US2666620A

US2666620A - Oil well coring method

Info

Publication number: US2666620A
Application number: US30102A
Authority: US
Inventors: Henry J Welge; Robert C West
Original assignee: Standard Oil Development Co
Current assignee: Standard Oil Development Co
Priority date: 1948-05-29
Filing date: 1948-05-29
Publication date: 1954-01-19
Anticipated expiration: 1971-01-19

Description

,patented Jan. 'iai-ess.

" sample of this core.

pany, a corporation A'The present;v invention relates to a novel procedu're for obtaining core samples of unconsolidated material in an oil well. In particular the invention relates to av method and'apparatus for injecting a, thermoplastic material vinto a porous* formation to be sampled thereafter. The thermo'- plastic material is allowed to harden, and by, conventional curing procedures a core is.recov ered.` By suitably extracting the thermoplastic A material the .core may then be returned to its original state as it exists inthe earth. l'rx accord-4 'ance -with the preferred adaptation ofthe present f invention a chemically inert naturally occurring petroleum wax is employed. This material will consolidate the formation' to be removed and tested 'and is itself readily removable when the consolidated sample is brought to the surface.

' Inthe expensive procedure of drilling into th'e K earth in an attempt to find oil` deposits, it is important to obtain all 'possible information as to the nature of the earth through which drilling operations'are conducted. An important procedure generally .conducted in this connection is to obtain core samples of the strata penetratedv by the drilling tools. Thus in conventional operations whena point has been reached in the bore hole at which it is desired toobtain a core sample, the drill bit is removed from the' drill stem and a core barrel substituted.' The 'core barrel will cut a. core of the particular` stratum existingl at the bottom of the drilling hole, andwill permit bringing to the surface ofthe eartha the conventional procedures heretofore known to Y the art are not suitable for recovering an unaltered sample of the core.` Thus, if the stratum to besampled consists of. a loose sand formation,

' such as a type of sand in which oil may be found,

due to the loose unconsolidated material of the sand, penetration by a coring .tool disrupts the structure of@ the sand and defeats all attempts to obtain an unaltered coresample. It' is, therefore, the principal object of this invention to-provide apparatus and procedure for suitably obtaining an unaltered sample of unconsolidated material encountered in drilling operations. Y

` In accordance with the present invention. therefore', when conventional drilling operations l `lowered to the bottom o t the hole, suitable for injecting a' thermoplastic material into the un- YPersianas. (61. s-LFD However, when the stratum. u at the bottom of the drill hole is 'not a consolidated stratum, butis a loose and porous stratum.

of llelaware 0,102

consolidated formation. The thermoplastic ma v terial is lowered to the bottom ofthe Well Vso that it may be injected at a. temperature of about-1 5' to 60"A F., above the temperature existing at bottom of the bore hole, The thermoplastic ma.,

terial is so chosen than itwill harden at the be tom holetemperature After the material has vsuitably been injected into the unconsolidated" stratum, and time has been allowed for it to-` :o hraden in the stratum, the material will bind the particles of the porous formation sufficiently.. to' permit a conventional curing tool to 'cut and' rey in move on unaltered sample of the formuion,

When this samplehas been brought to :the'sur- .l5 face of the earth, it may be removed from the caring tool .and may be suitably imbedded in a hard plastic or metallic cylinder so as to support it during heating or solvent 'operations adapted o remove the thermoplastic material. The 'result will be a true. and unaltered Sample 0f the' .{inconsolidatedstratum, just as it existed atthe; "lott'om of the bore hole. It is possible to examine this sample forv porosity, permeabilityyconnate. water. and production characteristics as well as other properties. It is apparent that information will be extremely valuable -in assessing the oil producing possibilities of the particular stratum sampled'. It is contemplated that any thermoplastic maV Alerial may be injected into the unconsolidated stratum. lHowever, as pointed out heretofore the. i preferred adaptation of the present invention is to utilize a hydrocarbon or ester'wax, preferably a naturally occurring petroleum oil'wax. A basicv requirementA of the material to be injected is that it be liquid at a temperature somewhat above the i bottom hole temperature', andthat it solidify at. the temperature existing atthe bottom of the Well. It is apparent that the choice of the material will be' dependent onthe .particular .ternperature 'existing at the bottom of the bore hole,` which factor in turn is in large .part dependent ontlie depth of the well. It is presently thoiigiijsY that although other suitable-thermoplastic ma- Y terials may be generally employed, the preferred. material of theV present invention comprises pc- AtroleuinWax. 'A f Other waxes such as natural or synthetic waxes may also 'beutilized in practicing this ln- 59. vention but'normally no advantage be 'gained thereby. and a petroleum wax of suitable zneltinx point will be found to be ventirely satisfactory from the standpoint of both oost gnd'avaiiabinry. of course. where bottom hole temperatura; 'are 55 higher than the melting points of available.

