US2565794A

US2565794A - Directional drilling of deviated boreholes

Info

Publication number: US2565794A
Application number: US619749A
Authority: US
Inventors: Garth L Young
Original assignee: Signal Oil and Gas Co
Current assignee: Signal Oil and Gas Co
Priority date: 1945-10-02
Filing date: 1945-10-02
Publication date: 1951-08-28
Anticipated expiration: 1968-08-28

Description

Aug. 28, 1951 Filed Oct. 2, 1945 G. YOUNG 2,565,794

DIRECTIONAL DRILLING OF DEVIATED BOREHOLES 4 Sheets-Sheet 1 INVENTOR.

GARTH L. Youmc;

Aug. 28, 1951 ca. YOUNG DIRECTIONAL DRILLING OF DEVIATED BOREHOLES Filed Oct. 2, 1945 4 Sheets-Sheet? INVENTOR. y uNG A TTO NE Y- 4 Sheets-Sheet 5 IN VEN TOR.

flTToRA/E y.

( l/JETS; L. YOUNG 1951 G. L. YOUNG DIRECTIONAL DRILLING 0F DEVIATED BOREHOLES Filed Oct. 2, 1945 Aug. 28, 1951 G. YOUNG 2,565,794

DIRECTIONAL DRILLING 0F DEVIATED BOREHOLES Filed 001:. 2, 1945 4 Sheets-Sheet 4 IN V EN TOR.

GAR 7%??? give;

I or more below the surface.

Patented Aug. 28, 1951 DIRECTIONAL DRILLING OF DEVIATED BOREHOLES Garth L.

Young, Los Angeles, Calif., assignor to Signal Oil and Gas Company, Los Angeles, Calif., a corporation of Delaware Application October 2, 1945, Serial No. 619,749

Claims.

This invention relates to a method for drilling bore holes of great length or depth, such as are useful in the production of oil and gas.

The object of my invention is to reach producing horizons which may not be commercially reached by previously known techniques. Such prior methods are capable of reaching formation lying immediately below the well site. This method is the conventional vertical bore method. In order to reach formations at some distance away from the well site, directional drilling has been employed. In this method oriented whipstocks have been used.

In employing such oriented whipstocks, it is frequently necessary as a practical matter to drill a vertical bore through the unconsolidated formations before the whipstock may be set. This is because a deviated hole will form a bore the wall of which is inclined from the vertical, and in unconsolidated formations, which are soft and have little bearing power, the lateral pressure of the rotating drill stem forces the whipstock into the wall and thus buries it thus making the drilling impractical. Therefore, it is the common practice to drill a straight vertical hole through the unconsolidated formation. Such unconsolidated formations may extend down to 1000 feet It has been found that in setting whipstocks to form a, curved bore the maximum deviation is safely about 3 per hundred feet. This deviation is imposed in part by the friction resulting from running casing and drill pipe and rotating the drill pipe in uncased holes in such deviated bores. As a practical mat ter, casings ordinarily employed in bores cannot be introduced by present techniques into bores of greater degree of deviation extending for lengths beyond a practical maximum, dependin upon the size of the casing. The bend imposed on pipe run into such deviated bores of about 3 per hundred feet or greater creates such a large frictional resistance as to make running of casing in such bores impractical. Additionally, with this maximum degree of curvature, the total curvature permissible in order to permit drilling and running of casing in the bore hole is one which has a vertical deviation of 66. In other words, the tangent drawn to the curved bore at the maximum curvature may not, practically, make an angle more than 65 from the vertical. When this deviation has been reached, the hole below the arc thus formed must be drilled at no more than this maximum deviation from the vertical. This limit of curvature is in large part determined by the frictional resistance set up against the entry of easing into bores of greater vertical deviation.

This limit of curvature is set, in part also, by ing on the inner wall nearest the center of ourvature. Since the deviated hole may not be cased curvature. Since the deviated hole may be cased during drilling under present available techniques in the prior art, if the total vertical deviation of the hole exceeds a maximum of about 66, on turning the drill pipe and on withdrawing it from or advancing it into the hole, the drill pipe drags on passing or turning through the curved portion of the hole and forms a groove in the wall which is smaller than the drill. When the drill bit is withdrawn, it comes up against the key slot and is wedged in place. Sticking of the drill pipe has been experienced in such cases, causing loss of the pipe in the hole. This phenomenon, of the key slotting is also present, in somewhat reduced degree, when the total deviation is less than 66, where the degree of curvature is in excess of 3 per hundred feet. While key slotting may occur in bores of lesser degree of curvature, depending upon the softness of the formation, key slotting becomes a serious problem where the degree of curvature is greater than 3 per hundred feet.

