Figure 2 illustrates the results of compara- 4 GB2196665A 4 tive tests with steam and various steam-foam- sulphonates for use in the process according forming mixtures in sand packs containing to the invention, olefins are advantageously Oude Pekela Reservoir sand having a permea- applied in which at least 90% of the mole bility of 7 darcys. The backpressure was 21 cules are alpha-olefins.

bar, corresponding with a temperature of 215 70 Particularly attractive are sulphonates derived C. The steam injection rate was 600 from the SHOP alpha-olefins (trademark) sold cml/min, having in the water phase containing by Shell Chemical UK, in part for their linear 0.5 %w sodium C,,-C,,-di-alkyibenzenesulpho- structure and high alpha- olefin content, i.e., nate. The figure shows the variation of the greater than 95% in each case. The SHOP pressure difference in bar (Y-axis) with disalpha-olefins are prepared by ethylene oligom tance in centimetres (X-axis) from the pack erization. Products having a high content of inlet 2. The pressures were measured at the internal C11-C21-01efins are also commercially inlet 2, at the taps 4, 5, 6, 7 and 8, and at manufactured, for instance, by the chlorina the outlet 3 of the pipe 1 of Figure 1. Curve tio n-de hyd rochl ori nation of paraffins or by pa A relates to the displacement wherein a mix- 80 raffin dehydrogenation, and can also be pre ture of 85% quality steam, having in the water pared by isomerization of alpha-olefins. Inter phase containing 0.5 %w sodium C,-C,o-cli-alnal-olefin-rich products are manufactured and kylbenzenesulphonate, was used as a displac- sold, for example, by Shell Chemical UK.

ing composition. For preparation of di-alkylarenesulphonates, Curve B relates to using a steam-containing 85 the olefins as described above are subjected fluid having a steam quality of 85% and a to reaction with benzene, toluene or xylene.

water phase which contains 0.5% by weight The di-alkylbenzene, dialkyltoluene or di-al of a surfactant. In the Curve B test, the surkylxylene isomers are reacted with sulphur factant was a linear sodium C11-C12di-alkylben- trioxide. The term - sulphur trioxide- as used zenesulphonate. 90 in the present specification and claims is in-

Curve C relates to using the mixture used tended to include any compounds or com- for Curve B except that the surfactant was a plexes which contain or yield S03 for a sul linear sodium C,,-C,,Ai-alkylbenzenesulphophonation reaction as well as S03 per se. This nate. reaction may be conducted according to To all surfactant solutions 0.25 %w sodium 95 methods well known in the chemical arts, typ- C,,-C,, a-olefin sulphonate had been added in ically by contact of a flow of dilute SO, va order to merease the solubility of the sodium pour with a thin film of liquid alkylate at a di-alkyibenzenesulphonates. temperature in the range of about 5 to WC.

The greatly improved steam permeability re- The reaction between the SO, and the alkylate duction performance of the presently de- 100 yields a sulphonic acid which is neutralized by scribed C,,-C,O-di-alkylarenesulphonate-contain- reaction with a base, preferably an alkali metal ing surfactant component is clear from the hydroxide, oxide, or carbonate.

Curves B and C as compared to the Curve A The specific composition of dialkylarenesul- in Figure 2. phonates prepared as described above (and 105 also, for instance, the methods used for sul Compositions and procedures suitable for use phonation, hydrolysis, and neutralization of the in the present invention specified olefins) have not been found to be a For purposes of the present invention, the critical factor to the performance of the sur- surfactant component of the steam-foam-form- factant in the steam foam process according ing mixture is necessarily comprised in sub110 to this invention. In this regard, it is observed stantial part of linear C,,-C,,-cli-alkylarenesul- that factors which have conventionally gov phonate. Materials of this class but with a erned the choice of sulphonation conditions, much shorter alkyl chain have heretofore found e.g., product colour, clarity, odour, etc., do 1 commercial utility, for example, in detergent not carry the same weight in the preparation formulations for industrial, household and per- 115 of dialkylarenesulphonates for purposes of sonal care application. use in the process according to the invention A class of di-alkylarenesulphonates very that they have been accorded in detergent suitable for use in the present invention is that manufacture. Consequently, reaction conditions derived from a particular class of olefins, outside of those heretofore considered desir which may be defined for present purposes in 120 able for alkylate sulphonation are still suitably terms of the configuration and number of car- applied in the preparation of surfactant com bon atoms in their molecular structure. These ponents suitable for use in the steam-foam olefins preferably have a carbon number of forming mixture.

13-14. For purposes related to maintaining product In terms of molecular structure, these olefins 125 stability, conventional manufacture typically are aliphatic and mainly linear. Either alpha- or yields a dilute solution or dispersion of the di internal olefins are considered suitable for the alkylarenesulphonates, for instance, products alkylation route chosen to produce the pro- with a 15-30 %wt active matter content in ducts to be used according to the invention. water. Such products may be directly applied For purposes of derivation of the di-alkylarene- 130 to the preparation of steam-foam-forming mix- GB2196665A 5 tures for purposes of this invention. above-mentioned USA patent specification

Suitable alkylarenesulphonates, generally pre- 4,086,964. An aqueous solution may be ap- pared by methods such as described above, plied that contains an amount of electrolyte are themselves commercially available pro- substantially equivalent in salting-out effect to ducts. 70 a sodium chloride concentration of from 0.001 The strength of the foam formed by,the to 10% (but less than enough to cause signifi- steam-foam-forming composition including di- cant salting out) of the liquid-phase of the alkylarenesulphonate tends to increase with in- steam. Some or all of the electrolyte can com creases in the proportion of the surfactant an- prise an inorganic salt, such as an alkali metal d/or electrolyte components of the compo- 75 salt, an alkali metal halide, and sodium chlo sition. Also, there tends to be an optimum ride. Other inorganic salts, for example, hal ratio of surfactant and electrolyte components ides, sulphonates, carbonates, nitrates and at which the surface activity of the compophosphates, in the form of salts of alkaline sition is maximized. earth metals, can be used.