troleum waxes. use must be made of higher melting hydrocarbon or ester waxes such. as carnauba wax, ozocerite. montan wax, or of other materials thctic4 waxes are v commercially available with melting points in therange of G-300 E. and

in the vicinity ofv 400 F.

By employing waxes, particularly naturally occurring petroleum waxes, in accordance with .i the preferred embodiment of the present i'iven` iondesirable results are obtained. outheretofor'e the temperature in the bottom of at least one is reported having amelting point aeseaofff.. l

such as asphalt, rosin, etc. A number of syn As pointed any particular bore hole will vary generally with the depth. Thus the particular wax fraction whiclris most suitable for an particular depth may be readily selected. Furthermore by employing these waxes as consolidating mediums it -is easy to remove the same readily without damage to the core at the surface, thus preserving the core sample in its natural condition. This represents a distinct' improvement and advancement over other consolidating 'mediums since in l many cases removal of the mediums from the core at the surface results in damage to the sand grains. '-At thepresent time waxes are commercially available having any desired melting pontfwithreadily choose and obtain a suitable wax for a `particular bottom. hole temperature. For `instance the following data illustrate the variance of bottomY hole temperature with depth and the melting point of the wax to be used as well as the desirable temperature for injecting the Wax into the bottom hole formation.

1 in a. fairly broad range making it possible t0- l I t .is to bc understood that bore hole temperatures' vary widely l with depth depending upon the particular area ox the earths surface.

The above are merely illustrative. When with the area the melting points of the waxes utilized should be Y adjusted accordingly.

" l -Although the melting point of the particular wax fraction'use'd may vary appreciably it is evident that the melting point must be higher than the temperaturewhich exists ln the bottom of the bore hole. In general it is preferred that the particular wax fraction used have a melting point in the range from about l5`to 25 F. higher than bore hole. Similarly it is preferred that the particular wax fraction be injected into the bore hole at a. temperature in the'range. of about 20 tlcular fractionA employed. l The apparatus employed to inject the wax into the unconsolidated stratum may be of great-.sim-

- pllcity, although it lspreferred to employ a' more the temperatures vary large heating coil may be employed together with the temperature-existing in the bottom of the F; to 50 F. above the melting point o the par- Acouzipllcated apparatus. hus'all that is actually wax injection.

required to inject the wax is a suitable 'wax con taining chamber. suspended at the bottom of a drill stem, having valves on either end of the chamber to control the influx and out-now of liquid through the chamber. As the was; may be heated at 'the surface of the earth to a fairly z high temperature before placing the Wax the' chamber referred to, the chamber may be lowered to the bottom of the bore hole while the -wax still in a liquid stage.

In the event that the distance through which the chamber must be lowered is so great that 'r considerable time be spent in the descent,

solidiiicationA of the wax may be prevented by providing the wax containing chamber with sumcient insulation for this purpose, or by iitting the chamber with suitable heating means. as for ample electrically heated coils.

When the chamber has been suitably lowered to the bottom of the hole, by opening the valves of the chamber, drilling, mud or other iluid may Y be pumped' down the drill stem to force thewax out o the chamber and into the stratum at the;

When the wax has suitably been injected in this manner, it only remains to remove the chamber from the -well and to use conventional caring equipment to'- bottom ofthe bore hole.

recover a suitable sample of the formation.

As stated,` however, it is preferred to use af somewhatA more complicated apparatus for the the bottom of the bore hole.Vv For this purpose it is convenient to employ an electrical heating l coil in the wax containing chamber, suitably supplied with electricity through conductors leading` to a source of current at the surface of the earth so that the wax may be maintained in aliquid state regardless of the time, or cboling eiects encountered on lowering the wax con- I taining chamber to the bottom of the bore hoie.