These considerations therefore impose a limit on the oil and gas formations which may be reached from any well location, or, conversely, on the minimum distance which the well head at the earths surface must be from a point immediately above the bottom of the hole.

In many locations, due to the physical or legal impediments, it is impracticable to place the drill site as close to the well bottom as is required by these limitations. Thus, many formations lie at depths too shallow and at distances too far removed from the drill sites to be reached by whipstocking. Frequently, also, zoning restrictions or the presence of water do not permit the erection of well derricks sufficiently close to permit the well to be bottomed in the formations underlyin the prohibited or restricted surface zones.

It is an object of my invention to increase the range of rotary drilling equipment positioned at well locations or surface drill sites.

It is an object of my invention to increase the horizontal distance at which the well head or drill site may be located from a point on the surface immediately above the objective formation to permit deviated bore holes to be drilled from such locations to reach such formations at various depths below the earths surface.

In my method, instead of starting the well as a vertical bore through the unconsolidated formations, I start the deviation substantially from the surface. The bore is drilled, advancing casing and drill pipe together, with the bit slightly in advance of the casing. f'he advancing casing is fabricated so as to have a pre-set curvature conforming to the degree of curvature required.

The degree of'curvature will depend on the point to be reached by the bore hole and also on the permissible curvature allowable for the size of the casing, its wall thickness, material, heat treatment, etc. casing should alsobe of such rigidity that the pre-set curvature will not be substantially varied by the weight of the casing in the hole or by the load imposed in advancing the casing.

By starting the hole as a deviated hole at or near the surface Where the'conductor pipe terminates, or by casing the hole from the surface by means of a control or guide casing having a curvature corresponding to the deviated hole to be drilled, the drillzpipe is encased and cannot key-hole in the wall surface of the curved section of the bore. Since the degree of curvature in this control or guide casing is kept below the permissible maximum deviation throughout the totali curve, the drill pipe or casing to be run inside the guide casing the drill pipe and casing passed through the guide casing is never flexed beyond its permissible safe limit. Drill pipe and casing then pass through the curved guide readily.

By pre-setting the curvature of the control ing and employing a substantially rigid pre-set pipe as casing, its curvature is not altered substantially in the bore by the action of gravity or the pressure created by the pipe advancing mechai nism, or other loads or stresses arising during drilling, and a bore of the above describedcharacteristicscan be obtained.

It is therefore an object ofmy invention to direction Without using 'whipstocking.

It is another object of my invention to control the deviation of the bore hole by employing an" advancing casing of pre-set angularity of curvature, and to maintain the angularity of curvature of the bore equal to the degree of curvature of the pre-set casing for a. predetermined distance into the ground.

When the penetration thus obtained has reached preferably to a point below the unconsolidated formation, drilling may proceed further with the advancing curved casing or the drilling may be continued without further deviation; or further deviation may be made, for example, by employing ordinary whipstock operations beyond this point.

It is frequently desirable to set casing. in the bore either after completion of the drilling or before the bore has reached the objective formations. This may occur even before the curved portion of the bore is completed. In that case the pro-curved casing is cemented in the hole. A se cause the drilling of a bore hole deviated from nd string of casing, also pre-curved in thesame c manner, is introduced inside the original casing and advanced with the drill in like manner. It maybe necessary to set a series of pre-curved casing in this manner before the curved portion is completed. Additionally, straight casing,

i. not preset or pre-curved, may be introduced through the guide casing or the most innermost casing of the plurality of concentric precurved casings, and it will flex to'the desired degree of curvature in accordance with the principles set forth herein. If the casing extends from the surface through the curved portion of the hole and into the straight portion of the hole, then the portion of the inner casing in the curved portion of the hole can be either of a pre-set curvature or be under fiexure, i. e., originally straight. If it is of a pro-set curvature similar to the guide casing, the bending stresses may, like in the pre-formed guide casing, be relieved by normalizing the bent casing. In that case the casing in the curved portion will not be under any material stress imposed by bending in the curved portion.

This invention will be further described and better understood by reference to the accompanying drawings, in which:

"Fig. l is a graphic, schematic illustration of the range of the drill employing my invention;

Fig. 2 shows one method of drilling according to my invention from an initially horizontal direction;

Fig. 3 shows a-section of the cellar for drilling horizontally;

Fig. 4 is a fragmentary section showing the position ofthe drill and guide casing-in the bore;

Fig. 5 is a fragmentary section showing the outer and inner guide casing in position in the bore; and

Fig. 6 is a vertical view of one apparatus for carrying out my drilling method from an ini" tially vertical direction.

