US3088520A

US3088520A - Producing fluid from an unconsolidated subterranean reservoir

Info

Publication number: US3088520A
Application number: US719951A
Authority: US
Inventors: Alexander B Hildebrandt
Original assignee: Jersey Production Research Co
Current assignee: Jersey Production Research Co
Priority date: 1958-03-07
Filing date: 1958-03-07
Publication date: 1963-05-07
Anticipated expiration: 1980-05-07

Description

y 1963 A. a. HILDEBRANDT 3,088,520

PRODUCING FLUID FROM AN UNCONSOLIDATED SUBTERRANE'AN RESERVOIR Filed March 7, 1958 2 Sheets-Sheet 1 FIG. 2

FIG?! Alexander B. Hi ldebrcmdr Inventor Attorney y 7, 1963 A. s. HILDEBRANDT 3,088,520

PRODUCING FLUID FROM AN UNCONSOLIDATED SUBTERRANEAN RESERVOIR Filed March 7, 1958 2 Sheets-Sheet 2 P o a 47 v a n Alexander B. Hildebrand: Inventor 8% 4 Attorney Patented May 7, 1963 ice 3,088,520 PRODUCING FLUID FROM AN UNCONSOLI- DATED SUBTERRANEAN RESERVOIR Alexander B. Hildebrandt, Tulsa, Okla, assignor, by

mesne assignments, to Jersey Production Research Company Filed Mar. 7, 1958, Ser. No. 719,951 8 Claims. (Cl. 16633) The present invention relates to a system for the completion of oil, gas, water or other type wells. It relates more particularly to a method of completing a well which has penetrated an unconsolidated subterranean reservoir. It is known in the art of producing hydrocarbons from underground reservoirs that many wells are sometimes drilled into or through a loose or unconsolidated formation containing hydrocarbons. When such a well is placed on production, sand is eroded from the formation and is carried into the well by the flow of fluid and is produced concurrently therewith. This entrained sand presents many producing and operating problemsamong them being severe erosion of the tubing, valves and other equipment which are used in the production of oil and gas. In severe cases sufficient sand may be carried from the formation with the flow of formation fluids to cause the overburden to crumble, thus filling the cavity through which the fluids are produced. This crushing of the overburden can have the effect of stopping the flow of hydrocarbons from the producing formation to the well bore completely. This in turn causes expensive workovers and in some instances even abandonment of the productive formation. In more severe cases the casing may be caved in by crumbling or failure of the overburden resulting in the loss of the entire well.

Multiple completion and permanent type well completions are not practical in those instances in which large amounts of sand are produced concurrently with oil or gas. This type of completion is discouraged largely due to probable increased mechanical difliculties and failures caused by the sand carried in the formation fluid. If sand is produced with this fluid, surface equipment for the removal of sand must be installed and used before the formation fluids can be further utilized. It should also be noted that the problem of sand control is also often encountered in the production of water from water Wells.

Broadly the invention concerns a system of completing a well which has penetrated an unconsolidated subterranean reservoir in a manner such that fluid may be produced from the unconsolidated formation without encountering the problem of sand entrainment in the production of reservoir fluid, as such entrainment of sand is eliminated or at least reduced sufliciently as not to be a problem. In the practice of this invention the casing will normally be set through the unconsolidated formation in which the completion is desired. The casing is then preferably uniformly perforated throughout the length of the casing which is opposite the unconsolidated formation. The perforations are also preferably uniformly spaced circumferentially. The perforations at the level at which it is desired to for-m the plastic wafer are isolated from the remaining portion of the perforations by suitable packers and tubing arrangements. A plastic is then injected through these isolated perforations into the formation and at the same time a fluid not harmful to the reservoir and which Will not later impede the flow of fluid therefrom is injected into the formation through the other perforations. For example the fluid injected into the other perforations in an oil reservoir may be oil and in a water reservoir water. The plastic and the oil should have essentially comparable flow characteristics so that the rate of injection of the plastic and the oil is in essentially the same proportion as the volume of the formation which each is to occupy. This technique is used to obtain a substantially horizontal wafer or disc of sand which has been consolidated by the plastic. The plastic is allowed to set, forming a cemented or consolidated wafer of sand. The well is then produced at a rather high rate through perforations below the water in order to clean out the sand immediately adjacent the casing below the bonded mas-s or wafer. This high rate of production leaves a cavity about the casing below the plastic wafer. After a period of time the entrainment of sand normally ceases. However, in most cases it is desirable that the rate of production be reduced, as at a reduced rate the Well can he produced sand-free in a much shorter period of time. This system eliminates the sand production problem as described above primarily for the following two reasons. First, the cavity which is formed beneath the plastic mass increases the area of entry of the fluid from the unconsolidated formation to the well bore or cavity. This increase in area greatly reduces the velocity of the fluid as it leaves the unconsolidated formation and because of this greatly reduced velocity, the amount of sand thus carried will be practically nil, therefore, no sand will be produced; and second, the plastic consolidated mass supports the overburden to a degree sufficient to eliminate any crumbling thereof.