^ The heat requirements 'may be .calculated sufciently accurately' so thata given size electrical cuil may be used. or alternatively an excessively a thermo'c'ouple placed in the' wax to enable control of the heat input, and, thus, to permit control of the temperature of the wax. It is also preferred that the apparatus have means forv sealing off the bottom of the bore hole to prevent any' bypassing of the wax released from the chamber, and to insure suitable penetration of the wax to the `porous stratum. The bore hole may be' conveniently sealed off by employing an elastic packer which may be inflated to contact and thoroughly seal the lowermost part of 'the bore hole. Thereafter on applying pressure to force the wax from the wax chamber, the wax will be positively driven into the lower part of the bore hole without any possibility of bypassing the wax upwardly in the drilling hole.

In this embodiment of the invention, it is preferred to employ an auxiliary air containing chamber which is 'opened simultaneously with the 'release o1 the wax se that displaced arming mud and fluids .trapped below the elastic packer may be pumpedinto the'air lled chamber.

A further extremely simple form of appara-tus that could be used consists merely of a section 'of tubing, filled with the melted wax. and closed at the bottoni by a'trip valve which would operate as described below. The section of tubing,v

with the liquid wax inside temporarily prevent- Y For example, it is preferred to heat the wax containing chamber to insure the ,liquefaction of the wax at the time it reaches hole,

ly be dropped into the open drill hole. Upon striking bottom, the trip valve would be ar.. rangedso as to open', and pumping iiuid into ,the hole thereafter as suggested hereinbefore would cause the melted wax to enter the unconvided4 at the top with a hook or other means,

' well known Ato the art, to facilitatewithdrawing it from the hole after the injection of Wax was nished, in order that-the operation of. cutting a core consolidated by the solidied wax .mightv next be carried out.

Aed from escaping by the'trip valve, would meresolidated formation. It will be understood that in practising" this embodiment of the invention,

the section of tubing would preferably be pro- As stated, once the thermoplastic material,`

such as wax has `seen injectedinto the unconsolidated stratum, and has been allowed to harden, any conventional coring procedure may; be em- 'played to recover a suitable sample. As the coring procedures used are no part of the present invention, they willnot be' described in detail. It is to be understood, however, that .the pror meduresused` generally are suitable for cutting a cylindrical core of the stratum to be sampled,

and of holding the corev in the core' receiving core by applicationlof suiclen heat to melt it, although the use of solvents isordinarily more desirable. It is also often helpfulto apply heat when employing solvents, so as to enhance the solvent action. After the wax has been re moved from the core, it is apparent that the core will be in exactly the condition in which it existed at the bottom of the bore hole prior -to sampling.

Preferred embodiments of the apparatus and procedure of this invention incorporating thematures vmentioned above ,are fully described' in connection with the accompanying drawings in which e Figure lrepresents the simplest form of apparatus' 1- suited for the purposes of this invention, in which Figure 2 represents a preferred form of apparatus incorporating an elastic packer :nid a .mud receiv- 1 ing chamber, and in which Figure 3 represents an chamber sov that the core may be brought to the surface of the-'earth'. If desired, orienting devices may be employed with the core cutter so Y as to get information as to the. exact orientation of the core in the earth. Knowing this, iniormaton it possible to tell exactly how the lore obtained is positioned iri the bore hole with respect to the directions of the compass, and

' with respect to the inclination of the core sampier.

When the core sample has been brought to th suriaceof the earth, any suitable procedure may be used for eliminating the thermoplastic mat-:rial so as to' restore-the sample to its originall condition, as it was in the bottom'of-the'bore However, it is'preferred to remove the j cylindrical core frointhe caring apparatus and to grind down the two faces of the core to provide atpara1lel faces. porous plate is placed over each of these faces andthe entire cores v imbedded in a suitable supporting material, the

exterior sides of the` porous platesbeing, of

course, left exposed. For example, the core may be imbedded in a hard plastic, suchlas phenolformaldehyde, urea-formaldehyde cr methylmethacrylate, or it may be imbedded in a' lowtemperature-melting alloy such as Woods metal.

