US2110779A - Oil well casing - Google Patents

Description

March 8, 1938. H. G. TEXTER OIL WELL cAsING Filed Aug. '7, '1936 2 Sheets-Sheet l ATTORNEYS.

Patented Mar. 8, 1938 W OIL WELL CASING:

Howard G. Texter, Tulsa, kla., assignor to The National Supply Company, Pittsburgh, Pa., a corporation of Pennsylvania Application August '1, 1936, Serial No. 94,779

1.o claims.

AThis invention relates to well casing programs, and more particularly to the construction of oil well casing strings formed of a plurality of lengths of pipes connected end to end by threaded joints.

In drilling deep wells, such as oil Wells, the usual practice is to estimate the depth to which the well must be drilled and the number of casing seal-offs between the surface and the final f depth. The desired finishing hole size is then selected and provision is made for the number of nested casing strings to provide the seal-offs and the starting drill size isthereby determined. Drilling is commenced with the selected starting size and proceeds until it becomes necessary to seal off the hole with the outer or largest string of casing. This rst seal-olf operation may be necessary to seal off surface water, for example, after the Well has been drilled from 100 to 1000 feet deep. Lengths of the outer casing are progressively connected together and lowered into the hole until the entire hole is lined with the casing string. The string of casing thus provided is usually sealed off by pumping cement down into the well, which ows up around outside of the casing and sets, thereby sealing and anchoring the casing firmly in place.

Drilling is then continued through the outer or largest diameter string of casing with a. bit which will pass through the casing and with this second size of bit the well may go, for example, to 6500 feet Where it again becomes neces-f sary to seal off gas, Water, sliding shale, or the like. The second string of casing is then progressively made up and lowered into the Well through the first string and is sealed o in. the same manner as the first. Drilling is continued in this manner with any number of additional seal-offs necessary (usually the estimated number) until the Well has been drilled to the de- 40 sired depth, at which time the final or inner string of casing of the smallest diameter is run in and sealed 01T in the same fashion as the other strings of casing. Usually each of the strings of casing extendsl to the top of the well and a nested assembly of strings is formed which is called a casing program.

From the foregoing brief description ofthe usual process of casing an oil well it is seen that a casing program usually consists of a'number of concentric strings of casing, and that these strings are of successively increasing length.

Myl invention applies particularly to vlong strings of casing and offers economies which increase as a direct function of the length of the string.

There are two limiting features which determine the proportions of a long 'string of casing of "a given diameter. It is the general practice to determine rst `the thickness of the casing- Wall which is required to resist collapse at the 4bottom with a predetermined factor of safety, under the arbitrary assumption that the casing is closed at the bottom and contains only air, While the casing is immersed for its entire length in salt water. The second step is' to determine the factor of safety with which the joints connecting the various individual pieces together will resist tension failure under the assumption that the entire length of the string is hanging in air and is supported by a joint located at the upper end of the string. This tension safety factor is` usually made large enough to withstand a pull considerably greater than the weight of the entire strir'ig. Very heavy pulls are sometimes imposed on casing. Some of the occasions demanding heavy pulls are: (a) recovering casing which stuck in the hole while being run in, and failed to reach bottom; (b) recovering casing froma'dry hole, i. e., a hole which failed to produce gas or oil; (c) recovering, casing from an exhausted well. To cover these contingencies a large number of operators consider a tension safety factor of 2.5 advisable.

As a result of the many factorsinvolved in the construction and use of'oil well casing various parties concerned, functioning through Vthe agency of thev American Petroleum Institute (hereafter referred to' as A. P. I.) have set up certain Well-recognized standard specifications on casing setting depths, sizes, Weights, joint constructions, and operating safety factors. These specifications cover the majority of the casing sizes commonly used in the United States. Sizes other than those listed by A. P. I. are manufactured but these sizes closely follow the A. P. I. specifications and are here considered as being of the same type. Tables are included in the A.P. I. specifications showing the lengths of strings that may be run using various factors of safety for both tension and collapse. In the past the practice has been to use the same grade of material and the same wall thickness throughout a casing string. This practice has. in general been satisfactory Vbut ,wells are now being drilled deeper and are more expensive and the operating organizations are paying more attention to costs. For the most economical casing practice in deep wells there are certain disadvantages in the A. P. I type of casing joint.

