US2146263A - Method of coring to preserve fluid content - Google Patents

Description

Feb. 7,1939. N. JOHNSTON 2,146,263

METHOD OF CORING TO PRESERVE FLUID CONTENT Filed May 31, 1958 NORRIS JOHNSTON INVENTOA QM w. g y

Patented Feb. 7, 1939 UNITED STATES METHOD OF CORING T PRESERVE FLUID CONTENT Norris Johnston,

Los Angeles, Calif.

Application May 31, 1938, Serial No. 210,842

Claims.

The invention pertains to the taking of cores from the formations at the bottoms of deep wells such as oil wells, and more specifically to methods of cutting such cores without allowing the intrusion of any drilling fluid into them and to methods for bringing such cores to the surface of the earth without loss of any part of their original content of gases or liquids.

In modern deep well drilling practice, coring ahead is resorted to in order to determine in advance the lithologic character of the structure to be penetrated by the drill. This practice becomes increasingly necessary and desirable as horizons suspected to be capable of producing oil or gas are approached.

The methods of coring heretofore used are adequate for obtaining accurate samples of the rocks from which the cores are taken, at least in most cases, but they are wholly inadequate to cut and bring to the surface a core having its original fluid contents intact and uncontaminated, This follows from the fact that all present coring methods expose at least one surface of the core to contact with the fluids in the well during the cutting of the core and its withdrawal.

Since in most cases it is found expedient to keep the hydrostatic pressure of the drilling fluid in the wall substantially higher than the fluid pressure in the formation, to avoid any possibility of the formation fluids flowing out of the hole and thus interfering with drilling, it is normally true that the drilling fluid, in direct contact with the core throughout the period of cutting, intrudes into the core throughout the period of cutting, in trudes into the core under a definite and substantial pressure differential, thus displacing connate fluids in an uncontrollable manner and to an unknown extent.

One of the purposes of the present invention is to remedy this inadequacy by providing that the core shall be out without contact at any time with the drilling fluid, and under such conditions as to be free from the influence of the excess pressure of that fluid over the formation fluid pressure. In general terms, the means for attaining this result is to core with a barrel of special design, through a small amount of molten metal at the bottom of the hole. This layer of metal, being about seven to ten times as heavy as most drilling fluids, will. completely displace them from the bottom of thehole 'where coring is in progress, thus effectively preventing core contamination. The metal itself will not intrude into the core as it does not wet the materials of the formation and its high surface tension prevents intrusion into even fairly porous core bodies, especially if the drilling fluid pressure is not allowed to exceed formation pressure by too wide a margin.

A further purpose of the invention is to bring the core to the surface with its original fluid contact intact. As the withdrawal of the core is effected, under present methods, with at least part of the surface of the core exposed to contract with drilling fluid, and under a constantly lessening hydrostatic pressure as the surface of the earth is approached, any gases in the core, which originally are highly compressed, are allowed to expand and escape, carrying with them the water or oil which may be present in the formation from which the core was taken. Thus, by the time the core reaches the surface, its original character as regards fluid contents is totally obscured, and ascertainment of the quantities and kinds of fluid originally contained is rendered impossible.

This loss of gas and other fluids I prevent by controlling the temperature of the mud circulation, thus freezing the metal into a solid plug at the bottom of the core holder. This step allows the sample to be brought to the surface-in a hermetically sealed container, the contents of which can be studied at leisure in the laboratory and from which no fluid can escape until it is opened deliberately.

While various types of coring apparatus may be used in the practice of the invention, I shall illustrate its general application with reference to the. particular form shown in the attached drawing and hereinafter described.

Referring to the figure, 1 is a drilling bit, preferably of such diameter as to form an undersized hole 2 at the bottom of the main hole 3. A core barrel centering ring is indicated at 4 and the core barrel at 5, the lower end of this barrel being formed as a coring bit 6 and being provided with the usual core catcher I and also with grooves 8, the purpose of which will appear. This barrel differs from the conventional in being open at the lower end only, the upper end being blind as indicated at 9. The entire inner surface of the barrel, and particularly its lower portion, should be tinned to permit it to be wetted by the fused metal hereinafter described and thus to form a gas-tight joint with the plug of metal after its solidification, As the barrel, after the solidification of this plug, forms a sealed bomb, it must be of such strength as to withstand internal pressures at least equal to the maximum pressure existing in the formation from which the core is to be taken.

Having provided the above simple apparatus, I

proceed in the following general manner to the taking of the desired core. I first obtain, by any suitable method, the temperature of the circulating mud at the bottom of the hole during normal drilling and also the static bottom temperature of the hole. I then drill an undersize hole one to two feet in depth as illustratedthis step is not essential but conserves the expensive alloy metal.

About two hours, or a sufficient time, before the core is to be taken, I start the circulation of hot mud. By hot mud I mean circulating mud heated to a temperature above the normal temperature of the circulation in the particular well and also above the melting point of the alloy metal to be used. The mud may be heated in any convenient manner, as by the introduction of steam into the circulation. When the well has become heated above its normal temperature, I set the kelly aside and drop into the well a slug (as for example, more or less thirty pounds) of the fusible alloy, which melts on contact with the hot mud and sinks to the bottom of the well to form a pool on its bottom.

