US2550005A

US2550005A - Well logging method and apparatus utilizing periodically variable spontaneous potentials

Info

Publication number: US2550005A
Application number: US22616A
Authority: US
Inventors: Doll Henri-Georges
Original assignee: Schlumberger Well Surveying Corp
Current assignee: Schlumberger Well Surveying Corp
Priority date: 1948-04-22
Filing date: 1948-04-22
Publication date: 1951-04-24
Anticipated expiration: 1968-04-24

Description

2,550,005 a PERIODICA LLY April 1951 HENRI-GEORGES DOLL WELL LOGGING METHOD AND APPARATUS UTILIZIN VARIABLE SPONTANEOUS POTENTIALS Filed April 22, 1948 3 n a 6 m 2 L W m 7 FIGJ.

A.C. ELECTRODE ELECTRODE PO TENTIA L POTENT/AL(RELAT/V) (RELATIVE) INVENTOR. HENRI-GEORGES DOLL HI ATTORNEYS.

Patented Apr. 24, 1951 WELL LOGGING METHOD AND APPARATUS UTILIZING PERIODICALLY V A R I AB L E SPONTANEOUS POTENTIALS Henri-Georges Doll, Houston, Tex assignor to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Delaware Application April 22, 1948, Serial N0. 22,616

6 Claims.

The present invention relates to well logging methods and apparatus. More specifically it relates to a novel method and apparatus utilizing alternating electro-filtration potentials for determining the levels of the more permeable formations traversed by a bore hole.

My prior Patent No. 2,433,746, discloses several different well logging methods and apparatus for locating permeable formations by means of alternating electro-filtration potentials. In all of the systems there disclosed, alternating electro-filtration potentials are produced by lower ing into a bore hole containing a column of liquid a mechanism for producing localized, periodic fluctuations in the liquid pressure at different depths. The location of the permeable formations is accomplished by lowering an electrode into the bore hole at substantially the same level as the mechanism for producing the liquid pressure fluctuations, and obtaining indications of the alternating potentials picked up by the electrode.

The principal object of this invention is to provide a novel, alternating electro-filtration potential well logging method and apparatus in which periodic pressure pulses are produced in the bore hole liquid without lowering special mechanism into the bore hole for this purpose.

Another object of the invention is to provide a new and improved method and apparatus of the above character in which alternating electrofiltration potentials are produced at the levels of permeable formations by applying a periodically varying pressure above the column of liquid contained in the bore hole.

A further object of the invention is to provide a new and improved well logging method and apparatus of the above character which requires little more than conventional equipment usually present at the well in the field.

In accordance with the invention, the bore hole containing a column of drilling mud or other liquid is substantially completely closed off, and suitable means is connected thereto at the surface for producing periodic and uniform pressure pulses in the bore hole liquid. The increment of pressure contributed by the pressure pulse producing means is large enough to insure the generation of alternating electro-filtration potentials of significant magnitude at the levels of permeable formations. The alternating potentials may be picked up by an electrode moved through the well on a suitable cable, stuffing box means being provided to facilitate movement of the cable through the well without adversely 2 affecting the operation of the pressure pulse producing means.

The invention may be better understood from the following detailed description of a typical embodiment, taken in conjunction with the accompanying drawings in which:

Fig. 1 is a schematic diagram of alternating electro-filtration potential well logging apparatus constructed according to the invention;

Fig. 2 is a typical curve of variations in alternating electro-filtration potentials such as might be obtained in a well traversing permeable formations; and

Fig. 3 is a typical curve of variations in alternating electro-filtration potentials superimposed on a standard spontaneous potential curve.

The theory underlying the production of alterhating electro-filtration potentials is fully discussed in my above-mentioned prior Patent No. 2,433,746 and it will not be necessary to repeat it herein. Sufficeit to say that electro-filtration potentials can be produced in the vicinity of relatively permeable formations surrounding a bore hole containing a column of liquid by producing periodic pressure variations in the bore hole liquid. For any given type of liquid and permeable formation material, the electro-filtration potential produced is known to be proportional to the pressure differential at the boundary between the bore hole liquid and the formation and a given increment in pressure produces a corresponding increment in the electro-filtration potential observed.