The tensile efficiency of the A. vP. I. type of casing joint is low due to the reduced cross sectional area at the root ofthe pipe threads. i The tensile eciency of a casing joint is the percentage quotient found by driving the load at which the joint will fail by the load at which the body of the casing will fail. Joint strength is the actual pull in pounds required to break a joint.V

Joint eiiciency in accordance with A. P. I. specification is practically constant for the same size of `casing regardless of the various wall thicknesses (weight per foot) commonly used. Therefore the tensile joint strength of any one size and grade of casing is roughly directly proportional to the wall thickness. This means that while a long string of light wall casing of a certain size and grade might be able to support itself, with a suitable safety factor it would have a lower safety factor if the bottom part of the string consisted of the same size pipe but with a heavier wall. In order to increase the joint strength a few sizes of the A. P. I. type of casing have been made with upset pipe ends for use inemergencies. For general"1 use this is not practicall because of the increase in the outside diameter of the joint which is highly undesirable due to the decreased clearances between the nested strings of casing forming a complete program. A re-establishment of the clearances would necessitate a larger starting casing and bit size, with the attendant increase in drilling costs.

It is the object of my invention to avoid and fovercome the foregoing and other diiculties ofknown casing practices as particularly exemplified in the use of A. P. I. casing by the provision of yan improved casing program in which the individual casing strings are formed of certain combinations of individual lengths of cas- A ing whereby the program is better adapted to resist collapse and tensile pull while maintaining kthe starting casing and drilling size at amini- Another object of the invention is the provi-y sion of a casing string for lining an oil well in which the actual strengthof' the joints connecting the individual casing lengths is independent of the wall thickness and is substantially constant for any given size and grade of material and wherein the Weight per foot of the casing increases downwardly from the top of the string and the eiiiciency lof the joints connecting the individual casing lengths increases upwardly of the string at one or more stepped points.

The foregoing and other objects of the invention are achieved by provision of a well-casing program including a plurality of nested strings .of casing extending to progressively greater depths into the earth, at least one of said strings comprising a lower portion formed of connected individual casing lengths having a suitable factor of safety against collapse, together with an upper portion connected to the lower portion and comprising individual casing lengths of less weight per foot than the lower portion, the joints between the individualY lengths of this upper portion are of approximately the same strength as the joints of the lower portion in the cases where both portions are of the same grade of material. In the the wall thickness of the pipe.

be less than that of the lower portion but in any case the joint efficiency ofthe upper portion will be greater than that of the lowervportion so that a suitable factor of safety in tension is provided.

Referring to the drawings which quite diagrammatically illustrate the invention, Fig. 1 is a vertical cross-sectional View through a well and illustrating a casing programl constructed in accordance with the principles of my invention; Fig. 2 is a view taken on line II-II but only partly in section of a typical joint construction particularly adapted for -use in the practice of my invention; Figs. 3, 4, and 5 are transverse cross-sectional views taken respectively on lines III-III,JIV`-IV, and V-V of Fig. l and illustrate the decrease at vertically spaced points of the wall thickness of the casing without change in outside diameter or joint strength.l

