Acid soluble coating for well screens

A well liner for a subterranean well is provided comprising an elongated tubular member which may be a wire wrapped screen having a plurality of slots or passages which are disposed longitudinally and circumferentially of and extending from the outer surface to the inner surface of the tubular member, and an impermeable inorganic matrix substantially filling said slots or passages and coated upon said inner and outer surfaces, said matrix being the reaction product of a first reactant consisting essentialy of magnesium oxide and a second reactant consisting essentially of magnesium chloride, in a solvent or solution therefor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a screen for producing fluids from subsurface 
wells and, more particularly, to a well screen having perforations, 
passages or slots packed with, and the interior and exterior surfaces 
coated with, an impermeable inorganic matrix. The invention further 
relates to methods for producing fluids from wells utilizing the screen 
coated with the impermeable inorganic matrix. 
2. Description of the Prior Art 
In the completion of wells that produce oil, water, or other fluids, 
production of fluids from loosely consolidated formations containing sand, 
silt, or clay presents substantial problems. Such formations contain 
little or no naturally-occuring binding or cementing materials. As a 
result, normal fluid flow conditions tend to cause particles from the 
formations to run into the well bore with attendant deteriorative effects 
upon fluid production from the well. Clogging of well screens and damage 
to pumping equipment are examples of problems resulting from sand flow. 
For the foregoing reasons, proper well completion for production of fluids 
from loosely consolidated formations requires use of some means to prevent 
entry of any substantial quantities of formation particles into the well 
liner. Accordingly, control of sand flow in wells is commonly provided by 
a sand or gravel pack placed exteriorly of the well liner. One practice is 
to pack the space between the well liner and the unconsolidated formation 
with sand or gravel after the liner is set in place. Another practice is 
to use a pre-filled liner in which sand or gravel is packed within the 
annular space formed between inner and outer liners. 
In the use of sand or gravel packs, control of sand flow is accomplished 
because the majority of the formation particles are entrapped as they are 
carried by produced fluids through the interstices of the pack. This 
occurs as a result of "bridging," by which phenomenon a stable structure 
of solid particles is built within an opening several times the diameter 
of the particles that might be expected to flow through the opening. 
In many of these procedures of gravel packing a subterranean well, it has 
been desirable to temporarily close the openings or perforations through 
the screen in order to prevent contaminant from entering through the 
perforations within the screen while the liner is being placed within the 
bore of the well, such that the contaminant, especially particulate 
contaminant, will stabilize within the perforation itself, thus clogging 
the screen and rendering it ineffective or less effective for subsequent 
treatment and production of the well. 
Exemplary of prior art compositions for coating screens are those in U.S. 
Pat. No. 3,216,497, which discloses utilization of an aqueous slurry, 
paraffin wax, or an oil-soluble hydrocarbon resin. Additionally, this 
patent discloses, for use in situations where the screen will be contacted 
with oil, a coating of a water soluble material such as sugar, starch, 
polyvinyl alcohol, sodium chloride, or other similar compounds which can 
be crystalized on the screen, then subsequently dissolved with water. This 
patent also discloses utilization of a coating of a thin sheet of a 
metallic substance, such as magnesium, which is subsequently dissolved 
with hydrochloric acid. Polymers such as polyvinyl chloride or polyvinyl 
acrylate also may be utilized. These organic polymers are subsequently 
dissolved with tetrachloroethylene. 
U.S. Pat. No. 3,273,641 teaches the utilization of plugs of various forms 
preferably made from a fusible metal or metallic alloy having a melting 
point which is above the normal reservoir temperature or hydration 
temperature of cement. These plugs, which are contained within the 
interior of the perforated or slotted liner, have a melting point which is 
low enough that a heated fluid, such as steam, hot water, or hot oil can 
be injected into the interior of the liner or circulated exteriorly 
thereof to melt the plugs. The fusible alloys used in the composition of 
the plugs generally are eutectic mixtures of bismuth, tin, lead, cadmium, 
indium, and antimony in various compositions, combinations and 
percentages. 
U.S. Pat. No. 3,322,199 teaches the use of certain wax materials to reduce 
permeability and flow to groups of slots filling the well liner with a 
fluid at a temperature to sufficiently melt the waxy substance in a given 
group of slots. Preferable, the waxes will either be animal or vegetable 
waxes because of the property of slight or no solubility in crude oils. 