In Fig. 1 is shown a bore hole 10 drilled into the earth and traversing a plurality of subterranean formations such as relatively impermeable shales ll, i2 and I3 and relatively permeable formations M and i5. According to the usual practice, the upper part of the bore hole It may be lined with a metallic casing I5 which extends to the surface of the earth. The bore hole !0 contains a column of liquid ll such as drilling mud, for example.

Connected to the casing is at the surface of the earth is the usual flow line l8 having a discharge valve is therein which is normally maintained closed. The casing is is also connected to a conduit 20 which receives the output of conventional pumping means 2i driven by any suitable prime mover 22 in the conventional manner. The pumping means 2| is adapted to pick up mud or other incompressible liquid 23 from the mud pit 24 through an inlet 25 and to discharge it through the outlet 20 into the casing iii.

For the proper practice of the present invention, it is essential that the bore hole In be substantially completely closed off. This may be accomplished in any suitable manner as by securing a cap member 26 at the top of the casing, for example. The cap member 26 may be provided with a suitable packing gland or stuffing box 21 which is suitably designed to facilitate the passage of a cable 28 carrying an electrode 29 into the well Ill. The stufling box 21 may be of any convenient construction and it may comprise, for example, two cooperating halves 3D and 3| adapted to be fitted around the cable 28 to form a unitary assembly which can be threaded or otherwise secured in the cap member 26.

Formed within the stufling box 21 is a chamber 32 in the upper portion of which is disposed a snugly fitting packing member 33 designed to prevent any appreciable leakage of liquid around the cable 28. The packing member 33 may be of the split washer type and it should be made of a suitable resilient material such as rubber, for example.

Mounted in the lower portion of the chamber 32 is a second packing member 34, also made of resilient material such as rubber, for example. The packing member 34 may be provided with a bore 35 sufliciently larger than the diameter of the cable 28 to permit a certain amount of leakage of liquid around the cable 28 into the chamber 32 for a purpose to be described hereinafter. Fluid from the chamber 32 can flow through a passage 36 to an annular space 31 between the outside of the stufiing box 21 and the inside of the cap member 26. The annular space 31 communicates with an outlet 38 for discharging the liquid into the mud pit 24. A plurality of rollers 39 may be provided on the stufling box 21 to guide the cable 28 and prevent excessive wear of the

packing members

33 and 34.

In operation, the bore hole In is completely filled up to the level of the stuffing box 2! by operation of the pumping means 2|, if necessary. The operation of the pumping means 2| is then continued with the flow line discharge valve l9 closed so that periodic, substantially uniform pressure pulses are produced in the bore hole liquid II. By virtue of the stuffing box construction described above, it will be understood that a certain amount of the bore hole liquid II will continuously leak through the space between the cable 28 and the bore 35 in the packing member 34 into the chamber 32 in the Stuffing box 21.

Thus, each time the pumping means 2| produces a pressure increase in the bore hole liquid H, the leakage through bore 35 in the packing member 34 causes the pressure to decrease to a reference value before the next pressure pulse is received. As a result, periodic, substantially uniform pressure pulses are produced in the bore hole liquid I"! which result in the production of alternating electro-filtration potentials in the vicinity of the more permeable formations such as the formations l4 and I5. As is known, these alternating electro-filtration potentials will have the same frequency as the pressure pulses generated by the pumping system described above.

The alternating electro-filtration potentials produced as described above may be picked up by the electrode 29 and transmitted through a suitably insulated conductor 40 in the cable 28 to the surface of the earth where they can be indicated or recorded in any suitable manner. By connecting the cable 40 to a recording instrument 4| having its other terminal grounded at 42, a

curve of the type shown in Fig. 3 may be obtained. This curve is a composite of the normal spontaneous potential curve (shown in dotted lines in Fig. 3) and the alternating electro-filtration potentials which are superimposed thereon. As indicated in Fig. 3, the amplitude of the alternating component is substantially increased in the vicinity of the permeable formations l4 and i5 so that the depths and thicknesses of the latter can be readily determined.