In the use of combination strings of casing in a well-casing program, as herein' taught, the `strength and dimensions of the joint connecting the individual casing lengths together is important. The joint must be capable of being constructed on pipe of the same outside diameter but of varying wall thickness so that the thinner Wall casing has a greater joint efficiency (although the same strength) than any heavier wall casing and also so that complete interchangeability is provided between all joints regardless of A joint which is particularly adapted for use in the practice of my invention is that disclosed in United States Patent No. 1,927,656 to Eaton and Burnish and an improved form thereof is disclosed in United States PatentNo. 2,062,407 to George M. Eaton. This joint is known in the trade under the name Extreme line and accordingly is so identified in the tables hereinafter forming a part of my specification. The casing or pipe body proper is the same as that [of the A. P. I. standards but an Extreme line joint is formed integral with the body as hereinafter more fully described, instead of the usual A. P. I. joint. The particular advantage of the joint which renderseit available to the practice of my invention is that it is readily adapted to be formed with substantially any desired joint efiiciency and strength by establishing the proper upsetting, machining, and threading operations. l

A joint ofthe Eaton type has been illustrated in Figs. 2 to 5 of the drawings and includes threaded male and female portions 5 and 6 formed integrally with the casing lengths. In making this type of joint the pipe ends are upset to provide the necessary thickness for the desired strength and are then machined to form the trapezoidal thread and sealing surfaces indicated. As hereinafter more fully explained, the casing of the joint of Figs. 2 and 3, like the remainder of thefcasing at the lower portion of the string, is formed with relatively thick walls and a small inside diameter to provide a casing very resistant to collapse. The casing of the joint of Fig. 4, typical of the casing at the middle portion of the string, is of a medium wall thickness and a somewhat larger inside diameter, as will be seen by comparing Figs. 3 and 4.' The casing of the joint oi Fig. 5, typical of thatC at the top of the string, is of a smaller wall thickness and a greater inside diameter and is formed with the upset male portion 5 of the joint being of equal strength and radial thickness to the female portion B of the joint. In the three joints of Figs. 3 to 5 the outside diameter is the same and the joint strength (actual breaking strength in tensile ypulll is the same since the radial thickness of the female portion 6 is constant. The joint `eiliciency 'of the joint of Fig. 5 is greater than .the upset pipe end the dimensions of the joint i are maintained within narrow limits and thus the clearances between nested casing strings are greater than with the A. P. I. type of joint so that in many instances the starting drilling size can be reduced.

Reference should now be had to Table I which contains data regarding some of the commonly used weights and grades of standard A. P. I., 7" outside diameter, well casing and the A. P. I. joint and also data'concerning Extreme line joints constructed in accordance with the principles of my invention.

of safety in collapse of less than 1.5. Sometimes where all conditions are favorable, a smaller factor of safety, as, for example, 1.25 orveven less, has been used with success. Also, it is .considered good practice to providea factor of safety in `tension of at least 2 to 21/2.

Referring .again to Table I it will be seen that the 30.3 1b. Grade D, A. P. I. casing lcan be set to a depth of 8960 feet with a factor of safety-in collapse of 1.5. Also 28.3 1b. Grade D, A. P. I. casing can be set to 8090 feet and 24.3 lb. Grade D, A. P. I. casing to 6340 feet with a factor of safety in collapse of 1.5. These figures are calculated from tables published in A. P. I. Code No.. 5, Supplement No. 1.

As previously explained, the general practice has been to use the same"wall thickness andV grade of material throughout the string, the

safety factors in tension and collapse being based TABLE I 7" Casing 24 26 28 30 24.3 26.3 28. 3 30.3 .332 .362 .393 .423 '.332 .362 .393 .423 Grade A. P. I. specs C C C C D .D' D D Tensile strength P S I--- 82000 82000 82000 82000 105000 105000 105000 105000 Pipe wall area square in.. 6.955 7.56 8.16 8. 74 6. 955 7. 55 8.16 8. 74 Pipe tensile strength lbs. 285000 309500 3341500r 358000 365000 396000 428000 Setting depth collapse it. F. S. 5010 5700 6400 7080 6340 7200 8090 8960 a.. r. i.