` The only requirement for the material to be used is that it harden at 'a .relatively low temperature and that it have sufdcient strength to hold the core sample rmly. When this has been done it is possible tn force a suitable solvent as for ex-z "ample'benzene, toluene, or an alcohol, preferably butyl or amyl, through the perforated plates so as to remove the wax material originally used to consolidate the porom formation sample. Certain other solvents-may also be used for the above purpose, such as a naphtha fraction or a. chlorinated hydrocarbon, for example carbon tetrachloride or tricl'iloroethylene. Selection of a.

forwnx, crude oil. andaspiialt. In addition',the

material' other than wax is used, suitable solvente Aso suitable solvent is governed by its solvent power solvent used must not attack or react with theA -rnetalor plastic used to encase the core. It is o1 -course to be understood that if a thermoplastic necessarythat thematcrial be in liquid state.v

This may-be readily accomplished by heating the' kwat: material to a-high temperature before plac-Y for removing that particular material will be se- L l lected; It is'also possible to remove the wai; vfrom there additional element which may be attached- -to thelbottom portion of the form of apparatus repre` sented by either Figure l or'Fgure 2.

Referring now toFigure 1,' numeral l designate a bore hole drilled into `the earth. It is to be un-- derstood thatthe bore hole is drilled until an unconsolidated foi-matiou 8 is encountered. At time the method and apparatus of the present invention are employed 'for the purpose of obtain-l ing an unaltered sample of the unconsolidated stratum. The drilling apparatus is removed from the bore hole and the apparatus illustrated in Figure `1 is threaded on to the drill stem 2. The

apparatus comprises a case 3 which may be threaded on to the drill stem 2, or may be. at-

tached to the drill stern in any desired manner. Aninlet port 4 to the chamber 3Vv is closed by a. solenoid operated lvalve 5, While the outlet 6 at Y the lower end of the chamber is terminated by a.

Similar solenoid `valve 'L These valves are of con vcntional construction,v being suitable for operation on activation by an electrical current supplied to the valves through-conductors not illus-1 trated in the drawing. it i's to be understood that'- the chamber 3, rn ade of any suitable material, has been nlled with a. thermoplastic material, pref' f erably wax, before the chamber is lowered into l the bore hole. As stated, this wax material is a' wax which will solidify atthe temperature existent` at the bottom of the bore hole. This temperature..

will generally be known with sufficient accuracy by ascertaining the depth of the bore hole so as to permit choice of a suitable waxmaterial. Alternatively, prior to the choice o f the wax, a thermocouple or other temperature measuring mea-ns" maybe lowered tov the bottom of bore hole l tol accurately determine the temperature. In

case wax is introduced into chamber 3 prior to loweringl the chamber into the bore hole. When chamber-3 has been lowered intothe bottom of the' bore hole; and suicient'weight bas been 'puton the chamber to forc'e the opening 5 somewhat-into v the porous formation. solenoid valves 5 and l are opened and-a. hydrostatic pressure is supplied. to the drill stern in order to force the wax from 'Chambord 'into the Porous formation 8. The hy.-

dro'static pressure to vforce the material from chamber 3 may be readily supplied by filling the' drill stem with drilling mud and then applying a e drill pumping force to. the. drilling mud in th stem. It is tobe understood that'while the ci amber is lowered and up tothe time the therznoplas4 tic material is f oreed'4 into the chamber 3 it isi lng it in the chamber 3. Alternatively, the material 'can be kept in a liquid .state by vsupplyingheat.

yirig eiexfants m' une chamber 3, as'wn be eey scribed in connection with Figure 2.

` paratus, adjacent the drill stem.

Referring now to Figure 2, a preferred embodiment of' the apparatus of this invention is illu'strated, Again the apparatus is threaded on to, l 'or suitably connected to the drill stem 2 to support the apparatus, and to permit it to be lowered into the bore hole I. In the embodiment illustrated, an inner chamber I' is provided with an annular spaced outer chamber l l and an elastic.

packer l2, which is shown to be in the inated pof a'oric. The packer is thus of such anature that it can be inated or de ated so as to permit sealing the packer against the bore hole so as to eiectively seal the hole.' As illustrated, an inlet port I3 maybe'provided at the upper end of the ap- This opening provides a fluid passageway into the annular space between the elastic packer i2 and the outer part Vof 'chamber ill. The passageway i3 is provided with a solenoid operated valve l to control influx A of fluid beneath the elastic packer.' It is apparent that when drilling mud or other uid is pumped down the drill stem 2 while valve i4 is open,the.