If desired, indications may be obtained separately of the alternating electro-liltration potentials by connecting the conductor 43 in series with a D. C. blocking condenser 43 to a suitable recording instrument 44, the other terminal of which may be grounded at 45 in the usual manner. With this connection a curve of the type shown in Fig. 2 is obtained. In this figure, it will be noted that the amplitude of the alternating electro-filtration potentials is substantially increased in the vicinity of the permeable formations l4 and I5 so that the depths and thicknesses of the latter may be readily located from the log obtained.

In order to provide liquid pressure pulses of maximum amplitude for any given pump, the packing member 34 should be properly designed to permit sufiicient leakage of bore hole liquid through the bore 35 to restore the liquid pressure within the top of the casing substantially to atmospheric pressure during each stroke. Obviously, the rate of leakage to provide maximum pressure pulses will depend upon the displacement of the pumping means 2| and the rate at which it is operated.

In practice, the packing member 34 should preferably be designed to provide sufiicient leakage through the bore 35 to insure the production of pressure pulses of maximum amplitude with the smallest pump that may be expected in the field. Then, in cases where larger pumps are available, the additional leakage required, if any, may be provided by manually adjusting the flow line discharge valve IE1, or by providing another valve on the stuffing box.

In an actual test made in the field, pressure fluctuations from a level of about 25 to 50 pounds per square inch to a level of about pounds per square inch were obtained with a frequency of pulsation of the order of one cycle in two seconds. Under these conditions, the levels of permeable formations traversed by the well could be readily detected by the increase in amplitude of the alternating electro-filtration potentials obtained, the maximum amplitude being of the order of 5 millivolts.

The above described method is of special utility in regions where the type of formation is rather continuous but has relatively permeable zones at unknown locations. This condition often obtains in limestone areas where the limestone is substantially impervious but has permeable sections of limited extent at different levels through which oil is produced. In limestone areas of this type, the usual spontaneous potential curves are sometimes difficult to interpret with the degree of accuracy required. However, curves of alternating electro-filtration potentials produced by pressure pulses in accordance with the present invention have been found very helpful in locating such producing zones in limestone areas.

Generally speaking, the attenuation of the acoustic waves through the mud column will increase with increases in frequency so it is usually desirable to use rather low frequencies such as a few cycles, or a few tens of cycles, per second although the method is not limited to low frequencies. Also the bore hole liquid should preferably be substantially incompressible. It will be apparent that the presence of large gas bubbles in the mud column might dampen the pressure pulses. Where a great quantity of undissolved gas is present, it is recommended that it be re-' moved. A conventional mud shaker or screen is usually sufficient for this purpose.

It will be understood from the foregoing description, that the invention provides an effective method and apparatus for locating relatively permeable formations traversed by a well. The

method and apparatus are especially suited for use in areas where the formations are relatively thick and continuous as in limestone areas, for example. Further, the greater part of the equipment required for practising the method is usu ally present at the well in the field, and only a small amount of additional equipment is required.

The specific embodiment described herein and illustrated in the drawings is merely illustrative and can be modified considerably within the spirit of the invention. For example, other suitable means can be used for producing periodic pressure pulses in the bore liquid. In case a stuffing box is used which does not permit any appreciable leakage of the bore hole liquid, a conventional pressure responsive valve designed to open when a predetermined fluid pressure is reached and to close when the pressure has dropped to a desired value may be mounted in the flow line IS, the discharge valve l 9 being kept open. Other modifications will be apparent to those skilled in the art.

I claim:

1. In a method of logging the uncased portion of a well containing a column of conducting liquid and having an upper portion lined with a casing extending to the surface of the earth, the steps of lowering an electrode on a supporting cable into the well, ailixing at the top of the well a cap having a stuifing box to facilitate the movement of the cable through the well, pumping a substantially incompressible liquid into the well to increase the hydrostatic pressure in the well, permitting controlled leakage of liquid from the top of thewell to permit the increased hydrostatic pressure of the well liquid to decrease substantially to a reference level, repeating said liquid pumping step while permitting controlled leakage from the well to produce periodic pressure pulses in the well liquid, the pulse repetition rate and the relative rates of pumping and leakage being suitably chosen to produce periodically Variable electro-filtration potentials in the well at the levels of relatively permeable formations, moving the electrode through the well during the production of said pressure pulses, and ob.- taining indications only of periodically variable potentials picked up by said, electrode.