One method of reducing the sand problem which has heretofore been tried is the use of a gravel pack. In that method, gravel is packed into the Well bore adjacent the unconsolidated formation and in some instances the well is produced to form cave-ins or small cavities surrounding the well bore. In order to support the overburden or formation and prevent it from crumbling into the well bore, gravel is packed into such cavities. For the fluid to be produced it must pass through a space between the grains of gravel. As the flow area between the gravel grains is relatively small in comparison to the flow area of the cavity, the velocity of the fluid being produced is still sufiicient to entrain considerable sand. Accordingly, while gravel packing is of some temporary benefit, it definitely does not permit ideal flow conditions, i.e., the production of sand-free fluid. However, this ideal flow condition is obtained in the use of applicants system.

The objects and a complete understanding of the invention will become apparent from the following detailed description taken in conjunction with the accompanied drawings in which:

FIG. 1 is a schematic diagram of an underground unconsolidated reservoir penetrated by a well bore illustrating one manner of completing the well in accordance with the invention;

FIG. 2 illustrates a cavity formed beneath the plastic wafer; and,

FIG. 3 is a schematic diagram illustrating a different embodiment of the invention.

Referring to the drawing and in particular to FIG. 1 there is illustrated the best mode contemplated for carrying out the invention. Numeral 12 illustrates an underground reservoir which has been penetrated by Well bore 14. Reservoir 12 is typically overlain by a layer of shale 16 and underlain by a layer of shale 18. Casing 20 is set in hole 14 and cemented as at 22. This cement 22 may be only above the producing formation 12 or it may be set throughout the vertical length of the casing opposite the formation 12. An inner string or tubing 24 is sus pended within casing 20; and a packer 26 is used to seal the annulus between inner string 24 and casing 20 and is set at a point or slightly below where the plastic wafer is to be formed as will more fully be explained hereinafter.

For the purposes of explaining this invention it is assumed that reservoir 12 is a homogeneous unconsolidated formation or sand body which has a vertical thickness of approximately 20 feet and that it is desired to form a wafer of consolidated sand particles in the upper portion of the reservoir cemented together by a plastic with the wafer being approximately 2 feet thick and extending for a radius of 4 or 5 feet from the borehole. The casing 20 is preferably perforated circumferentially and uniformly through the vertical length of the reservoir 12 with perforations 28 being above packer 26 and perforations 30 being below packer 26. The perforations per unit length above packer 26 are thus seen to be the same as the perforations per unit length below packer 26.

A suitable plastic is selected and is injected through annulus 32 between the inner string 24 and the casing 20. This plastic is injected through perforations 28 and is under sufficient pressure to force it several feet into the formation (approximately 4 or 5 feet radius from the well bore) in a rather short period of about two or three hours or less. Oil is injected through perforations 30 below packer 26. The plastic is injected through perforations 28 and the oil injected through perforations 30 simultaneously and in a manner to obtain a fairly horizontal interface 36 between the injected plastic and the injected oil. This is accomplished by selecting a plastic which has approximately the same flow characteristics as the oil which is injected, and by controlling the rate of the injection of each. The amount of plastic or the rate of plastic injection is proportional to the thickness of the desired plastic wafer compared to the total thickness of the oil reservoir. In the example given herein the rate of the injection of the plastic to the rate of the injection of the oil would be 1:9. The plastic and oil injected should normally be in proportion to the space each is to enter, and in a uniform reservoir it will be in the ratio of the thickness of the desired plastic wafer and the remaining thickness of the reservoir. The lateral distance from the well bore which the plastic is injected is dependent upon the vertical distance below that wafer through which it is desired to produce the formation fluid as will more fully be shown.

