Title:
Method of and apparatus for investigation of cased drill holes
United States Patent 2400593


Abstract:
This invention has to do with electrical exploration conducted through drill holes, and has special application in connection with mapping of formation penetrated by oil wells, water wells and the like. Drill holes, particularly those intended for the exploiting of oil-bearing strata, before...



Inventors:
Jacob, Neufeld
Application Number:
US14136337A
Publication Date:
05/21/1946
Filing Date:
05/07/1937
Assignee:
WELL SURVEYS INC
Primary Class:
International Classes:
G01V3/22
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Description:

This invention has to do with electrical exploration conducted through drill holes, and has special application in connection with mapping of formation penetrated by oil wells, water wells and the like.

Drill holes, particularly those intended for the exploiting of oil-bearing strata, before being put into operation, are generally provided with a tubular metallic casing cemented to the walls.

Up to the present time the electrical exploration has been usually conducted through uncased drill holes. In some instances, when casing was provided and extended only partly into the drill hole, the electrical measurements could begin only below such metallic casing. In many instances, however, it has been found desirable to investigate the conditions in a well provided with metallic casing throughout its length in order to determine all other formations previously shut off when the strings of casing were run to complete the well at lower depths. Among those formations lying at present behind the casing some valuable strata may exist which are capable of producing oil and the existence of which has been ignored by the operator.

My invention has therefore as an object to explore and ascertain the said valuable strata located behind the casing in view of producing additional quantities of oil and gas and increasing the total productivity of the well.

My invention can also be utilized in other instances, in connection with some recent methods of completing gas and oil wells, the said methods consisting in running the casing to the bottom of the hole and then shooting the casing to admit the gas or oil. By using these methods it is possible to case off several sands and produce them one at a time in any predetermined order, usually from the bottom upward. Or one producing formation can be treated in this manner and in handling high pressures the well can be kept under control at all times.

Prior to my invention various attempts have been made in order to investigate the physical properties of geological formations lying behind the casing of a drill hole and thus to determine the depths at which the valuable strata are located and at which the casing should be perforated in order to produce oil or gas. Some of these attempts consisted in making a temperature survey in a cased hole or in studying the electrochemical phenomena, presumably caused by the strata lying behind the casing. These results have not been however fully described and a satisfactory justification of the attempted methods has not been found. (See in that connection: "Electrical logging and its applications in moder petroleum prospecting," published in "The Petroleum Engineer," February 1937, pages 158 and 159.) It is therefore the main object of my invention to provide a simple and reliable method for electrical exploration conducted through cased drill holes with a special application in connection with mapping of formations penetrated by the drill holes and located behind the casing.

It is another object of my invention to provide a method for determining the location of producing zones behind the casing in order to indicate the depths at which the casing should be perforated and oil or gas produced.

It is also an object of my invention to provide a method for preventing perforation of casing in possible water sands in order to obviate the loss of the well or valuable production areas.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention, itself, however, both as to its organization and method of operation, together with objects and advantages thereof, will be better understood by reference to the following description taken in connection with the accompanying drawings, in which: ig. 1 represents an arrangement according to my invention for investigating electrical properties of geological formations behind the casing in cased drill holes.

Fig. 2 shows a diagram representing the measurements performed in the drill hole.

Fig. 3 represents a modified arrangement for investigating electrical properties of geological formations behind the casing in cased drill holes.

Referring now more particularly to Fig. 1, two 4drill holes 10 and II provided with tubular metallic casings 12 and 13 are shown penetrating a formation to be explored. It may be assumed that the holes 10 and II are in an oil field where a strike was originally made in a deep formation as D and that there are very few records of existing structures. The earth is assumed to be composed of four sedimentary formations A, B, C, D.

Let the depths at which the formations B, C, D are located be designated by l, 12, 13 respectively.

Let also the conductivities of the formations A, B, C, D be designated by 71, 72, 73 respectively.

The present method is intended to detect the presence of the above formations and to indicate their electrical characteristics. This is accomplished by means of measurements of electrical resistivities of the formations the said measurements taking place inside the drill hole at various depths. The results obtained are given in form of a diagram of resistivities of various geological formations at different depths, such a diagram being usually designated as an electrical log.

The exploring apparatus for performing the measurements comprises a battery 14 connected to the casing 12 directly and also connected by means of a switch 15 to the casing 13, of the drill holes 10 and II respectively. The hole 10 is filled with a conductive liquid, such as salt water.

Two insulated conductors 16 and 17 are provided, and are connected to electrodes 18 and 19 which are suspended at different depths in the hole 10.