For example, animal waxes, such as crude grades of stearic acid, and 
vegetable waxes, such as carnauba, Japan, and Candelilla waxes may be 
utilized. Additionally, various synthetic paraffins, such as low molecular 
weight polyethylenes may be utilized. Gradation of melting points may be 
achieved by mixtures of the waxes of different melting points or by 
initial fractionation by means of solvents. 
U.S. Pat. No. 3,333,635 discloses the use of a filler material within the 
bore of the liner which can be easily removed by utilization of a mild 
solvent. For example, sulphur, heavy grease, and very low melting point 
metallic alloys which can be removed by hot water may be utilized. 
Additionally, eutectic mixtures of bismuth, tin, cadmium, indium and 
antimony may be utilized. 
U.S. Pat. No. 3,880,233 teaches the use of certain fatty alcohols, thermal 
plastic resins and waxes as plugging agents for perforated or slotted 
liners. 
U.S. Pat. No. 3,905,423 discloses plugging and coating materials such as 
hydrated nitrates of chromium, iron, mercury and nickel. Additionally, 
this patent discloses plugging agents of pure organic compounds, such as 
acids, paraffins, gilsonite, beeswax, and certain metals and metallic 
alloys. Additionally, the patent discloses the utilization of a blend of a 
wax and a polymer for a plugging agent. 
It has been found that these prior art waxes and organic polymer materials 
may dissolve at too low a temperature when exposed to fluid within the 
well bore while the screen is being placed immediate perforations in the 
well. Accordingly, these prior art materials are useful only at the given 
softening point or melting point of the sealant. 
The present invention obviates the problems associated with utilization of 
prior art compositions by providing a coating composition which may be 
readily and easily applied completely or partially to the exterior of the 
screen prior to insertion of the screen into the well bore. The 
composition is readily available, inexpensive to obtain and prepare, is 
easily applicable to the exterior of the screen, and has been found to be 
extremely durable, yet is easily dissolved by utilization of a strong acid 
or an acid material having a highly disassociatable hydrogen ion. 
Additionally, utilization of the present composition as a coating for the 
liner screen provides a screen which is substantially sealed and will 
prevent penetration therethrough of particulate contaminant when the liner 
is inserted within the bore of the well and eliminates the need for 
subsequent utilization of a wash pipe which would be carried within the 
interior of the screen. The composition protects the screen and the 
passageways or slots therein from damage and plugging during shipping and 
handling and while the screen is being run into the well. 
SUMMARY OF THE INVENTION 
In the present invention, a well liner is provided which comprises an 
elongated tubular member which has a plurality of slots or passageways 
disposed longitudinally and circumferentially of and extending from the 
outer surface to the inner surface of the tubular member. An impermeable 
inorganic matrix substantially filling said slots or passageways and 
coated upon inner and outer surface is the reaction product of a first 
reactant consisting essentially of magnesium oxide and a second reactant 
consisting essentially of magnesium chloride, in a solvent therefor. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention provides an inorganic matrix used as a temporary 
cement filler or coating which is applied to the exterior of a ported, 
perforated or slotted well liner which, in turn, is subsequently inserted 
within a well bore for treatment and, thereafter, production, of a 
producing zone in the well. Preferably, the matrix consists of the 
reaction product of an equal portion, by weight, of magnesium oxide and 
magnesium chloride. The inorganic matrix is a reaction product of the 
above active ingredients, the reaction normally taking place within an 
aqueous solvent. Numerous ancillary additives may be added to the reaction 
product, either during or after the initial reaction in the aqueous 
solvent, for numerous purposes. For example, hydroxyethylcellulose, sodium 
chloride, polyvinyl alcohol and magnesium carbonate each may be added, as 
fillers, extenders, etc. 
Of course, the magnesium oxide reactant is provided in the form of a solid 
powder for reaction purposes. The magnesium chloride also may be in the 
form of a solid, but is preferably utilized in a hydrated state. 
It has been found that little difference will be seen in the order of 
addition of additives, and the particular order of addition is not 
critical to the invention. After the addition of the desired amounts of 
magnesium oxide and magnesium chloride, the reaction will take place by 
the addition of water, thus forming an aqueous slurry or paste. 