Joint strength tension lbs. F. S.=2 1 182600 197300 211420 259250 281400 304050 V1125300 lSetting depth tension it. F. S.=2-.----- 7010 7024 7047 7048 10668 10690 10744 10762 Joint ei. 59 59 59 7l 7l 71 71 EXTREME LINE Joint strength tension lbs. F. S.=,2.... 276000 276000 291000 291000 353500 353500 372000 372000 Betting depth tension it. F. S.'=2 11530 10600 10390 9680 14750 13560 13310 12300 Joint en. 07 89 87 Si 07 87 si structed in accordance with my invention in which'the eillciency increases from about 81 per cent for the heaviest weight per foot/to about 97 per cent for the lightest weight per foot in casing.

I of 7" diameter. The efficiency of my joint is so established that the joint strength is substantially the same in the same diamete and grade of pipe regardles of the weight per foot of the pipe. This means that the lightest weight ofmy casing can support a. heavier string of casing than can be supported bythe same weight equipped with the A. P. I. joint, and moreover that the lightestv weight of my casing ca n support a stringi of heavier weight peri'foot' casing with the same factor of safety as the A. P.' I.

casing.

'Ihe cardinal principle of my. invention is the use of a casing string formed of a combination oi individual casing lengths of different wall thicknesses and connecting joints.' The particular combinations made are predicated uponl the 4 resistance to collapse of the casing lengths in the on the data published :by A. P. 1. -As directly distinguished from this previous general practice is that of 'my invention wherein strings of casingare employed, at least certain of the strings in a program being formed with a plurality of different weights or grades of casing lengths connected in any one string' by interchangeably threaded joints of different but selected eiii' ciencies.

More particularly having reference to Fig. 1

the numeral I0 .indicates generally a casing proj ^gram comprising an outer string I2, anintermediate string, and an inner. string I6. The outer string of casing i 2 is generally employed to sealing-olf surface waterA and the like and is usually set only to 100 to 1000 feet. Because of the relatively short length of this casing and the comparatively small tensile and collapsing forces to which it 'is' subjected as compared with the casing set much deeper into the earth, I generally provide the same grade of casing and con-v nect the individual lengths with joints ofthe same emciency throughoutv the entire length of thisl string. However, the principles of myinvention l as hereinafter explained in conjunction with the strings of casing set to agreater depth may` likewise be employed in conjunction with the sur-l face-sealing casing string i2.

The intermediate casing string idis formed with a lower portion ida of connected easing lengths particularly adapted to resist collapsing pressures. The upper portion Mb of the'string .is formed of casinglengths designed to resist tensile pull and havg joints ,of increased em.-

/ ing the combinations employed in each individual intermediate string of casing may extendl 6500 feet or more into the well. The exact manner of selecting the individual casing' lengths and associated joints for both the lower and upper portions of the casing string M will be describedv in greater detail hereinafter.

'I'he casing string I6 extending from the top of the well clear down to the oil strata is made in accordance with my invention with two or more portions of differenty Weights and joint strengths all connected together to form a continuous casing string. For example, the lower portion I6a of the string is formed of connected casing lengths particularly adapted to resist collapsing forces. A middle portion |611 is adapted to resist medium collapsing forces and medium tensile forces. The upper portion [6c made of individual casing lengths and connecting joints particularly adapted to resist tensile forces inasmuch as the joints of this portion of the string support the weight of the entirestring. The casing lengths at the top of the string, however, need not be particularly strong so far' as resistance to collapse is concerned. 4,

Particularly illustrating the manner of practicing my invention and the method of determinstring of casing,\Table II illustrates several typical examples of combination strings and the resulting casing program.

. TABLE 2,110,779 ciency connecting lthe individual lengths. The' foot to casing of 40 lbs. per foot,4 but that the grade of the material used can be changed from Grade D to the less expensive Grade C. This is due particularly to the fact that I make the joint eciency in the 40 1b. Grade C casing approximately 93 per centso that the joints of Grade C casing are able to support the weight of the entire casing string.