Fig. 1 graphically illustrates how by employing my method of drilling I may reach areas not possible to explore by conventional drilling operations, and how therange of the'drill'bit is increased from any fixed drill site. In'Figl, I is a drill site, and 2, 3, and 4 are oil formations. Such formations are, of course, reachable if the drill site can be placed suificiently close to the area overlying the oil zone. I-Ioweven-h this is not possible, as, for example, should the oil zone be beneath a restricted area 5 such-as a city or town in which-drilling is prohibited, or should the oil zone underlie water, a mountain, or other topographical hinderancewhich pro-- hibits the positioning of the drill site near the surface overlying the objective formation, then the" zone may not be reached by conventional straight hole drilling or by directional drilling by prevailing methods. Such a'situation is illustrated in Fig. 1.

In this figure the drill site is positioned as close to the restricted or prohibited area as topography or legal restrictions permit. 1 From this location I may drill by prior art methods either a straight, vertical hole 5 or a straight hole through the unconsolidated formation l, and thereafter an uncased deviated hole by whipstocking with various degrees of curvature up to about 3 per feet, as in 8, until a maximum of'66 from the vertical is obtained. Thereafter the bore hole must be drilled straight and uncased, as at 9, at a tangent to curve 3 for the total permissible depth not to exceed about 10,000 to about 15,000 feet by present drilling techniques. By variously adjusting the degree of curvature to less than 3 per 100 feet and the total vertical deviation to 66 or less, and the depth of the penetration up to the maximum attainable by prior art techniques, illustrated by line i2, as exemplified by bore holes is? and ii, the area defined by lines 5, i2, 9, and 3, and, of course, the volume defined by the revolution of this plane around 6, is reachable by present techniques. It will be observed that the oil zones 2, 3, and 4 are not thus reachable.

By employing my drilling method I can reach, from location 1, portions of the earth lying outside this zone reachable by ordinary conventional techniques.

Thus, by starting my drill horizontally at the surface and deviating the bore from substantially its surface penetration, with various but constant degrees of curvature, by simultaneously advancing a guide or control casing, fabricated (i. e., pre-set) to such curvature, with the advancing drill, up to the maximum permissible degree of curvature, and continuing this degree of curvature to various total curvatures, i. e., deviations from horizontal, and thereafter, if desired, drilling straight bores, I may reach various penetrations from the well location I outside of the zone reachable by conventional drilling. Thus, I may drill a bore starting horizontally and deviating continually until the deviation of up to 90 from horizontal is obtained, and thereafter I may discontinue the addition of and advance of the pre-set casing and drill a vertical bore at the end of the encased curve. This is illustrated by the well bore l3, in which, for purposes of illustration, the degree of curvature is shown as 3 per 100 feet. Or the deviation of the curved encased bore may be interrupted at some intermediate deviation from horizontal less than 90 and the advance of the pre-set curved casing discontinued and the bore drilled straight from thereon, as is illustrated by bore 14.

By reducing the degree of curvature other portions of the earth may be reached. Thus, l5 de. fines a bore hole in which the degree of curvature is at less than the present attainable maximum degree of curvature, specifically shown for illustrative purposes as less than 3 per 100 feet, and the curve is continued until a maximum deviation of about 66 is obtained, and the advance of the pre-set curved pipe stopped, and thereafter the bore is drilled straight to the objective sand 3. By adjusting the degree of curvature and the total curvature, it is possible to reach zones 2 and 3 by continuing the curved casing in a continuous arc, as is illustrated by IS, until the objective formations 2 and 3 are reached; or by interrupting the curvature at any degree of vertical deviation, stopping the ad vance of the pre-set curved casing and thereafter drilling a straight hole, as illustrated at H, an upper sand may be reached, as at 4. In drilling the curved section of such bore, in all cases 1 advance a casing, pre-formed to the desired degree of curvature before introduction into the bore and oriented during drilling, to maintain a predetermined direction of the bore.

Instead of starting the hole horizontally, I may start the hole vertically, setting a conventional surface, short, conductor pipe vertically instead of horizontally, which conductor pipe may itself be pre-eurved to the desired degree of curvature, and thereafter drill a deviated bore, using the pre-curved advancing control or guide casing as previously described. I may continue the deviation until a vertical deviation of up to 90 is reached and at any such vertical deviation discontinue the advance of the pre-curved pipe and drill a straight bore at a tangent to the terminal portion of the curved bore. For example, bore 19 illustrates the drilling of a curved bore (for illustrative purposes) shown as 3 per 100 feet until a vertical deviation (-for illustrative purposes) shown as 66 is obtained and thereafter drilling a straight bore l9. Or I may drill a bore of curvature less than about 3 per 100 feet, then drill a straight hole, as in 20. By continuing the curvature at the same degree of curvature, as in bore l9, until a total vertical deviation of is obtained, I may drill a horizontal bore as in i8 and its straight tangent 20'. Or the bore may be of constant curvature from the original vertical to the objective formation, as is illustrated by 20 and its arcuate extension 2|.