`elastic packer will be inlated, thus forcing the packer against the walls of the bore hole, as illustrated. 'A passageway l5 is provided from the in-` ner part of the elastic packer to the well, external o f the drill stem 2.- 'I'his passage is provided with a check valve 16. This check valve may consist j Vvalve I6 will open at a pressure sufciently high to insure full inflation of the 'packer i2 without permitting.` rupture of the packer.

when drilling mud is pumped down the drill stem sitiom.vr The elastic packer l2 may be of conventiene? construction The-packer comprises a deinv- `ble membrane such as rubber, clothor metallic sign. The header maycontain attached thereto f al; inch. As the apparatus is lowered into the bore Vof a spring operated valve so placed as to prevent 2 through passageway i3 into the packer l2, checl;

valve i8 will remain ,closed until the packer is fully iniiatedl at which time the valve willA open to permit bypassing of the ii-uid: The inner chamber IU of the apparatus is the' chamber to be illed with the `Wax materiaL chamber to be lled with air prior to lowering the apparatus into the bore hole. Both of these chambers are supplied with valves to be operated as will be described. Chamber 'ID may also be The chamber ll is a supplied with an electrical heating coil l1 which may operate through electrical conductors extending to the surface of the earth.

these conductors as well as the conductors lead#- For simplicity heat may be supplied through coil l1. When the ing to the solenoid valves have not been shown in the drawing. As in Figure l the wax lled chamber is sealed at the upper end by solenoid operated valve I8, closing inlet passageway I9, while the h'ole has been v'drilled until an unconsolidated for- 2 is placed on the drill stem and chamber l0 is chamber isterminated `at the lower end by passageway 20 provided with a. solenoid valve 2l. In using the apapratus of Figure 2. when the bore lled with wax or other thermal setting material' as formerly described. .The chamber 1s filled with; air, or if desired may be evacuated. The apparatus is then lowered into the bore hole,

' it beingunderst'ood that packer l2 ls-.in the dei flated position while lowering. f Figure 3 illustrates a, header element which can be attached'to the base ot the apparatus shown sageway 2U controlled by valve 2l.. Attached` to ments or prongs 3l. These prongs or tubes may tion. For convenience, they are depicted as of valve 14 may be closed. It will be noted that inas Wax may then be Yextruded from the* chamber Therefore,

out into'the surrounding forma-tion through suitl able openings 32 in the prong elements.

' material has been adjusted to insure that the ma- It is apparent that sulicient pressure is applied. I to the drill stein 2 while extruding the wax to in-v es' j mation 8' is encountered; the apparatus of Figure thermoplastic material, as described, it is only hole and to permit suihcient time for the thermomaterial hardens. Theresult will, therefore, bo j ln either Figure 1 or Figure 2. For convenience, this header element is shown .connected to the base of the apparatus of Figure 2; header element 30 communicates with chamber i0 through pas the header element are a plurality of tubular elebe of any suitable shape, that is, they may he of circular, square. triangular or similar cross sec- Y circular cross section, that is, they are of gen-i erally cylindrical shape, l 1

The header element may be of any suitable deany number of hollow tubes or prongs. In general it is preferred that the number of these prongs range from 2I to 8, 5 being the preferred hole these prong elements 3i penetrate readily into formation 23'.' When the apparatus has been lowered .until the prongs 3| are imbedded in the porous formation 23, the elastic packer l2 is inhated by pumping fluid, such as drilling mud through the drill stem, valve I4 having been pre-- viously opened. Pumping of uuid is continued until enough fluid has been provided to insure the' opening of the bypass chamber l5, at which time flation of the packer l2 eectively seals' the lower part of the bore hole l so as 'to provide a sealed portion of the well immediately above the porous stratum 2S. The. thermo plastic material such IU by opening valves i8 and 2| and by applying suitable pressure through the drill stem 2. The.` Wax iiows into header element 35 and into the prongs previously described. The wax then iiows It ist@ be understood' that the heating, effect' of coil i7 prior to extruding the thermo-plastic teri'al is in liquid state. As formerly indicated, this may readily be achieved by placing a thermocouple in chamber IB, not illustrated in the drawing, so as to indicate the temperature of the thermoplastic material in order that sufhcient thermoplastic material is forced from the chamber ID, the pressureapplied will cause the material to be forced downwardly into the unconsoldated stratum 23, tending to displace fluid contained in the stratum.