Although the particular percentages by weight of initial reactants are not 
particularly critical, it has been found that an equal percentage by 
weight of magnesium oxide and magnesium chloride will make a very 
desirable inorganic matrix reactant which is easily applicable upon the 
exterior of the screen, has strong endurance and hardness characteristics, 
and is readily removable from the exterior of the screen upon exposure to 
a solution of strong acid, such as hydrochloric acid. The amount of water 
added to the reactants to provide an aqueous solution for the reaction may 
be varied, with amounts as low as about 16% by weight of the total 
reactants and as high as about 50% by weight of the total reactants, being 
preferred. 
The preferred matrix will contain about 16.3% by weight water, about 39.3% 
by weight saturated magnesium chloride, and about 44.4% by weight 
magnesium oxide. A lighter and finer powdered product may be used by 
increasing the water and magnesium chloride content and by decreasing the 
magnesium oxide content. 
Subsequent to preparation of the slurry or paste, it may be easily applied 
to the exterior of the screen by utilization of a caulking gun, or the 
like. Preferably, continuous strips of about one-half inch width of the 
matrix are squeezed from the caulking gun along the surface of the screen 
and into the perforated or slotted openings or passageways within the 
screen by means of a trowel, rubber spatula, or by hand. Depending upon 
the concentration of active ingredients within the matrix and the amount 
of water initially added to the reactants, the inorganic matrix can be 
expected to dry and harden and adhere to the exterior surface of the 
metallic screen from between about 2 hours and about 4 hours, at room 
temperature. 
After the matrix has been coated onto the screen as described above, 
sections of the screen may be secured together in conventional fashion and 
run into the well in a known manner. Thereafter, hydrochloric acid or 
other strong acid may be circulated in the well in a conventional fashion. 
This strongly acidic solution will readily dissolve the inorganic matrix 
to expose the interior of the liner or screen to the well fluids and 
carrier and treating fluids used in the remedial treating of the 
production zone of the well, or for similar purposes. 
Only a very small amount of acid solution has been found necessary to 
completely dissolve the inorganic matrix from the exterior of the screen. 
For example, it has been found that approximately 1/2 gallon of a 15% 
active hydrochloric acid solution will dissolve as much as 1 pound of the 
inorganic matrix consisting of approximately equal amounts by weight of 
magnesium oxide and magnesium chloride. The amount of acid which is 
required to dissolve a given amount of matrix of the present invention 
from a given length and size of screen is easily calculatable for a given 
length of screen. For example, assuming that a 30 foot length of screen of 
2-7/8 inch outside diameter is to be utilized, a satisfactory coating of 
the screen exterior may be obtained by application of approximately 0.30 
pounds of the inorganic matrix per foot of the screen. Accordingly, 
approximately 9 pounds of the inorganic matrix will be utilized to coat 
all of the exterior of the screen and, by calculation, approximately 41/2 
gallons of 15% active hydrochloric acid solution may be utilized to 
completely dissolve the matrix and discharge the matrix from the 
perforations in the screen. Since a typical acid treatment of a production 
zone containing 15 feet of perforations may be expected to use 
approximately 1,125 gallons of hydrochloric acid, it can be seen that very 
little amount of that acid will be utilized in dissolving the matrix off 
of the screen. 
Of course, the dissolving rate of the matrix in the acid depends upon the 
selected acid, the activity of the selected acid, the bottom hole 
temperature of the well, and whether the acid is flowing by the screen or 
is ambient in the well bore. For example, it has been found that a 15% 
active solution of hydrochloric acid will remove a matrix prepared from 
equal portions by weight of magnesium oxide and magnesium chloride in an 
aqueous solution on a screen at a temperature of 75.degree. F. in 30 
minutes if the acidic solution is flowing around the screen, and in 60 
minutes if the screen is exposed only to a bath of the acidic solution. 
Comparatively, if the same acidic solution of the same inorganic matrix is 
exposed to a screen at a temperature of 100.degree. F., the matrix will be 
dissolved from the screen exterior in approximately 15 minutes if the 
acidic solution is flowing around the screen, and in 30 minutes if the 
acidic solution is provided in the form of a bath for the screen. If the 
temperature is raised to 120.degree. F. or higher, the time required for 
removal of the matrix from the screen if the acidic solution is flowing 
around the screen is reduced to just 10 minutes, and is reduced to 20 
minutes if the acidic solution is provided in the form of a bath for the 
screen. At 160.degree. F., the matrix is removed from the screen within 3 
minutes by a flowing acid, and is removed in just 8 minutes if the screen 
is in an acid bath. 