I then drill and core a short distance farther, as for example six inches, to get into formation which has not been touched by drilling mud, the

alloy remaining on the bottom of the hole. I

then remove the center bit and leave the melted alloy on the bottom protecting the newly exposed formation from contact with the drilling mud, which it does efiectively because of its high spe- ClfiC gravity.

As the next step I introduce the special form of core barrel above described and core down for a desired distance, as for example 12 to 18 inches, through the melted alloy. The dotted line Ill-l of the drawing indicates the position and contour of the bottom of the hole before the metal is introduced.

Circulation of cold mud is then started and is continued until the molten alloy, which now surrounds the core and also the lower portion of the core barrel, has solidified. In the use of the term cold mud I refer to circulating mud cooled below the normal temperature of the well and be low the solidifying point of the alloy used. The freezing of the alloy should require but a relatively short time, as for example 45 minutes. It is desirable at this point to raise the drill a few inches from the bottom of the hole and maintain the cold circulation for one or two hours to make sure of solidification of the metal.

The drill stem is now withdrawn and the special core barrel broken out. If the general course of the above procedure'has been followed, the portion of the barrel below the core will be found to be filled and hermetically sealed with solidified metal, the grooves 8 preventing the internal pressure from dislodging the metal plug. The core above this plug will not have been subjected to any lowering of pressure nor to contact with any foreign fluid, and will contain whatever kind and quantity of oil, gas, and water which it contained when in place in the formation.

The entire barrel may now be placed in a sealed bomb, the air surrounding the barrel swept out with some suitable gas such as carbon dioxide, the bomb heated to such temperature as to fuse the metal plug and cause the molten metal to drain away, and the evolved gases vented from the bomb through measuring and analytical apparatus suited to the determinations of kind and quantity which are to be made. The bomb may then be opened for the removal of the core at atmospheric pressure and for the determination of any quantities of water or oil not removed by the escaping gases.

The choice of alloy melting point rests on the temperatures existing in the particular well from which the core is to be taken. For example, if the static or formation temperature is F. and the normal temperature of the circulatin fluid is 150 F., a suitable melting point for the alloy would be 155 F. The metal would then be melted with some rapidity by raising the mud fluid temperature to 175 F. and would be solidified within reasonable time limits by lowering the temperature to 135 F. Alloys ranging in melting point, in steps, from 0 F. to F. would cover the entire range of wells, so far as I am aware, and could be provided in advance or made up to suit each specific case.

Alloys having definite melting points and sufficient mechanical strength to withstand the internal pressures realized on withdrawing the core barrel from even very deep wells may be made from suitable combinations of bismuth, lead, tin. cadmium, and antimony. The preparation of such alloys at desired melting points is a well known art, and it is sufficient to say that the proportions of these five constituents, in the order stated, would range from 50:27:13;10;0 to 50: 5:12:8:5, the first named having the minimum and the last the maximum melting point.

The density of these alloys is high, of the order of 600 pounds per cubic foot; so that core cuttings will float up to the region of mud circulation and the mud jets will not readily disturb the upper surface of the pool of molten metal. Penetration of the mud downwardly, through or around the metallic pool, is obviously impossible. Loss of alloy by leakage into the formation is minimized by the high surface tension of the liquid metal, but care should be exercised to avoid excessive pressure difference between the mud fluid and the formation at' the bottom of the hole. On the withdrawal of the core barrel the portion of the metallic pool outside it may remain in the hole as a metallic ring or lining. This may readily be drilled away or may be recovered with a spear-head fishing tool if of sufficient value to repay the cost of removing it.

In order to prevent the intrusion of mud into the core barrel while lowering it into the hole it may be desirable to fill the core barrel with the low melting point alloy. When solidified this will retain its position until the temperature is raised to its melting point. If the fluids in the hole should be of such temperature that the metal will melt while the barrel is being lowered to position, the filling may be retained by'a thin cap or membrane of some higher melting point metal placed over the end of the barrel. This membrane should be of such slight thickness as to be immediately destroyed when the core barrel is revolved on the bottom of the hole.

I claim as my invention:

1. The method of coring the bottom of a drilled well which comprises: elevating the temperature of the circulating fluid in .said well above its normal temperature when drilling; introducing into said well a metal fusible at said elevated temperature; forming a core at the bottom of the hole and below the pool of molten metal collected thereon and within a core barrel open at only its lower end; lowering the temperature of said circulating fluid below the solidifying point of said molten metal and thereby sealing said core within said barrel, and withdrawing said barrel with said core sealed therein.

2. The method of coring the bottom of a drilled well which comprises: forming a pool of molten metal on the bottom of said well; forming a core from said bottom and within a core barrel open at only its lower end; detaching the core from said well bottom, and sealing the open lower end of said core barrel below said core by solidifying said molten metal before said barrel is withdrawn from said well.

3. A method of taking a core from the bottom of a drilled well which includes the step of solidiing the core within an otherwise closed core barrel by solidifying a fused metal Within the open end of said barrel.

5. A method substantially as in claim 2, including the additional step of forming a hole of reduced diameter in the bottom of said well with- 10 in which said molten metal may collect.

NORRIS JOHNSTON.

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