The plastic which is used is preferably both water and oil insoluble, particularly after it has set. The plastic must also be capable of adhering to the minute sand particles and binding the unconsolidated particles into a consolidated mass. This consolidated mass may be permeable although this is not necessary as the oil or gas will be produced primarily from below the wafer as will hereinafter be explained more fully. The plastic must also be capable of withstanding the temperature of the borehole formation of interest. The formation temperature normally is in the range of about 100 F. to 200 F. although in some cases it may reach about 400 F. The plastic selected or used must also have a controllable setting time and must be able to flow into the formation rather freely. The flow characteristics of the plastic as it is injected into the formation should be comparable to the flow characteristics of the oil selected to be injected to insure the predetermined voidage desired for the plastic and for the oil. The rate at which a fluid can be forced into a formation depends primarily upon the pressure exerted on the fluid, the viscosity of the fluid, and the permeability of the formation to the injected fluid. Those skilled in the art can readily select a plastic whose viscosity is such that by applying the proper pressure to the plastic and to the oil as they are being simultaneously injected, a desired rate of flow of each may be obtained. Laboratory tests can be run on cores obtained from the formation of interest to select a plastic or plastic mixture having the desired flow characteristics. Satisfactory cores can be obtained by use of special coring devices. In this regard attention is directed to a coring apparatus described in my co-pending application Serial No. 701,840,

4 now Patent No. 2,927,725, Unconsolidated Formation Core Barrel."

There are many plastics known to those skilled in the art which meet the requirements set out hereinbeforeone such plastic is phenol formaldehyde. A phenol formaldehyde mixture can be prepared with a setting time of six to eight hours or more which permits it to be injected before any appreciable viscosity change occurs. Longer setting time, of course, may be obtained. A phenol formaldehyde mixture can readily be prepared which has a viscosity range of about 2 to about cps. It is thus seen that it is a simple matter to select a phenol formaldehyde mixture which has a similar viscosity to most natural petroleum oils which are available for injection. The rate of thickening of the phenol formaldehyde mixture can be controlled by the amount of catalyst added.

It should be noted here that previous attempts to use plastics to consolidate a reservoir from which production is desired to be obtained have attempted to retain at least a portion of the permeability of the reservoir. The attempt to retain the permeability of the plastic was most frequently made by use of an acid catalyst such as stannic chloride. This requirement is not necessary in the present invention and a basic catalyst such as sodium hydroxide can be used. If a basic catalyst is used, the Wafer formed by the plastic consolidaing the sand particles is essentially impermeable. If the wafer is impermeable, the fluid will be produced from that portion of the formation other than through the wafer which will give a longer life to the wafer as it is known that if a fluid is produced through a permeable plastic wafer, the fluid will tend to erode the plastic and eventually the sand will no longer be cemented together into a consolidated mass. The use of a basic catalyst has a further advantage in that a basic catalyst is much easier to handle and is less hazardous than an acid catalyst.

Other plastics meeting the above requirements may be selected by those skilled in the art. Thermosetting plastics are especially contemplated. Such plastics include the various phenolics or phenol (cresol)-aldehyde resins and the amino-aldehyde or urea-formaldehyde resins. Epoxy resins and polyester resins may also be used but as a practical matter are more expensive and might be slightly soluble to some types of crude oil, but for use with most crude oil they too are acceptable. In selecting a plastic or plastic mixture, those skilled in the art would take into consideration the reservoir characteristics and other conditions set out hereinbefore.