The depth to which the 40 lb. Grade C casing is set is determinedby the collapse setting depthv -ports the lower portion Ila of the string formed of 47 lb. casing, Grade D, which has a factor of safety in tension well above 3.1. The factor of safety in collapse of the 3000 feet of 47 lb., Grade D, forming the portion Ha of the casing string is approximately 1.54 and the factor of safety against collapse of the upper portion Hb of the casing string formed ofr40 1b. Grade C is approximately 1.71.

Example l of Table II likewise indicates that thel inner or smallest string i6 of casing is set to 10000 feet. The tables, such as Table I, are in- IlI / Example l Emgple Exaglple Example4 Exaile Type I Extreme Extreme Extreme Extreme une une une une A. 1. I. A. P. I. A. P. I.

Length fsu-ingrat 0500 1 10000 11000 0500. 10000 0000 4700 5800 D 3500 4300 3000 280 4700 17 4700 Lengthfeet 2500 5300 awo 0500 10000 650.0 10000 11000 900 4700 5000 i 3500 4700 4300 De in safest 7500 7500 0500 10000 .p 650 10000 100 11000 i 900 .0. D. easing in gj i Z i 0% 7 Z A f 7 7 1 24 n4 40 24 24 Wei ht 01min lbs. foot as 2s 47.0 so

E W A 47 30 2s 30 30.3

Gmdeofsingc4-r-1.) g 3 8 g D D g 1.0 1.04 Factor ofsafety collapse 1.02 -gl 1.02 1.54 1.34 Hg Factor orsafry tension a1 d 208 212 v2.43 2.70 1.74v 1.38

, L, In Examplel it is found necessary to Set cas-y vestigated to determine the necessary weight ing having an outsi e diameter of 9%" to the depth of 6500 feet. s string of casing might, for example, be the intermediate string Il shown in Fig. 1 of the drawings. Looking into the A. P. I. tables we nd that 9%" `O. D. and 47 lbs. per foot can be set to 6500 feet with a safety factor of 1.54.

Now instead of running 47 lb. casing for the -entire'0500 ,feet to which it is to be set, I have discovered. that by using the Extreme line joint and making it of the proper strength and efficiency, not only can the wall thickness of the casing be reduced in its upper portion, as, for example, by changing from casing of 47 lbs. per

- factor of safety in collapse of 1.62. As a result of these figures and inaccordance with the prin- .ciples of'my invention I form string |6of an upper portion lic formed of 4700 feet 'of 24 lb. Grade C 'casing having a. factor of safety against collapsefof 1.6. To the bottom of this upper-portion of the string' is secured the Ymiddle portion I .against 1.74). Grade Cv material is also used in OflGb Of 2800 feet (7500-4700) of 28 1b. Grade D casing which as above stated can be set to '7500 A factory.- The relatively high factor of safetyin tension of the upper portion 16e of the combina.-v

tion string is due to the high joint efllciency which compensates amply for the lower grade and less weight-per-foot casing employed.

In this manner I am able to reduce the cost of the casing program without reducing thefactor of safety of the casing in tension and collapse and without increasing the starting drilling size.

Example 2 in Table IIy indicates a combination string set to 10000 feet in which the upper portion of the vstring comprises 4700 feet of 7"-24 lb. vGrade C casing having a .factor of safety in tension for the complete string of at least 2.12 and a factor of safety in collapse of at least 1.6.l The bottom or lower portion of the string comprises 5300 feet of 28 lb. Grade D casing having a factor of safety in collapse of at least 1.21. Since much less tensile weights carried bythe lower portion of the casing the factor of safety in tension will of course be far above 2.12.

Example 3 inj Table II illustrates my method of setting 7 casing to'the depth of 11000 feet while providing aisafetyl factorin collapse of at least 1.2. In this embodiment of the invention 24 lb. cuece` D casing is set to the depth of 5800 feet and provides a safety factor in collapse of 1.64.