The conductors 16 and 17 have individually a total length somewhat in excess of the depth of the hole to be explored and are normally wound upon a drum 20 positioned adjacent the top of the well.

The terminals of the conductors 16 and II are connected to the recording voltmeter 21.

The operation of the device and the method employed can be explained as follows: Assume that the switch 15 is closed. Consequently a current is made to flow from the positive terminal of the battery 14 to the casing 12 to traverse the various geological formations between the casing 12 and the casing 13 and to return through the casing 13 to the negative terminal of the battery 14.

Let the output voltage of the battery 14 be vi, the output current be ii, and let a designate the depth below the surface of the earth.

It is apparent that the difference of potential between the casing 12 and the casing 13 will vary with the depth x and can be represented symbollically as v(x). Then, at the surface x=0; and the difference of potential between the casing 12 and 13 is substantially equal to the output voltage of the battery, i. e. v(o) =vi. For increasing depths, however, the difference of potential between the casings decreases i. e. when x becomes larger, v(x) becomes smaller, and this is due to the voltage drop caused by the current flowing in the casing 12 in the downward direction and in the casing 13 in the upward direction.

It is also apparent, that the current flowing in the casings 12 and 13 in the downward and the upward directions respectively varies with the depth x and can be represented symbolically as i(x). Then, at the surface of the earth x=O, and the current flowing in the casing 12 or out of the casing 13 is substantially equal to i(o) =ii. However, for increasing depths, the current i(x) is smaller than ii. This is because a great part of the current ii has already passed from the casing 12 to the casing 13 by traversing the layer of earth between the surface and the depth x. Thus we have to consider two current components; one flowing in casings 12 and 13 in the vertical direction and designated by i(x), and the "leakage" current traversing the earth which can be designated as di(x) It is obvious that the intensity of the current varies with the depth and depends among other things upon the conductivity of the earth formation at a given depth x.

Let p designate the resistivity of the casing, and - the conductivity of the earth formation at a given depth x. It will then be obvious to those - dv() =pi(x) di(x) dz =7v(s) The relation (1) expresses the fact that the rate of decrease of the voltage v(x) between the 10 two casings is proportional to the current i(x) flowing through the casing in the vertical direction at a given depth x.

The relation (2) expresses the fact that the rate of decrease of the current flowing through 15 the casing in the downward direction at a given depth x is proportional to the voltage between the casings and to the conductivity of the earth formations lying between the casings.

From the relations (1) and (2) it follows that d2i(x) dx-'t x) d2v(x) , i(s) K v(W) =K2e where Ki and K2 are constants of integration.

It has been thus shown by means of the equation (5) that the current i(x) passing through the casing decreases exponentially with the depth 35 x in a manner which is determined by the value V-yp. Since the specific resistivity p of the casing is substantially constant and only the conductivity Y of the earth formation between the casings varies with the depth-the variation of N/'yp 40 is influenced only by the conductivity. Consequently, the rate of the exponential decay indicates the conductivity of the corresponding geological formation.

Consider now in that connection Figure 2 4 representing diagrammatically the variation of the current i(x) with the depth x. It will be apparent that for the depths x

  • 70 It is noted that the points Mi, N1, Ri corresponding to the values of x equal to 1, 12, h1 respectively, represent the transition points at which the curve i(x) changes its character.

    Consequently these points may be easily found 75 and identified on the record. It may be also understood by those skilled in the art that from the records of the functions represented by PIMi; MINi, NiRi, etc., the relative values of the conductivities of the corresponding formations may be found and a resistivity log of the strata traversed by the drill hole may be determined.

    For instance, from the curve MiNi we have: Mim=1 K12e VpYl (7) Nin =Ke I2-/211 (8) and from the relations (7), (8) I1 1 1 MmT (9) p - 1-2 log n ](9) In a similar manner it is possible to determine yi, y3, etc.

    It is apparent that the variations of the current i(x) with the depth x substantially as shown in Figure 2 may be obtained by means of the arrangement shown in Figure 1 and consisting of the exploring electrodes 18, 19 lowered into the hole at various depth and connected to the recording voltmeter 21. The recorded voltage between the exploring electrodes 18, 19 is substantially proportional to the current i(x) corresponding to the depth at which the electrodes are located.

    Thus the diagram of voltages between the exploring electrodes, obtained by means of the recording voltmeter 21 is substantially similar to the Figure 2 and exhibits the variations of the resistivity of various formations traversed by the drill hole in the manner explained above.

    In describing my invention I have limited myself to the study of the direct current electrical characteristics i. e. to the specific conductivity of the earth formations. It is however obvious to those skilled in the art that I can easily modify my arrangement in order to make it adaptable for the study of the alternating current electrical characteristics of the various strata traversed by the hole.