After wafer 40 has ben permitted to consolidate into a consolidated mass, fluid is produced at a very rapid rate through perforations 30 to clean out the sand immediately surrounding the well bore, forming a cavity 42 below wafer 40. It will normally be desired to produce fluid from the formation through perforations 30 which are immediately below wafer 40. The lower perforations (below cavity 42) are closed or plugged by known means such as squeeze cementing. Production is limited to those perforations immediately below wafer 40 in order to limit the size of cavity 42 so that the maximum radius of the cavity is less than the radius of the wafer. This permits the undisturbed portion of the reservoir to give support to wafer 40 as shown at 41. As a general guide in determining through what vertical segment of the well production can be obtained, production should normally be obtained through perforations located a distance below wafer 40 not greater than two-thirds of the product of the radius of the wafer times the tangent of the angle of repose. The angle of repose as used herein means the angle which the sloping bank of sand or other material in the cavity makes with the horizontal. If the exact angle of repose is not known, it is normally safe to assume that the minimum angle of repose is 45. With an assumed angle of 45 then the distance below wafer 40 through which production may be obtained is approximately equal to two-thirds of the radius of the wafer 40. Thus by mathematical analysis it is seen that the distance the wafer is to extend laterally is equal to or greater than one and one-half the cotangent of the angle of repose of the material of said reservoir times the vertical distance below the circumferential segment through which it is desired to produce fluid from the reservoir. The lower perforations may be closed off in a conventional manner to prevent production therethrough. After the well is produced at this high rate for a suflicient length of time to create a cavity 42 as illustrated in FIG. 2 with a radius of 23 feet at its widest point, the production of the well is then cut back to a slower rate to obtain essentially sand-free production. The volume of the cavity can be determined by the amount of sand produced. This can conveniently be accomplished by producing the fluid through a sand catcher at the surface and measuring the sand thus produced. The velocity of the oil leaving the formation and entering cavity 42 is thus seen to be greatly reduced from the previous velocity of the oil from the sand to the well bore. When the velocity is reduced to this point, it is thus seen that essentially no sand particles will be carred by the fluid being produced from the formation into the cavity. As the oil in the cavity is essentially sand-free, the rate of velocity of the oil from the cavity to the interior of the casing is then relatively unimportant and sand-free oil will then be produced.

In some instances, especially in reservoirs having a thick section, it may be desirable to create plastic wafers at intervals along the vertical section of the formation. FIGURE 3 illustrates a system which may be used for creating a Wafer at other than the top of the reservoir. A straddle packer assembly 44 is placed in the casing such that packer 46 is above the location of the proposed wafer and packer 4 8 is below such location. The straddler packer assembly illustrated in FIG. 3 is a simplified version of commercially available units such as manufactured by Halliburton Oil Well Cementing Company, PO. Drawer 1431, Duncan, Oklahoma, and illuslustrated on page 2249 of Composite Catalogue of Oil Field Equepment and Services, 1957, 22nd edition, volume 2 published by World Oil, PO. Box 2608, Houston, Texas. For example it is assumed that it is desired to form a second wafer in reservoir -12 which is approximately half way from wafer 40 to the base of the reservoir. Before setting straddle packer 44, casing 20* was perforated at 50, 52, and 54. These perforations were so arranged and located that perforations 50 would be above packer 46 after it was set, perforations 52 would be between

packers

46 and 48, and perforations 54 below packers 48. As explained above, the ratio of the perforations 52 to the number of perforations 50 and 54 (assuming uniform sized perforations) would be equal to the ratio of the desired thickness of proposed wafer 56 to the thickness of the reservoir beneath wafer 40- minus the thickness of proposed wafer 56. As it is desired that wafer 56 be located centrally in the reservoir section below Wafer 40, it is thus seen that perforations 50 should equal perforations 54.

In the system illustrated in FIGURE 3 a plastic such as described hereinbefore is injected through tubing 24 which is attached to and supports straddle packer 44 at 58. The plastic passes through inner tube 60' of straddle packer assembly 44 and thence out through perforations 52. Oil with similar flow characteristicst as the plastic is injected through the annulus between inner string or tubing 24 and casing 20. The oil enters the interior of straddle packer 44- through ports 62 and thence down the interior 64 where it then passes out through ports 66 in the lower portion of the straddle packer and below packer 48 and thence out through perforations 54 into the formation. The

packers

46 and 48 are set with the straddle packer assembly in the normal or conventional manner before the injection of plastic and oil. In order to maintain

interfaces

68 and 70 in a substantially horizontal direction, it is desired that the rate of injection of the plastic to the rate of injection of oil through

ports

50 and 54 be the same as the ratio of the volume of the reservoir to be voided by each. In a homogeneous sand such as reservoir 12v this ratio would be the same as the ratio of the thickness of the desired wafer to the thickness of the remaining portion of the reservoir beneath wafer '40. In this example the distance or thickness of the sand or reservoir below wafer 40 is 18 feet, as wafer 40 was made 2 feet thick. To make a two-foot wafer it is thus seen that the ratio of the oil to the plastic to be injected is 8:1. The volume of oil being injected would of course be divided between

perforations

50 and 54 thus assuring that one-half the oil would be above wafer 56 and one-half below. It is seen that as many Wafers as desired may be reproduced in this manner. After wafer 56 has set in such a manner that it consolidates the sands, the Well is produced at a rather high rate from perforations beneath wafer 56 in the manner described above for the production below wafer 40.