While I have explained the principles of my invention employing, in Fig. l, bores starting from an initial substantially horizontal or substantially vertical direction, I may also start my bore at an acute angle to the horizontal or vertical, and in that manner obtain curved bores substantially identical with those illustrated in Fig. 1, but differing from these curved bores by an acute angle to the vertical or horizontal.

I may, by starting the bore at an initial acute angle to the horizontal, drill a curved bore which will be directed to traverse substantially in a plane which is inclined to the horizontal. In such case the axis of the bore will have a constantly varying azimuthal direction.

Essentially, I may choose the plane which the bore is to traverse by pointing the axis of the guide casing and by rotation of the curved pipe to advance the casing in the chosen plane, and I determine the arc of the bore in the chosen plane by adjusting the degree of curvature of the pre-set, i.e., pre-curved, casing.

The above illustrates the principle of my invention and illustrates how I may advance a drill farther away from a drill site to tap formations not reachable from such locations or drill sites by conventional drilling. By employing my meth- 0d of drilling I may, from one location, explore a far greater volume of the earth than can be explored by employing prior art methods.

This also illustrates how by employing my preset curved control casing and advancing the drill with the casing I may, from a given location, reach to depths and regions remote from the drill site not now attainable by present drilling techniques.

After drilling has progressed to the required depth and total degree of deviation by running of pre-curved casing or casings in the hole, I may run smaller diameter casing through the control or guide casing, having removed the drill pipe. This inner casing may be ordinary straight casing, i.e., of no pre-set curvature. The outer control casing will guide the inner casing being introduced, and since the degree of curvature of the outer control casing does not stress the inner casing being run beyond its yield point, it will straighten out as it passes out of the outer control or guide casing and enter any straight bore forming an extension of the curved bore. After a section of inner casing equal to the length of the straight bore section is introduced, additional sections are then added to be placed within the curved section, i.e., to be positioned inside the pre-set curved outer guide casing. This portion of the inner casing may be straight and thus be under flexure inside the control casing, or may itself be pre-formed and stress relieved to conform to the arc of the outer guide casing, and thus may, like the outer guide casing, be under no stress imposed by bending. Where the entire bore isof constant curvature, then, since both the control casing and the inner casing will assume the same curvature, both casings may be preformed. and. stress relieved, if desired, to the same degree. of curvature.

While stress-relieving of the casing which is bent tothe desired degree of curvature prior to introduction is advantageous in order to increase the loads which the casings may withstand in the bore, it is not essential where the degree of curvatures does not exceed that which will give the safe working stresses as previously described.

Another important advantage of my drilling process with casing of pre-set curvature arises from the fact that I can orient the casing into the hole so that it is pointed in a given azimuthal direction. Thus, I start the first casing section with its axis pointed me given direction and pass the drill through the casing. I then advance the casing and the'drill through the surface conductor pipe, keeping the axis properly oriented. The diameter of the conductor pipe is sufficiently greater than that of the pre-curved casing so that the pre-curved casing passes freely through the conductor pipe. By preventing the rotation of thecasing as it advances or correcting the rotation so that the curved axis advances always in the same plane, the direction ofthe bore may be maintained as the bore is advanced. Additionally, since the bore is cut to gauge slightly greater than the gauge of the casing and the bit maintained but a short distance in advance of the casing, the bore is cut at a vertical and horizontal deviation determined by the pre-set curvature of the casing immediately above the drill bit, and the casing may be advanced only into the drilled bore which forms a continuation of the arc established by the cased guide section of the bore.

My method for drilling will be further described in connection with the drawings. In Figs. 2, ,3, and .4 is illustrated my procedure for drilling in an initial horizontal direction. Acellar 3,8 is prepared by digginga trench of proper size and preparing a suitable concrete apron ofsuitable curvature. A conductor pipe 39a of suitable dimension and pre-curved .to the proper degree of curvature desired for the bore is jetted and pushed into the ground. It is properly oriented to the desired horizontal and azimuthal ,,direc-, tion for the desired bore andcemented in place at 39. Supports 38a are mounted on the apron 38' and pre-curved sections of conductor pipe in proper orientation are then joined and mounted on the supports and bolted to head 31 of the drilling machine. The conductor pipe is thus oriented to the desired degree of curvature and in the desired direction to act as a guide to the preformed guide casing.