sure displacement of uidapproximately equal to'l the volume of chamber Hl. By this means saturatin of the stratum 23 with the wax in the vicinity of the apparatus described will be insured. Having suitably filled a porous stratum with a necessary to remove the apparatus from thev drill plastic material to reach the temperature existent at the bottom of the bore hole, which as has been stated is a. temperature at which the e i wr 1o that the porous stratum 23 adjacent to the bot-Y are bound, cutting a -sample of the said stratum f tom of the bore hole will be saturated and sup- J imbedded in the thermoplastic material, removported by the hardened plastic material. By eining the said sample to the surface of the earth,` vploying' conventional colin; tools, a core ofthv and removing the thermoplastic material from stratum may then be cut andthe sample cut 5 said 4samplewhereby a bore is obtained rherein i brought to the surface ofthe earth. It is prethe sand grains arein apcondition substantially, ferred that when the sample is recovered, the the same as existed in said s'traturn, said thermoendsof the sample'be ground to provide smooth plastic material bein?,r characterised by having a x faces, to accommodate porous supporting plates melting point in the range v0:1? from 15 F. to 25 such as fritted glass or wire screen. This asl0 F. above the temperature existing in Vthe bore i sembly of core and the two end plates with the hole at the point the sample istaken, said thermov exception of the face of each end plate that is plastic material being injected into 'said stratum v. not in contact with the core, is then entirely imat a temperature in the range of from about 20\.

bedded in a suitable plastic, such' asphenol- F. to'about 40 F. above' the temperature at which j aldehyde or urea-aldehyde resin, polvvinyl'aoe l5 said thermoplastic material melts. tate; plvmethylmethacrvlate. polystyrene,- or the 2. A process as defined by claim 1 wherein the like, or in a low temperature meltngalloy, such Vthermoplastic material comprises a wax.. as Woods metal- When this material has suit- 3. In oil well operations the process of obtainably solidified, the core is subjected to solvent ing unaltered core samples comprising the stepsv action through the perforated end plates to per- '2o of injecting al thermoplastic material into the ,mit removal ofthe thermoplastic material origistratum to be tested, hardening said. material in 1 nally used at the bottom of the bore hole. In-A the stratum whereby the particles of stratum are the casevvhere wax is used, for example, a suitbound, cutting a. sample of the said stratum ernable 'petroleum solvent or other solvent s'uch as bedded in the thermoplastic material, removing previously mentionedmay be employed to dis- 25 the said sample to the surface of the earth,. solve the wax and return' the core tothe original smoothing two opposite faces of said sample, apcondition it was in at the bottom of the bore hole. plying porous supporting plates to said faces, emf xThese steps having been carried out it will be bedding the resulting assembly, with the excepapparent that the grains of the original uncontio-n of the outer faces of raid plates, in a hard- A solidated 'stratum have been restored to exactly '30 enable medium, hardening said medium andretheir original condition underground with removing said thermoplastic material from said speci; to the roti; composition vand -orientation o.' sample through at least one of said plates, wherei v the grains.' Consequently, the core sample may by a sample is obtained wherein the sand grains 1 be examined for porosity, conn-ate water, per- Aare in a condition substantially the same as exmeability and any other properties as desired. 35 istedinsaid stratum'. Y

' As described,` therefore, the present invention HENRY J. WELGE. relates to a procedure and apparatus for obtain-' ROBERT C. WEST. ing an unaltered sample of unconsolidated strata I Q encountered at the bottom of a bore hole.. In References Cited in the le of this patent accordance with the invention a suitable appa.- 40 UNITED STATES PATENTS ratus is employed to inject into the stratum a Number Name i Date fin the stratum whereby the particles. of stratum

Claims

1. IN OIL WELL OPERATIONS THE PROCESS OF OBTAINING UNALTERED CORE SAMPLES COMPRISING THE STEPS OF INJECTING A MOLTEN THERMOPLASTIC MATERIAL INTO THE STRATUM TO BE TESTED, HARDENING SAID MATERIAL IN THE STRATUM WHEREBY THE PARTICLES OF STRATUM ARE BOUND, CUTTING A SAMPLE OF THE SAID STRATUM IMBEDDED IN THE THERMOPLASTIC MATERIAL, REMOVING THE SAID SAMPLE TO THE SURFACE OF THE EARTH, AND REMOVING THE THERMOPLASTIC MATERIAL FROM SAID SAMPLE WHEREBY A CORE IS OBTAINED WHEREIN THE SAND GRAINS ARE IN A CONDITION SUBSTANTIALLY THE SAME AS EXISTED IN SAID STRATUM, SAID THERMO-