Connectedl to the bottom of the 24 lb. casing is 1700 feet of 28 lb. Grade D casing having a safety factor in collapse of 1.62. Secured to the bottom of the 28 lb. casing is 3500 feet of ,30 lb. Grade D casing which has a safety factorin collapse of 1.22. The joint efficiency of the 24 lb.` casing is 97 per cent, the 28 lb. casing'? per cent, and the 30 1b. casing 81- per cent. The actual joint strength in each case isapproximately the same, namely, about 690,000 lbs. With the total weight of the combination string approximately 300,000. lbs. the factor of safety in tension is. at least 2.25. Example 4 'of Table II is illustrative of the standardmethod of usi-ng A'. P. I. casing to provide the strings of casing given in Example 1. vIt n will be recognized that the use of A. P. I. casing necessitates the selection of heavier pipe-to obtain the same factors of safety. `Asha matter'of fact, the difference in weight of the string I4 of `Example 1v and the first string of Example 4 is However, the factor of `safety in collapse is the same in bothfstrings (1.54) and the factor of safety in tension is gre ter (3X1 against 2.76) in, my combination string. oreover, in my combina- .tion string, less expensive metal (Grade'C as Yet the factor of safety in collapse is the samein both strings (1.34) and the factor of safety in tension in my improved string is greater (2.08

part of my combination string.

It will therefore be recognized that I have materially reduced the weight of a casing string and the grade of material employed in part of the string and still have not lowered, but in most cases have improved, the factor of safety of the string. The result is a'less expensive and a more satisfactory casing string and program.

Example 5, Table II, discloses the use of A. P. I. casing while practicing certain of the principles of the invention to provide a composite or combination string of casing similar to that disclosed in Example 3, except that the string is only 9000 feet long. However, the factor of safety in ten sion has dropped dangerously low.

While the standard A. P. I. threaded joint is 4relatively easy and inexpensive tomanufacture, it is open tonumerous objections which have been listed in detail in the Eaton patents referred to above. However, in proposing an improved form of well-casing joint it is necessary not only to overcome the inertia of the publics slowness to accept a new product, vbut also to compete, if possible, on a price basis with the old product, even though the new is superior. An important feature of my invention resides in the' savings effected by the use of combination strings whereby a joint of the Eaton type,'when constructed as herein disclosed, can not only compete with rthe standard A.P. I. joint but can often show denite savings in casing costs. This is due chiey to the fact that the weight of the casingV can be reduced upwardly of the string and a cheaper grade of materialcan be employed without reducing the factor of safety in collapse or in ten/sion to points below the necessary minimums. The lighter weight casing of cheaper grade forming the upper portions of my proposed combination strings isv provided with joints of y -greater efficiency than the joints in the lower portions and f ample strength so that the factor of safety'in tension is maintained.

It should be appreciated, however, that in certain instances some of the advantages of my invention are obtained by the practice of the principles thereof .with ordinary A. P. I. casing or other casing not having special high strength joints. Use in this manner is limited to conditions where the ordinary casing2 has sufficient strength in tension and collapse` to effect the desired end.

4It willbe seen from the foregoing that the objects of the invention have been achieved by the provision of an improved type of casing program in which the individual nestedl strings of casing are formed ofA combinations -of interchangeable lengths of casing possessing different properties such as pounds per foot, material used, and joint eiliciency. The invention provides for maintaining the factor of safety against collapse and in tension at relatively high and satisfactory levels while noticeably reducing the cost of the casing program, as well as ins'uriig the smallest Under the teachings of my invention the casing program for any well is easily planned by the man skilled in the artVI and comprises the rst scientific casing program utilizing the strength of the metal therein more emciently than was the general practice prior tomy invention.

05 possible drilling size for the largest finishing size. I

tion is not limited thereto or thereby but is dened in the appended claims.