Ini summary it is thus seen that this is a system for completing and producing a well that has penetrated an uncemented or unconsolidated formation. On portion of the unconsolidated formation is then cemented together to form a consolidated mass. 'Fluid is then produced from below this mass for a period of time and at a rate sufficient to create a cavity below the mass. After a cavity of desired volume is created, the rate of production is reduced and sand-free production is obtained. It is, of course, understood that by making the consolidated mass or wafer large enough and creating a correspondingly larger cavity, production at essentially any rate can be obtained without sand production. These consolidated masses or wafers can also be spaced throughout the vertical thickness of the formation about the well bore as desired.

Various modifications of the system for carrying out the principle of this invention may be employed without departing from the scope thereof. For example, it may be possible to have perforations located in the casing immediately below where the plastic wafer is to be formed which are suitable for both the injection of oil for maintaining the interface between the oil and plastic and also for subsequent production of oil therefrom; in this instance then there is no perforation in the casing which requires plugging before the production of oil can be started. It is also recognized that the radius of the wafer may be of such magnitude that production may be obtained from any or all of the vertical section of the reservoir below the wafer. It is therefore intended that the illustrations given herein are for the purpose of illustration and not for limitation.

The invention claimed is:

1. A method of completing and producing a well that penetrates an unconsolidated subterranean formation containing a producible fluid having particulate sand which comprises injecting a setting fluid through the well into said formation through a preselected first circumferential segment of the well, said setting fluid being injected in a volume and under suflicient pressure to penetrate into the reservoir a preselected distance laterally in all directions from said well, said setting fluid being of a character to set as a solid which is substantially insoluble in the reservoir fluid, simultaneously injecting through. the well a non-setting fluid of similar flow characteristics as said plastic setting fluid through said formation other than said segment exposed to said Well with the ratio of the rate of injection of said setting fluid to the rate of injection of said non-setting fluid being substantially equal to the ratio of the vertical thickness of said first segment to the total thickness of the formation less the thickness of said segment, maintaining the pressure required for injection of said setting fluid and said non-setting fluid until said setting fluid is set, thereafter producing said well at a rate to obtain essentially sand free fluid from that part of the formation adjacent said well immediately below said first segment and extending below said segment for a distance not greater than two-thirds of the distance of said lateral injection of said setting fluid and at a rate suflicient to entrain sand in the produced fluid thus forming a cavity under said segment, thereafter producing said well only through said cavity at a rate at which the produced fluid is free of sand.

2. A method of completing and producing a well that penetrates an oil-bearing, subterranean, uncemented formation composed of material having a determinable angle of repose which comprises the steps of injecting a plastic into an upper circumferential portion of said formation through said well, said plastic being injected in a suflicient volume and under suflicient pressure to penetrate into the reservoir a preselected distance laterally from the well, said plastic being of a character to set as a solid which is substantially insoluble in the reservoir fluid, simultaneously injecting a non-setting fluid of similar flow characteristics as said plastic into said formation other than said portion through said well with the rate of injection of said plastic to the rate of injection of said other fluid being substantially equal to the ratio of the thickness of said first portion with the thickness of said formation less the thickness of said portion, thereafter producing fluid from the formation through a vertical portion of the well immediately below said circumferential portion at a rate suflicient to produce sand with the fluid so as to form a cavity below said injected plastic, the height of said vertical portion and the vertical dimension of said cavity not exceeding two-thirds of the product of the tangent of the angle of repose of the material of said formation times said preselected distance and thereafter producing at a rate reduced to obtain sand free fluid and only from said cavity.