    It is well known that in an electric alternating field produced in the ground, phase displacements caused by certain formations constitute a measurable and characteristic quantity which renders it possible to distinguish various kinds of rock, ores, and the like from each other and to locate the boundaries between the same. Since said phase displacements depend, in part at least, on the dielectric constant of the formations which are traversed by the electric alternating field, my modified arrangement provides a method of investigating cased drill holes which, renders it possible to locate and identify various formations met with in the drill hole by their different dielectric constants. This is of importance when locating oil-carrying strata which is rendered possible by the large impedance of such strata as compared to that of other strata.

    In accordance herewith my modified method consists in applying an alternating voltage between the casings 12 and 13 of the respective drill holes and determining the alternating current I(x) as regards phase and amplitude at different depths x in the drill hole. This can be done by means of an arrangement substantially as shown in Figure 1, in which the battery 14 is substituted by an A. C. generator and the recording voltmeter 21 is substituted by an arrangement simultaneously recording the magnitude and the phase of the voltage between the exploring electrodes 18 and 19.

    Figure 3 represents another modified embodiment of my invention in which the exploring electrodes 118 and 119 are at the same depth x, the electrode 118 being suspended in the hole 10 and the electrode 119 in the hole 11 respectively. The insulated conductors 116 and 117 connect the exploring electrodes with a recording voltmeter 121.

    Both holes are filled with a conductive liquid, such as salt water.

    Assume that the switch 115 is closed. Consequently a current is made to flow from the positive terminal of the battery 114 to the casing 12, to traverse the various geological formations between the casing 12 and the casing 13 and to return through the casing 13 to the negative terminal of the battery 114.

    The distribution with respect to the depth of the voltage v(x) between the casings and of the current i(x) flowing through the casing in the vertical direction is the same as is the previously discussed case and is expressed by the formulas (5) and (6). In particular, the formula (6) expresses the voltage between the exploring electrodes I18 and 119 and consequently for a given formation this voltage varies with depth substantially in the same manner as the current i(x). It will be therefore apparent that the record obtained by means of the arrangement of Figure 3 is substantially similar to the one shown in Figure 2 and may be used for the determination of the electrical characteristics of the strata trayersed by drill holes in a manner which has been explained above.

    In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain part or all of the benefits of my invention without copying the structure shown, and I, therefore claim all such in so far as they fall within the reasonable spirit and scope of my invention.

    I claim: 1. An electrical process for determining the nature of the geological formations traversed by cased drill holes comprising transmitting electrical current through the earth between casings of two drill holes and determining the distribution of the said current as a measure of the varying character of the formation traversed by the drill holes.

    2. An electrical process for determining the nature of the geological formations traversed by cased drill holes comprising applying a difference of potential to the casings of two drill holes and determining the variation of the said difference of potential at various depths between the casings of the said holes as a measure of the varying character of the formation traversed by the drill holes.

    3. An electrical process for determining the nature of the geological formations traversed by cased drill holes comprising applying terminals of a voltage supply to casings of two drill holes, filling one of the drill holes with an electrically conductive liquid, moving two exploring electrodes to various depths in the liquid filled hole, and recording the voltage variations between the exploring electrodes as a measure of the varying character of the said geological formations.

    4. An electrical process for determining the nature of the geological formations traversed by cased drill holes, comprising applying terminals of a voltage supply to casings of two drill holes, filling the drill holes with an electrically conductive liquid, lowering an exploring electrode into each of the said drill holes and recording the voltage variations between the exploring electrodes as a measure of the varying character of the said geological formations.

    5. Apparatus for electrically investigating geological formations traversed by cased drill holes comprising a source of current, electrical connections between the source of current and the casings of the drill holes, two exploring electrodes, adapted to be suspended at different depths in one of the said drill holes, a recording instrument adjacent to the top of the said one of the said drill holes and insulated conductors connecting said exploring electrodes to said recording instrument.

    6. An electrical process for determining the nature of the geological formations traversed by cased drill holes comprising applying terminals of a voltage supply to casings of two drill holes, filling one of the drill holes with an electrically conductive liquid, moving at least one exploring electrode to various depths in the liquid filled hole, and recording the voltage of said exploring electrode as a measure of the varying character of the said geological formations.

    7. An electrical process for determining the nature of the geological formations traversed by cased drill holes, one of said drill holes being filled with an electrical conductive liquid, comprising applying terminals of a voltage supply to casings of two drill holes, lowering an exploring electrode into each of the said drill holes and recording the voltage variations between said exploring electrodes as a measure of the varying character of the said geological formations.

    JACOB NEUFELD.