1. A well casing string comprising a lower portion formed of connected individual casing lengths having a factor of safety in collapse of at least about 1.25 and an upper portion of connected individual casing lengths of less weight per foot than the lower portion but having joints between the individual lengths of approximately the sametensile strength as the joints of the lower portion but of an appreciably higher tensile joint eiiiciency sothat the'factor of safety in tension of the joints in the upper portion of the string is at least approximately 2.

2. A well casing string comprising a lower portion formed of thread-connected individual casing lengths having relatively. heavy walls adapted to resist collapse and an upper portion of threadconnected individual casing lengths of lighter walls than the lower portion but having joints between the individual lengths of a higher efiiciency but with 'approximately the same tensile strength as the joints of the lower portion of the casing.

' 3. A well casing string comprising a lower portion formed of thread-connected individual casing lengths particularly adapted to resist collapse and an upper portion of thread connected individual casing lengths of less weight per foot than the lower portion but having the individual lengths of casing connected by joints approaching 100% tensile efliciency so that the factor o f safety in tension of the joints in the upper portion of the string is at least approximately 2.

4. A well casing string formed of individual lengths of casing connected together by threaded joints at their ends, said string including a lower portion adapted to resist collapse, an intermediate portion' of less weight per foot than the lower portion and adapted to resist intermediate collapsing pressures andA medium tensile pull, and an upper portion of less rweight per foot than the intermediate portion and adapted to resist low collapsing pressures and high tensile pull, the strength of the joints in tension throughout the string being substantially constant but with the tensile joint efficiency of the upper portion beingY approximately 90%or more, the tensile joint eiiiciency of the lower portion being approximately or more, and the tensile joint `efilciency of the intermediate portion being in beportions.

5. A well casing string formed of individual lengths of casing connected together by joints, said string including a lower portion particularly adapted to resist collapse, an intermediate por`- tion particularly of less weight Jper foot than the lower portion and adapted to resist intermediate collapsing pressures and medium tensile pull, and an upper portion of less weight'per foot than the intermediate portion and particularly adapted to resist low collapsing pressures and high tensile pull, the strengthof the Joints in tension throughout the string being substantially constant but with the tensile joint efficiency increasing at stepped points upwardly of the string so that the factor of safety in tension is kept at least 2, and the factor of safety in collapse of each of the string portions not falling below about- 1.5.

6. A Well casing string formed of individual lengths of casing connected together by threaded joints, said string including a lower portion having a factor of safety against collapse of not less than 1.25, an intermediate portion of thinner lWalls having a factor of safety against collapse of not less than 1.25, and an upper portion having thinner walls than the intermediate portion and a factor of safety against collapse of not less than 1.25, the strength of the joints in tension throughout the string being substantially constant but with the tensile joint eiciency increasing at stepped points upwardly of the string.-

7. A well casing string formed of individual lengths of casing connected together by joints at their ends, said string including a lower portion having relatively thick walls adapted to resist collapse, and an upper vportion having thinner walls adapted to resist lower collapsing pressures, the tensilejoint emciency of the upper portion being not less than approximately 99%, and the tensile joint emcienoyof the lower portion being lower than about 8. A well casing string comprising a lower portion formed of connected individual casing lengths having a factor of safety in collapse of at least about 1.25, means joining the lengths together, an upper portion of connected individual casing lengths of less Weight per foot than the lower portion, means joining the individualindividual casing lengths of less weight per foot than the lower portion but having the individual lengths of casing connected by joints approaching 100% tensile eiiiciencyA so that the factor of safety in tension of the joints in the upper por-I tween the efficiencies of the upper` and'lower tion 0f the String i-S at least 1-5- 10. A well casing string comprising a lower portion formed of connected individual casing lengths particularly adapted to resist collapse and an upper portion of vconnected individual casing lengths of less .weight per foot and different grade thanfthe lower portion. but having the l individual lengths ofcasing connected by joints approaching 100 per. cent tensile eiiiciency so that the factor of safety in tension of the joints in the upper portion of uthe string is at least approximately 2.

HOWARD G. TEXTER'. 7

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