3. A method according to claim 2 in which said plastic is phenol formaldehyde.

4. A method as defined in claim 2 in which said plastic has a viscosity in the range of about 2 cps. to about 100 cps.

5. A method of completing and producing a well that penetrates an uncemented particulate reservoir containing a producible fluid, the particulate material of said reservoir having a determinable angle of repose, which comprises injecting through the well a plastic fluid into an upper circumferential portion of said reservoir adjacent said well, said plastic being injected into a suflicient volume and under sufficient pressure to penetrate into the reservoir a preselected distance laterally in all directions from the well, said plastic being of a character to set as a solid which is substantially insoluble in the reservoir fluid, simultaneously injecting through the well a non-setting fluid with similar flow characteristics as said plastic fluid into a circumferential portion of the reservoir below said upper portion with the rate of injection of said plastic fluid to said non-setting fluid being substantially equal to the ratio of the thickness of said upper portion to the thickness of said lower portion, maintaining the pressure required for injection on said plastic fluid and said nonsetting fluid until said plastic has set, thereafter producing fluid from said lower portion of said formation immediately below said upper circumferential portion and at a rate to produce entrained particulate material, the vertical distance through which fluid is produced not exceeding two-thirds of the product of the tangent of the angle of repose of the particulate material times said preselected distance so as to form a cavity below the por- 8 tion of the formation penetrated by said plastic; and thereafter producing the producible fluid through said cavity at a rate reduced to obtain production free of particulate material.

6. A method of completing and producing a well that penetrates an uncemented particulate reservoir containing hydrocarbons which has a material of a determinable angle of repose which comprises injecting through the well a plastic fluid into the reservoir through a preselected circumferential segment of said well within said reservoir, said plastic being injected in a SUffiCiCl'lt volume and under sufficient pressure to penetrate into the reservoir a preselected distance in all directions laterally from said well, said plastic being of a character to set as a solid which is substantially insoluble in the reservoir fluid, said preselected distance extending laterally at least one and one-half the cotangent of the angle of repose of the material of said reservoir times the vertical distance below said circumferential segment through which it is desired to produce fluid from said reservoir, simultaneously injecting through said well another fluid with similar flow characteristics as said plastic fluid into said reservoir other than said segment with the rate of injection of said plastic fluid to said other fluid being substantially equal to the ratio of the thickness of said preselected segment to the thickness of the reservoir less the thickness of said segment, said second fluid being of a character not to harm said reservoir or to impede subsequent flow of fluid therein, maintaining the pressure required for the injection on said plastic fluid and said other fluid until said plastic has set consolidating the reservoir opposite said preselected segment of said well, thereafter producing fluid from immediately below said preselected segment at a rate to entrain particulate material thus creating a cavity below said segment, the largest radius of said cavity not to be greater than about twothirds said preselected distance, and thereafter producing hydrocarbons from said well at a rate reduced sufficiently to obtain production free of particulate material.

, 7. A method of completing and producing a well that penetrates an unconsolidated subterranean formation containing a producible fluid which comprises: forcing a setting fluid into a wafer-shaped portion of said formation about said well, said setting fluid being of a character to set as a solid which is substantially insoluble in the fluid within the reservoir; producing the formation from immediately below said wafer-shaped portion after the fluid has set at a rate sufficient to entrain sand so as to form a cavity, said cavity not extending laterally beyond 7 said wafer; and thereafter producing said well from said cavity at a rate to obtain sand-free production.

8. A method of completing and producing a well that penetrates an unconsolidated subterranean formation containing a producible fluid which comprises: consolidating a wafer-shaped portion of said formation about said well, thereafter forming in the formation immediately below said wafer-shaped portion a cavity having a wall sloping upwardly and outwardly from the well, said cavity surrounding said well and in communication therewith and lying wholly beneath said wafer, and then producing said producible fluid through said well from said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 1,530,221 Uren Mar. 17, 1925 2,168,116 Crites et a1. Aug. 1, 1939 2,368,424 Reistle Jan. 30, 1945 2,713,906 -Allen July 26, 1955 2,784,787 Matthews et a1 Mar. 12, 1957 2,825,408 Watson Mar. 4, 1958

Claims

8. A METHOD OF COMPLETING AND PRODUCING A WELL THAT PENETRATES AN UNCONSOLIDATED SUBTERRANEAN FORMATION CONTAINING A PRODUCILE FLUID WHICH COMPRISES: CONSOLIDATING A WATER-SHAPED PORTION OF SAID FORMATION ABOUT SAID WELL, THEREAFTER FORMING IN THE FORMATION IMMEDIATELY BELOW SAID WATER-SHAPED PORTION A CAVITY HAVING A WALL SLOPING