In an actual well having 15 feet of perforations requiring 30 feet of 
screen at a bottom hole temperature of about 120.degree. F., it has been 
found that all of the inorganic matrix will be removed from the screen by 
a normal acid treatment of the well bore before injection of the gravel 
pack through the screen interior. Typically, an acid job may consist of 
26.8 barrels of 15% active hydrochloric acid solution followed by a second 
injection of 35.7 barrels of 12% active hydrochloric acid and 3% 
hydrofluoric acid, followed by a third injection of 13.3 barrels of 15% 
hydrochloric acid solution per foot of screen. If these solutions are 
pumped at a rate of 2 barrels per minute, the inorganic matrix will be 
removed completely from the screen before the first 26.8 barrels of acidic 
solution has been completely pumped past and below the section of screen. 
The inorganic matrix of the present invention is applied to the screen 
jacket or exterior in the form of a white cream paste that is non-toxic 
and non-flammable. It will set within approximately 2 hours and will be 
completely hardened within approximately 4 hours, but may continue to 
bleed some moisture for several days. However, the bleeding of moisture 
does not affect the utility of the matrix on the screen. Upon complete 
hardening of the matrix on the screen, the matrix has been found to have a 
density of 99.26 pounds per cubic foot and a compressive strength of 6,000 
pounds per square inch. The matrix bonds tenaciously to the steel screen 
wire and has been tested by repeatedly dropping a coated screen on the 
pavement without any apparent damage to the coating.

The present invention is further illustrated by the following examples, in 
which, reference to percentages is by weight: 
EXAMPLE I 
A mixture of 175 ml. of saturated magnesium chloride solution was added to 
150 g. of magnesium oxide and heated in a beaker at 115.degree. C. for one 
hour. The temperature was removed and the resulting matrix was permitted 
to hard set. Thereafter, the matrix was exposed to a 15% solution of 
hydrochloric acid and was found to be 100% acid soluble. 
EXAMPLE II 
A reaction matrix was prepared by adding to 100 g. of water, 15 g. of 
sodium chloride, 45 g. of magnesium chloride, 45 g. of magnesium oxide, 
and 3 g. of hydroxyethylcellulose. 115.degree. heat was applied to the 
reaction for approximately one hour, and the resulting matrix was allowed 
to set for 24 hours. Thereafter, only slight cracking of the compound was 
apparent. The compound was exposed to water and its resistance to water 
was noted, and no adverse affects were noted. The compound appeared to be 
100% acid soluble upon exposure to a 15% solution of hydrochloric acid. No 
visible residue remained after exposure to the acid solution. 
EXAMPLE III 
A reaction matrix was prepared in a solution of 100 ml. water containing 50 
g. of magnesium chloride and 130 g. of magnesium oxide. The reactants in 
the aqueous solution were stirred to provided a thick paste and applied 
manually onto the exterior of a wire wrapped screen and permitted to air 
dry for 24 hours. Upon visual observation thereafter, only slight cracking 
was noted and the hardened coating to the screen was water resistant and 
was 100% acid soluble when exposed to a 15% solution of hydrochloric acid. 
EXAMPLE IV 
An inorganic matrix was prepared in a 50 ml. solution of water by the 
addition of 1 gram of polyvinyl alcohol, 25 g. of magnesium chloride, and 
60 g. of magnesium oxide. Upon aging of the matrix, only slight cracking 
was noted. The matrix, as applied to the exterior of the screen, had good 
water resistance and was 100% acid soluble. 
EXAMPLE V 
An inorganic matrix was prepared in a solution of 50 ml. water by the 
addition of 55 g. of hydrated magnesium chloride and 95 g. of magnesium 
oxide. The material was applied to the exterior of a section of wire 
wrapped screen. Only nominal cracking was observed and good bonding to the 
screen was achieved. The matrix was 100% acid soluble. 
EXAMPLE VI 
A matrix was prepared in a 22 ml. aqueous solution by the addition of 53 g. 
of hydrated magnesium chloride and 60 g. of magnesium oxide to form a 
smooth paste. The material was applied to the exterior of a screen and no 
cracking was observed during setting. Excellent bonding characteristics 
were noted of the matrix onto the screen. The matrix appeared to be 
completely water resistant, and its compressive strength was tested to be 
in excess of 1,500 pounds per square inch. 