2. In a method for logging the uncased portion of a well containing a column of conducting liquid and having an upper portion lined with a casing extending to the surface of the earth, the steps of lowering an electrode on a supporting cable into the well, afiixing at the top of the well a cap having a stuffing box to facilitate the movement of the cable through the well, pumping a substantially incompressible liquid into the well to increase the hydrostatic pressure in the well, permitting controlled leakage of liquid from the top of the well to permit the increased hydrostatic pressure in the well to decrease substantially to a reference level, repeating said pumping step while permitting controlled leakage from the well to produce periodic pressure pulses in the well liquid, the pulse repetition rate and the relative rates of pumping and leakage being suitably chosen to produce periodically variable electro-filtration potentials in the well at the levels of relatively permeable formations, moving the electrode through the well during the production of said pressure pulses, and obtain ing indications of the total potential picked up by said electrode.

3. In apparatus for logging an uncased portion of a well containing a column of conducting liquid, the combination of an electrode movable in the well on a cable extendin to the surface of the earth, well cap means having stuiiing box means to facilitate passage of the cable therethrough, means for periodically injecting into the top of said Well a substantially incompressible fluid under pressure so as toincrease the hydrostatic pressure in the well, means responsive to said increased hydrostatic pressure for permitting controlled leakage of liquid from the well to release the pressure in said well gradually to a reference level after each injection of fluid therein, thereby producing periodic substantially uniform pressure pulses in the well liquid, the injection repetition rate and the relative rates of injection and leakage being suitably chosen to produce periodically variable electro-filtration potentials in the well at the levels of relatively permeable formations, and means for providing indications of periodically variable potentials picked up by said electrode.

4. In apparatus for logging the uncased portion of a well containing a column of conducting liquid and having an upper portion lined with a casing extending to the surface of the earth, the combination of an electrode movable in the well on a cable extendin to the surface of the earth, a cap member affixed at the top of said casing and having a stuffing box to facilitate movement of the cable in the well, pumping means for periodically injecting a substantially incompressible fluid under pressure into the well to increase the hydrostatic pressure therein, means responsive to said increased hydrostatic pressure for permitting controlled leakage of liquid from the top of the well to permit the well liquid pressure to decrease substantially to a reference value after the creation of each pressure pulse, the injection repetition rate and the relative rates of injection and leakage being suitably chosen to produce periodically variably electro-filtration potentials in the well at the levels of relatively permeable formations, and means for providing indications of periodically variable electro-filtration potentials picked up by said electrode.

5. In apparatus for logging the uncased portion of a well containing a column of conducting liquid and having an upper portion lined with a casing extending to the surface of the earth, the combination of an electrode movable in the well on a cable extending to the surface of the earth, a cap member aflixed at the top of said casing and having a stufiing box to: facilitate movement of the cable in the well, pumping means for periodically injecting a substantially incompressible fluid under pressure into the well to increase the hydrostatic pressure therein, means in said stufiing box and responsive to said increased pressure for causing controlled leakage around said cable of liquid from the well to permit the Well liquid pressure to decrease substantially to a reference value after the creation of each pressure pulse, the injection repetition rate and the relative rates of injection and leakage being suitably chosen to produce periodically variable electro -filtration potentials in the well at the levels of relatively permeable formations, and means for providing indications of periodically variable electro-filtration potentials picked up by said electrode.

6. In a method for logging an uncased portion of a well containing a column of conducting liquid, the steps of applying to the top of said column of liquid periodically varying pressure of such frequency and intensity as to produce periodically varying electro-filtration potentials in the well in the vicinity of relatively permeable formations traversed thereby, and obtaining indications of such periodically varying potentials as a function of depth in the well.

HENRI-GEORGES DOLL;

REFERENCES CITED lhe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,167,066 Elliott July 25, 1939 2,190,686 Slichtel Feb. 20, 1940 2,304,051 Beers Dec. 1, 1942 2,317,039 Ennis Apr. 29, 1943 2,347,615 Shelley Apr. 25, 1944 2,433,746 D011 Dec. 30, 1947