Tracks

23 and 22 on which travel cross heads 24 and 25 are moved in and. out by tWo-wayhydraulic jacks 26 and .21. Mountedon cross head 24 is a motor and mud and rotary drill head and swivel assembly 28 connected for driving the drill pipe. The drill pipe 29 passes through slips 30 in the stationary head 3! and slips 34 in cross head 25. The slips 34 are formed of two parts, one for advancing casing and the other for withdrawing casing from the hole. curved guide casing 33 passes through slips .34 in the travelling cross head 25 and through slips 36 in the stationary cross head 31. The drilling machinery is mounted in a cellar 38 so that the drill advances through the face 39 into the earth through the conductor pipe 3%.

The drill pipe carries a guide 42 slidable in casing 33 and making a snug but sliding fit with the casing. The surface of the guide 42, if desired, is suitably shaped to slide in the casing 33 and The prefluted for returning mud through the casing. The guide 42 is of smaller diameter than the casing, and in this manner the resistance to sliding is diminished, thus minimizing loading of the drill pipe or casing due to any differential vertical motion of the drill pipe and casing. The spacing also permits the rotation of the guide in the casing, thus preventing localized wear on the guide. The drill pipe is rotatably mounted in the guide 32 and carries a reamer H to cut a hole slightly greater than the outside diameter of the pipe and carries a conventional bit 43. The curve of the pipe and the casing shown in Fig. 4 is greatly exaggerated for illustrative purposes.

I separately pre-form the guide casing of de-, sired diameter to the degree of curvature as previously described. Such bending of casing may be made by procedures conventional in other arts, such as by bending or rolling either at the drilling location by suitable rolls or at the mill. The bent casing is then, preferably, stress relieved. Sections of the casing (for example, from 10 to 30 feet in length) are then employed as guide casing.

While I find that pipe bent to a uniform curvature is most desirable, I may also obtain many of the benefits of my method by employing curved pipe made of short, straight sections. Thus, I may cut casing sections into short lengths, cutting at such angle to the axis and welding them together so that the axis ofthe straight pipe sections is tangent to the arc of the curved bore which it is desiredto drill. In other words, an are drawn through the intersectionof the axis of the straight sections and the plane of their joints would be the desired arc. A similar result may be obtained by wrinkle bending the pipe by locally heating the pipe and bending it to the desired degree to obtain such a curved pipe section made up of straight sec" tions. The individual straight pipe sections should be of such length thatthey will not be flexed or bent in the bore under the loads imposed on drilling or running of the pipe sections, but remain rigid and straight. This requires that the adjustment of the length of the individual straight sections to the size, wall thickness, and strength of the metal be such as to give a beam which will not be deflected under the loads imposed on the section during its running or drilling. For example, 5-foot sections would be desirable for most such casings as employed in drilling. In this manner the pipe will pass through the bore drilled ahead by the drill in the same manner as a curved pipe of uniform curvature and may in this respect be considered the equivalent thereof.

I mount the curved guide casing in the slips 36 and 34 properly oriented for the desired direction of the bore in the earth. This curvature is not sufficient deflection to interfere. with the passage of the pipe through the axial slips 3 and 36 at their appropriate spacing. Since, however, the curve of the casing is uniform, the slips 34 and 35 may be placed in a position or be suitably shaped to vconform with the curve of the pipe passing through the slips, i. e., co-axial. In doing this I retract the cross head 25 and pass the casing through the slips 3 and 36 and set these slips. When 25 has been retracted to the right, I introduce another section of easing by setting slips 36, opening slip 3.4,.and withdrawing the head 25 to the left while threading the casing section through the

slips

30 and 34, head 24 being out of the way. I orient the added section to conform to the previous sections of casing and unite the sections, for example, by welding. When sufficient sections have been thus added to fill the conductor pipe 33a and the bore, I then introduce the drill string.

With the head 24 moved to the left and the drill-swivel head 28 removed, I pass a length of drill pipe carrying the reamer 4| and the bit 43. The reamer is of conventional design and will pass through the casing and be extended when it emerges from its end. The reamer will also retract when withdrawn into the casing. I preferably employ flush drill pipe and flush casing so that no external protuberances in the form of joints are presented, although the method may be employed with any conventional drill pipe or casing.

The pipe is passed into the casing by mounting the pipe in the head 24 and passing it through the slips 30 with the slips 30 unseated. The hydraulic jacks move head 24 to the right, thus introducing the pipe into the casing.

With sufiicient drill pipe in the hole to place the drill in drilling position, the drill head 28 is attached to the drill pipe 29 and mounted on head '24. The drill head rotates pipe 29, while mud passes through line through the swivel and drill head 28 and into pipe 25 exiting from the drill and returning between the drill pipe and casing to be discharged into the cellar.