EXAMPLE VII-A 
Into a 35% aqueous solution, 15% by weight hydrated magnesium chloride was 
added, together with 10% by weight of magnesium carbonate and 40% by 
weight magnesium oxide. A reaction occurred after application of heat. The 
reactants were found difficult to mix in solution and after application to 
the screen, numerous cracks were visually observed. 
EXAMPLE VII-B 
Into a 14.7% aqueous solution was added 35.3% hydrated magnesium chloride 
(near saturation), 40% magnesium oxide, and 10% magnesium carbonate. The 
matrix, prepared as above, was applied to a screen section and adhered 
easily thereto. The setting of the matrix onto the screen was hard and 
satisfactory, without cracks and with good bonding characteristics. 
EXAMPLE VII-C 
A 19% aqueous solution was utilized to prepare a matrix containing 46% 
hydrated magnesium chloride (near saturation) and 35% magnesium oxide. The 
matrix, prepared as above, appeared to be too thin upon visual 
observation, and the bonding characteristics to the screen were 
unsatisfactory. 
EXAMPLE VII-D 
Into a 17.6% aqueous solution was added 42.4% near saturated magnesium 
chloride in hydrated form and 40% magnesium oxide. The matrix, prepared as 
above, mixed easily into a paste, was found to adhere to the screen 
surface in a satisfactory manner, and upon setup, no cracks were 
visualized. 
EXAMPLE VIII-A 
Acid solubility tests were conducted on screen sections coated with the 
inorganic matrix of the present invention prepared in a 14.7% aqueous 
solution utilizing 35.3% magnesium chloride (hydrated), 40% magnesium 
oxide, and 10% magnesium carbonate. This matrix was found to be completely 
soluble in a 15% solution of hydrochloric acid after approximately 8 
minutes in solution. The results of this test are set forth in the table 
below: 
TABLE VIII-A 
______________________________________ 
Time In Acidic Solution 
Weight of Coated Screen 
______________________________________ 
0 240.2 g. 
4 min. 235.8 g. 
8 min. 233.4 g. 
12 min. 233.2 g. 
16 min. 233.2 g. 
20 min. 233.2 g. 
______________________________________ 
EXAMPLE VIII-B 
Water resistance tests were conducted upon screen sections coated with the 
inorganic matrix of the present invention made in a 15% aqueous solution 
by adding thereto 39.3% magnesium chloride and 44.4% magnesium oxide. The 
resulting paste was applied to a section of screen and permitted to set. 
Thereafter, the screen was inserted into a water bath for a given time and 
weighed. The original coated screen, before exposure to the water bath, 
weighed 264.5 g. After addition of the screen to the water bath, the bath 
was heated to 200.degree. F. for 11/2 hours. The screen was removed and 
weighed, at 264.5 g. The screen was permitted to set overnight at room 
temperature in the bath for a period of 17 hours, and then weighed, at 
264.5 g. The screen then was reheated in the bath at 200.degree. F. for 25 
hours and weighed, at 263 g. 
EXAMPLE IX 
A test was conducted to determine the correlation of time and volume of the 
dissolution of the inorganic matrix in a 15% active solution of 
hydrochloric acid. Three beakers were utilized, containing 200 ml., 150 
ml., and 100 ml. of acid, respectively, at room temperature (74.degree. 
F.). 10.09 g. of inorganic matrix prepared from an aqueous solution of 
approximately equal amounts by weight of magnesium chloride and magnesium 
oxide, the matrix being in the form of a hardened, flattened sheet, was 
found to be completely dissolved within the acid solution in 20 minutes. 
To the second beaker was added 10.15 g. of a similar flattened inorganic 
matrix, which was found to be completely dissolved into the acidic 
solution in 47 minutes. Into the third beaker was added 10.14 g. of the 
inorganic matrix which was flattened and thereafter shattered into small 
pieces, which were found to be completely dissolved into the acidic 
solution in approximately 55 minutes. 
EXAMPLE X-A 
A test was conducted in which were dissolved 50 g. of magnesium chloride 
into 50 ml. of tap water. The magnesium chloride appeared to dissolve 
readily into the water. Thereafter, the magnesium chloride aqueous 
solution was poured into 60 g. of magnesium oxide. The reactants appeared 
to blend without undue difficulty, thus indicating that the variation of 
the order of addition of initial reactants is not critical to the 
invention. 