The steam-foam-forming composition ac- 80 Generally stated, an electrolyte concentration cording to the present invention can form a may be applied which has approximately the steam-foam capable of reducing the effective same effect on mobility reduction of the foam mobility of the steam to less than about as does a sodium chloride concentration of 1/10th and even to 1/50th-1/75th of the mo- between 0.001 and 5 percent by weight (but bility it would have within a permeable porous 85 less than a salting outinducing proportion) of medium in the absence of the surfactant. the liquid phase of the steam- foam-forming The steam used in the present process an- mixture. The electrolyte concentration may be d/or compositions can be generated and sup- between 0.001 and 10 percent calculated on plied in the form of substantiall any dry, wet, the same basis.

superheated, or low grade steam in which the 90 In compounding a steamfoam-forming mix- steam condensate and/or liquid components ture or composition in accordance with the are compatible with, and do not inhibit, the present invention, the steam can be generated foam-forming properties of the foam-forming by means of substantially any of the commer- -components of a steam-foam-forming mixture cially available devices and techniques for according to the present invention. The steam 95 steam generation. A stream of the steam be quality of the steam as generated and/or ing injected into a reservoir is preferably gen amount of aqueous liquid with which it is erated and mixed, in substantially any surface mixed be such that the steam quality of the or downhole location, with selected propor resulting mixture is preferably from 10 to tions of substantially noncondensable gas, 90%. The desired steam-foam is advantage- 100 aqueous electrolyte solution, and foam-forming ously prepared by mixing the steam with surfactant. For example, in such a mixture, the aqueous solution(s) of the surfactant compo- quality of the steam which is generated and nent and optionally, an electrolyte. The water the concentration of the electrolyte and surfac content of these aqueous solutions must, of tant-containing aqueous liquid with which it is course, be taken into account in determining 105 mixed are preferably arranged so that (1) the the steam quality of the mixture being formed. proportion of aqueous liquid mixed with the Suitably, the noncondensable gas advantage- dry steam which is injected into the reservoir ously used in a steam-foam-forming mixture is sufficient to provide a steam-containing fluid according to the present invention can com- having a steam quality of from 10-90% (pre prise substantially any gas which (a) under- 110 ferably from 30-85%); (2) the weight propor goes little or no condensation at the tempera- tion of surfactant dissolved or dispersed in the tures (100-350 Q and pressures (1-100 bar) aqueous liquid is from 0.01 to 10.0 (prefera at which the steam-foam-forming mixture is bly from 1.0 to 4.0); and (3) the amount of preferably injected into and displaced through noncondensable gas is from 0.0003 to 0.3 the reservoir to be treated and (b) is substan- 115 mole fraction of the gas phase of the mixture.

tially inert to and compatible with the foam

Claims (8)

1. forming surfactant and other components of CLAIMS that mixture. Such a gas

   is preferably nitrogen 1. A process for displacing oil within an oil- but can comprise other substantially inert containing subterranean reservoir by flowing a gases, such as air, ethane, methane, flue gas, 120 steam-containing fluid in conjunction with a fuel gas, or the like. Suitable concentrations of surfactant component through a relatively noncondensable gas in the steam-foam mix- steam-permeable zone within said reservoir, ture fall in the range of from 0.0001 to 0.3 characterized in that a surfactant component is mole percent such as 0.001 and 0.2 mole employed which comprises in substantial part percent, or between 0.003 and 0.1 mole per- 125 at least one sulphonate of the formula RS03X cent of the gas phase of the mixture. in which R is di-alkylaryl aryl being phenyl, Suitably, the electrolyte used should have a toluyl or xylyl having attached thereto two lin- composition similar to and should be used in ear alkyl groups with equal or different chain a proportion similar to those described as length, but each containing 11- 20 carbon suitable alkali metal salt electrolytes in the 130 atoms in the alkyl chains and X is sodium, 6 GB2196665A 6 lithium, potassium or ammonium.
2. 2. A process according to claim 1, charac- terized in that an electrolyte is employed in the flow within the reservoir in conjunction with the steam-containing fluid.
3. 3. A process according to claim 1 or 2, characterized in that a substantially noncon clensable gas is employed in the flow within the reservoir in conjunction with the steam containing fluid.
4. 4. A process according to any one or more of the preceding claims, characterized in that the surfactant component comprises in sub stantial part sulphonate obtained by reacting a linear di-alkylbenzene, linear cli-alkyltoluene an d/or linear di-alkylxylene of which each of the alkyl chains contains from 11 to 20 carbon atoms with sulphur trioxide followed by neu tralization of the sulphonic acid.
5. 5. A process according to claim 4, charac- terized in that the sulphonate is derived from linear C13-14-di-alkyltoluene, C13-C,,-di-alkylben zene or CII-CI4-di-alkylxylene.
6. 6. A process according to any one or more of the preceding claims, characterized in that the aqueous liquid phase of the steam-foam forming composition contains between about 0.01 and 10 percent by weight of alkylarene sulphonate.
7. 7. A process according to any one or more of the preceding claims, characterized in that in addition to or instead of nitrogen or another non-condensable gas electrolyte is used up to 10% in the liquid phase to enhance the per- formance of the surfactant.
8. 8. A process for displacing oil within an oil- containing subterranean reservoir according to claim 1 substantially as hereinbefore described with reference to the laboratory tests.

   Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD Printed by Burgess & Son (Abingdon) Ltd Con. 1/87