It is preferable to circulate the mud back to the surface in the circular space between the drill pipe and the guide casing, and preferably not between the guide casing and the open bore of the ground. This is because the velocity of this mud through the restricted annular space causes a wash-out of the earth wall. Additionally, if the circulation stops, as, for instance, where :pipe or casing is added, then the mud may set up in the restricted annular space to freeze the casing in the bore and to make movement of the casing difficult.

By rotating 28 and advancing the cross head 24 by means of the jacks 26, with slips 30 in open position, and holding the casing 33 stationary in the slips 34 and 36, the drill is advanced in a direction guided by the casing. The drill pipe, guided by 33, is extended beyond the casing and cuts a bore of gauge determined by the reamer 4|. The axis of the bore is an extension of the axis of the pipe 33. When the drill pipe has been advanced sufliciently to move 42 to the end of the casing, by moving the head 24 to the right, further advance of the head 24 is stopped and with pipe 29 held in slip 30, the slips 36 are loosened. With slips 34 set, the head 25 is moved to the right, thus moving the pipe 33 into the pre-formed bore. Since the curvature and direction of the bore fit that of the pipe section 33, it is moved easily into the pro-formed bore.

Drilling is continued in this manner in a controlled direction and at a controlled degree of curvature of the bore until either 24 approaches 30 or 25 approaches 31, or the approach is made simultaneously by 24 and 25. If '24 approaches 30, then by setting slips 3!], 34, and 36, the drill head is unscrewed from the drill pipe and a new section of drill pipe is introduced. If the head 25 is in close proximity to the head 31, then the slips 36 are set and slips 34 opened and the head 25 is withdrawn to the left, leaving the casing 33 in the slips 36 with its orientation unaltered.

A new section of casing is then introduced. To do this, the following procedure may be followed. With the head 24 retracted to the left, the sections of. pipe between t e heads 24 and 3| are removed. During this operation the drill pipe is positioned on the bottom of the bore. With the head 24 to the extreme left, or removed entirely, a section of casing is threaded over the drill pipe and passed through

slips

33 and 34 and welded to the casing section held in slips 36. This orients the new section and con,- tinues the predetermined curve. The drill head and pipe section are then united on 24, and between 24 and 30 the drilling may continue.

If before completing the curved section it is found desirable to set the guide casing, as, for instance, in order to make a water shut-off, the outer casing may be cemented in place. The curved portion may then be continued by .running an inner string of pre-formed guide casing in the same manner as described for the outer guide casing, employing drills and reamers and guide collars of suitable size. In fact, a number of such pre-curved guide casings may be run inside one another before completing the curved casing, as may be dictated by the requirements of the bore, as will be understood by those skilled in the art.

When the curved section has been completed and it is desired to continue the bore as an undeviated bore, the further introduction of precurved casing is discontinued. The casing may be secured in the bore by cementing or other well known techniques. I may then drill an uncased bore at a tangent to the terminal portion of the drilled curved bore.

I may employ a guide casing in order to drill this straight tangent bore at the undeviated vertical deviation of the tangent bore. Thus, I may introduce straight inner casing, i. e., an initially uncurved casing of sufficient flexibility as described above to follow the curve of the precurved outer guide casing inside the pre-curved casing to enter the straight bore, and advance this inner guide casing and the drill to drill the straight bore.

In this manner the inner guide casing 45, after having passed from the curved portion 33, will tend to assume a position the axis of which is at a tangent to the terminal portion of the curved axis of the fixed casing, due to the release of the flexure force. Thus, this will assure the drilling of a straight bore at a tangent to the terminal section of the curved bore, even without setting a whipstock, and will insure the accuracy of the direction of the bore if whipstocks are set. Fig. 5 illustrates an inner pre-curved guide casing within an outer pre-curved guide casing which has been cemented in place. The outer guide casing terminates near the entrance into the earth and the irmer guide casing is mounted in the slips in the manner described for the outer guide casing. The advance of the curved guide casing and the drill pipe is similar to that described for the pre-curved casing. The drill pipe 29 is held centrally of the inner guide casing 45 (see Fig. 5) by means of a guide collar 42 and a short distance beyond the inner guide casing and carries a drill bit 43 and reamer 4| to drill a bore of gauge sufficient to permit the advance of the inner guide casing 45.

In Fig. 6 is illustrated means and methods for drilling according to my method whereby I drill a bore of constantly varying vertical deviation starting in an initially vertical direction. The conventional drilling derrick 45 is mounted on a derrick sub-structure 41 carrying a conventional rotary table 48 driven in the conventional manner. The sub-Structure carries uprights 49 which support a stationary'beam 59 on which two-way jacks are mounted. The piston rods 52 are connected to a yoke 53 which moves on wheels 54 in tracks 55 mounted on the uprights 49 by structural connection not shown on the drawing but as will be clearto those skilled in the art. Mounted on yoke 53 is a two-way slip 56 carrying a lifting slip 56a adapted to operate, when set, to grip the pipe when the yoke moves upwardly to the left of the pipe, and a lowering slip 56b operative, when set, to grip the pipe when the yoke moves downwardly to force the pipe into the bore.