EXAMPLE X-B 
Fifty grams of magnesium chloride were mixed with 60 g. of magnesium oxide 
and thereafter 50 ml. of tap water was added to the reactants to provide a 
pasty material. Although there was minor difficulty in initial mixing, and 
some magnesium chloride crystals remained undissolved, the end reaction 
appeared smooth. This test again indicates that the variation of the order 
of addition of the additives is not critical to the invention. 
EXAMPLE X-C 
Eighty-three grams of magnesium chloride were dissolved into 50 ml. of tap 
water. This solution was poured into 60 g. of magnesium oxide reactant and 
was found to mix readily. The resultant inorganic matrix was an extremely 
creamy fluid. This example again indicates that the variation in the 
weight of addition of magnesium chloride and the order of combination of 
the reactants is not critical to the invention. 
EXAMPLE XI 
Several tests were conducted to determine the amount of time required to 
completely dissolve an inorganic matrix in a solution of hydrochloric 
acid. 10.0 g. of a flattened sheet of a matrix obtained from an aqueous 
solution of substantially equal proportions of magnesium chloride and 
magnesium oxide was reacted and permitted to harden. Thereafter, the sheet 
was placed into a beaker containing 150 ml. of a 15% active solution of 
hydrochloric acid and stirred. The matrix visually appeared to be 
completely dissolved in the acidic solution in 7 minutes. A second test 
was conducted wherein 0.60 g. of the flattened sheet of the matrix was 
placed in a still beaker containing 350 ml. of 15% active hydrochloric 
acid and was visually observed to be completely dissolved therein after 
41.8 minutes. A third test was conducted utilizing 0.60 g. of a sheet of 
the matrix, as above, in 350 ml. of 15% hydrochloric acid, but the 
contents of the beaker were continuously stirred. The matrix was visually 
observed to be completely dissolved within the hydrochloric acid solution 
after approximately 3.5 minutes. 
EXAMPLE XII 
A test was conducted to determine the ability of the inorganic matrix of 
the present invention to satisfactorily dissolve in a solution of sulfamic 
acid. A 13/8 inch O.D., 2 inch length of screen was coated with the matrix 
material, as described in the above examples, and placed into a beaker 
containing 350 ml. of water with varying amounts of sulfamic acid. A 
stirring bar was inserted into the beaker, the stirring bar being 
activated by magnetic means and responsive to a second magnet contained 
within a heating element, utilized in the test. The stirring bar provided 
agitation of the solution to simulate circulation of the solution. At 
concentrations of 3 pounds per barrel and 7 pounds per barrel of sulfamic 
acid solution, satisfactory removal of the matrix from the screen was not 
obtained, even though the solution was heated to a temperature of 
152.degree. F., with agitation. However, a similar test utilizing 15 
pounds per barrel of sulfamic acid indicated that the screen next to the 
stir bar was completely free of matrix after 30 minutes and a temperature 
of 146.degree. F. The interior slots of the screen were cleared of matrix 
after 35 minutes, and approximately 1/3 of the screen was free of the 
matrix after 44 minutes and a temperature of 152.degree. F. A third test 
series was conducted utilizing 30 pounds per barrel of sulfamic acid. The 
matrix on the exterior of the screen section completely dissolved within 
the inorganic solution after 4 minutes and a temperature of 122.degree. F. 
The interior protrusions of the matrix within the interior of the screen 
appeared visually to be completely dissolved into the acidic solution 
after 8 minutes and a temperature of 132.degree. F. Moreover, the section 
of screen immediate the stir bar appeared to be completely free of matrix 
after 15 minutes and a temperature of 142.degree. F., while the screen was 
80% free of the matrix after 22 minutes. Complete cleaning of the screen 
was accomplished after exposure into this acidic bath for a period of 25 
minutes and a temperature of 154.degree. F. 
Although the invention has been described in terms of specified embodiments 
which are set forth in detail, it should be understood that this is by 
illustration only and that the invention is not necessarily limited 
thereto, since alternative embodiments and operating techniques will 
become apparent to those skilled in the art in view of the disclosure. 
Accordingly, modifications are contemplated which can be made without 
departing from the spirit of the described invention.