Mounted above conductor pipe 5'! in a cross beam 58 positioned between uprights 49 and carrying a holding slip 59.

With the yokes 53 and slips removed from. the assembly,.the drill pipe 59 is set in the rotary table and spudded in and a bore of about 30 feet in length is' drilled, sufficient to permit the setting of the conductor pipe 51. This procedure .is the same as .is now conventional in drilling vertical bores.

The yoke or cross head 53 and the slips are then assembled, as previously described. The pre-curved casing 6| is oriented in the desired azimuthal direction andthreaded over the tubing 60 and passed through the conductor pipe 57. Thedrill tube 60 passes through a guide, and the emerging tubing carries a drill, such as 43, and an expendable reamer, such as 4|.

The pre-curved casing passes through

slips

59 and 56 andcarries below the rotary 48 a removable mud overflow chamber 62 to which is attached a flexible mud overflow hose 63.

The usual. swivel and equipment for handling and runningcasing and tubing are provided. Mud circulation is provided by introducing mud through the .drill tubing. Circulation of mud is returned back between the tubing guide casing 6| and the drill tubing 50. Assembled in this manner, the rotary rotates the drill pipe within the guide casing and is advanced.

With slips 59 and 56a open and slips 56b closed, the jacks advance the casing. As the casing advances, the mud overflow chamber advances also.

In this manner the casing and the drill are advanced until the full extension of the jacks has been reached. The holding slips 59 are then set and. slips 5619 released, and the jacks retracted to take-another grip by re-setting 56b. The casing and drill pipe are thus advanced until the topof the'casing arrives immediately above the slips 56a. In this position the kelly 64 has made its maximum travel through the rotary table and themudoverflow 62 isat its extreme lower travel position.

-In order to add another joint of tubing and easing the holding slips 59 are set and slips 56 and 56b are released, and the jacks 5| retracted. With the drill pipe resting on bottom, the kelly drive bushings 55 are removed and another section of pre-curved casing is threaded through the opening in the rotary over the kelly, the swivel 66 having been removed. The overflow sump 62 is unclamped from-the casing and slid over the threaded new section and clamped in its new position at the top of the added section in the position shown in Fig. 6.

The swivel is then set in the kelly and the kelly is lifted so that its end is above the rotary and a section of tubing protrudes above the table. The tubing section is then secured in the rotary by setting slips in the rotary. The kelly is then disconnected from the tubing anda new section of i2 tubing is added and the kelly again joined to the new section of tubing. The tubing is lowered into the well with the kelly set in drilling position in the rotary table and drilling may proceed,

When the required amount of pre-formed casing has been lowered into the hole, the drilling may proceed in the conventional manner as a straight bore at a tangent to the terminal portion of the curved bore by either setting whipstocks or by running inner guide casing, as described in connection with Figs. 1 and 5.

I may also obtain an initial penetration of the drill at an acute angle to the horizontal by tilting the axis of the horizontal drilling machine, illustrated in Fig. 2, so that the axis of the drill stem and the guide casing have the desired acute angle to the horizontal. This I may do by building the foundation of the machine to give this initial tilt.

Other variations of the procedures will be understood by those skilled in the art from the principles and embodiments thereof as set forth herein.

While I have described a particular embodiment-of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, introducing into said bore hole, in accordance with the advance of said drill, casing having a pre-curved section of substantially constant curvature, advancing said casing immediately behind said drill, circulating drilling fluid into and-out of said bore hole during drilling to' remove drill cuttings, thereby guiding=said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said precurve'd casing, and thereafter removing the-drill pipe from said bore hole.

2. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earthwhich comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at apredetermined degree of deviation from the vertical, introducing into said bore hole, in accordance with the advance of said drill, casing having a precurved section of substantially constant curvature, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantiallyequal in degree to the degree of curvature of said pre-cu-rved casing, terminating the advance of said pre-cu-rved casing and continuing the advance-or said drillpipe to drill a straight bore at the :end of said curved bore and at substantially atangen-t to said curved bore and thereafter removing the drill pipe from said bore hole.

3. A method for drilling deviated boreholes into the earth for the production of formation fluids from the earth-:whichcomprises drilling. a bore hole in the earth by advancing a drill at the end of a drill pipe ata predetermined degree of deviation from the vertical, introducing into said bore hole, in accordance with the advance of said. drill,

casing having a pre-curved section of substantially constant curvature, advancing said casin immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole, introducing a second string of easing into and through said first mentioned string of easing, drilling a bore hole in extension of said curved bore hole beyond said first mentioned curved casing and advancing said second string of casing through said first mentioned curved casing in accordance with the advance of said drill and thereafter removing the drill pipe from said bore hole.

4. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, introducing into said bore hole, in accordance with the advance of's-aid drill, casing having a pre-curved section of substantially constant curvature, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole durin drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole, introducing a second string of easing, precurved to the same degree of curvature as said first mentioned pre-curved casing, into and through said first mentioned string of easing, drilling a bore hole in extension of said curved bore hole beyond said first mentioned curved casing and advancing said second string of precurved casing through said first mentioned curved casing in accordance with the advance of said drill and thereafter removing the drill pipe from said bore hole.

5. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, introducing into said here hole, in accordance with the advance of said drill, casing having a pre-curved section of substantially constant curvature, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole, introducing a second string of easing straight into and through said first mentioned string of casing, drilling a bore hole in extension of said curved b'ore hole beyond said first mentioned curved casing and advancing said second string of straight casing through said first mentioned curved casing in accordance with the advance of said drill and thereafter removing the drill pipe from said bore hole.

6. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, of at least 3, introducing into said bore hole, in accordance with the advance of said drill, casing having a precurved section of substantially constant curvature, of at least 3 per feet, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole.

'7. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, of at least 3, introducing into said bore hole, in accordance with the advance of said drill, casing having a pre-curved section of substantially constant cur vature, of at least 3 per 100 feet, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said precurved casing, terminating the advance of said pre-curved casing and continuing the advance of said drill pipe to drill a straight bore at the end of said curved bore and at substantially a tangent to said curved bore and thereafter removing the drill pipe from said bore hole.

8. A method of drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, of at least 3, intro- .ducing into said bore hole, in accordance with the advance of said drill, casing having a precurved section of substantially constant curvature, of at least 3 per 10-0 feet, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole, introducing a second string of casing into and through said first mentioned string of casing, drilling a bore hole in extension of said curved bore hole beyond said first mentioned curved casing and advancing said second string of casing through said first mentioned curved casing in accordance with the advance of said drill and thereafter removing the drill pipe from said bore hole.

9. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, of at least 3, introducing into said bore hole, in accordance with the advance of said drill, casing having a pre-curved section of substantially constant curvature, of at least 3 per 100 feet, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially-equal in de'gree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole introducing a second string of easing, ore-curved to the same degree of curvature as said firstmentioned pre-curved casing, into and through said first mentioned string of easing, drilling a bore hole in extension of said curved bore hole beyond said first mentioned curved casing and-advancing said second string of pre curved casing through said first mentioned curved casing in accordance with the advance of said drill and thereafter removing the drill pipe from said bore hole.

10. A method for drilling deviated bore holes into the earth for the production of formation fluids from the earth which comprises drilling a bore hole in the earth by advancing a drill at the end of a drill pipe at a predetermined degree of deviation from the vertical, of at least 3, introducing into said bore hole, in accordance with the advance of said drill, casing having a precurved section of substantially constant cur-' vature, of at least 3 per 100 feet, advancing said casing immediately behind said drill, circulating drilling fluid into and out of said bore hole during drilling to remove drill cuttings, thereby guiding said drill and forming a curved and cased bore of a curvature substantially equal in degree to the degree of curvature of said pre-curved casing, and thereafter removing the drill pipe from said bore hole, introducing a second string of normally straight casing straight into and through said first mentioned string of casing, drilling a bore hole in extension of said curved borehole beyond said first mentioned curved REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 550,783 Elliotte Feb. 28, 1895 1,283,191 Hughes Oct.29, 1918 l',"36.7',04'2 Granvil1'e' Feb; 1,1921

1 1 6075613 Upsonanew NOVl 23; 1926 1,919,931 Craig July 25, 1933 1 ,944,838 'Hill s Jan. 23', 1934 1,948,707 Gilma ne Feb. 27, 1934 1997 312 Satre V Apr; 9,1 1935 2,018,007 Brewster Oct. 22, 1935 2,198,016 Rogers et a1. 1 Apr. 23; 1940 2;296.,161 7 Hall, Jr. l -.'S'ept.15 1942 2,336,334 Zublin s Dec. 7, 1943 FOREIGN PATENTS Number Country Date 20;61l Great Britain Sept. 30, 1908 OTHER REFERENCES The OiIlWeekly, Oct. 1, 1945, p,p."35'-39,' Hori- Zontal Drilling. I

Brantly, Rotary Drilling Handbook, 4th .ed/. (1948), pp. 273-286.