US3412815A - Electromagnetic radiation method for guiding the drilling of oil wells after the borehole has entered a massive earth formation of chemically deposited material, by a mistake, accident, or the like - Google Patents

Description

412 815 ILLING OF OIL WELLS AFTER THE BOREHOLE HAS ENTERED A MASSIVE EARTH BYA N 6. 1968 w. T. HOLSER ETAL ELECTROMAGNETIC RADIATION METHOD FOR GUIDING THE DR FORMATION OF CHEMICALLY DEPOSITED MATERIAL MISTAKE, ACCIDENT, OR THE LIKE 4 Sheets-Sheet 1 Filed Nov. 14, 1966 F|G.1A

INVENTORS WILL/AM r. HOLSER ROBERT R. UNTERBERGER STANLEY B. JONES ATTORN Y5 Nov. 26, 1968 w r. HOLSER ETAL 3,412,815

ELECTROMAGNETIC RADIA'IION METHOD FOR GUIDING THE DRILLING OF OIL WELLS AFTER THE BOREHOLE HAS ENTERED A MASSIVE EARTH FORMATION OF CHEMICALLY DEPOSITED MATERIAL, BY A MISTAKE. ACCIDENT OR THE LIKE 4 Sheets-Sheet 2 Filed Nov. 1966 K18 lololzlvlzk O 2 @EHEB /2 \\%$&& v v, 4/ w //\A\\' 1 A J L=T l I I L Y=I I J l I l I r A AA A A A A A A I A A A A A A I: A A I: A A

A A A A A A A A A A A A A I\ A A A A A A A A A l A A A A A A q A AA A A A 2 A A A A I\ A E A A A A A A i 4Z l I I I l A A I A A A A 7 I l l fil A n A AA fiI=l=l=l=i=l=l=l\' A AA A A A I I I =j I '.l-:,'- .7,

INVENTORS WILL/AM T. HOLSER ROBERT R. UNTERBERGER STANLEY B. .JONES- ATTORNEYS Nov. 26, 1968 w. T. HOLSER ETAL 3,412,815

ELECTROMAGNETIC RADIATION METHOD FOR GUIDING THE DRILLING OF OI ELLS AFTER THE BOREHOLE HAS ENTERED A MASSIVE EARTH FORMATION OF CHEMICALLY DEPOSITED MATERIAL, BY A MISTAKE, ACCIDENT, OR THE LIKE Filed Nov. 14, 1966 4 Sheets-Sheet 3 0 TIME A I AVG O W Y Y Y- TIME E 7 2' POWER ---7 SUPPLY SWEEP 5s GENERATOR L ISOLATOR I 58 OSCILLATOR 54 ATTENUATOR 62 6.3 BALANCED 59 I MIXER DETECTOR/6,

ATTENUATOR G---- as AMPLIFIER DISTANCE V FREQUENCY E METER INVENTORS AZ T WILL/AM r. HOLSER H 9 /28 ROBERT R. UNTERBERGER 27 STANLEY 5. JONES FIG.

DEPTH United States Patent 3,412,815- ELECTROMAGNETIC RADIATION METHOD FOR GUIDING THE DRILLING OF OIL WELLS AFTER THE BOREHOLE HAS ENTERED A MASSIVE EARTH FORMATION OF CHEMICALLY DEPOS- ITED MATERIAL, BY A MISTAKE, ACCIDENT, OR THE LIKE William T. Holser, La Habra, Robert R. Unterberger,

Fullerton, and Stanley B. Jones, Whittier, Calif., assignors to Chevron Research Company, San Francisco, Calif., a corporation of Delaware Continuation-impart of application Ser. No. 253,339, Jan. 23, 1963. This application Nov. 14, 1966, Ser. No. 594,077

13 Claims. (Cl. 17541) ABSTRACT OF THE DISCLOSURE A method for aiding a driller-operator to redirect a borehole that has, by mistake, accident, or the like, entered a first massive rock formation of chemically deposited materials, such as a salt dome, into contact with a second earth formation having oil reservoirs in locational association with the interface formed between the first and second formations. The first-mentioned formation is irradiated with electromagnetic energy from an electromagnetic generator in the borehole. By measuring the two-way travel time of the irradiated energy with respect to the time of reception of reflected energy from the interface, the distance to the interface from the borehole is determined, whether the interface lies below or at a lateral distance from the borehole. With this knowledge, the operator redirects the well bore to encounter the second earth formation and its associated oil reservoirs.

This application is a continuation-in-part of application Ser. No. 253,339, filed Jan. 23, 1963.

The present invention relates to drilling oil wells. More particularly, it relates to a method for aiding a drilleroperator to redirect a borehole that has, by mistake, accident, or the like, entered a massive rock formation of chemically deposited materials, such as deposits of rock salt (halite), anhydrite, limestone or the like, into contact with oil reservoirs adjacent to the side or bottom of the borehole. By the term chemically deposited materials, it is meant not only to include material such as rock salt (halite) and anhydrite formed by precipitation, but,

also to include all types of limestones including those in which the chemical deposition has included biological processes as Well as those limestones precipitated inorganically.

In accordance with the present invention, the extent of sidetracking of a well bore :in order to encounter the oil reservoirs associated with these types of formations is guided by information obtained from irradiating the formation with electromagnetic radiation to measure the distance from the borehole to the interface of the formation, whether the interface lies below or at a lateral distance from the well bore and, with this knowledge, the operator redirects the well bore to encounter the oil reservoirs.

In exploration for oil and gas, it is known that commerical accumulations of oil and gas may be present in reservoirs associated with massive rock formations of chemically deposited materials, such as rock salt, anhydrite, limestone, or the like. For example, in the southcentral section of the United States, commercial accumulations of oil are usually present, if at all, in the sedimentary formations directly adjacent to a salt dome. It is generally believed that these accumulations occur because the sedimentary beds are uptilted by the upward intru- Patented Nov. 26, 1968 sion of the salt dome into the sedimentary layers. The upward tilt of the beds at the interface creates a pocket, or trap, where oil and gas can accumulate by gravity separation from other formation fluids.

Similarly, in exploring for gas and oil in earth formations that were once below sea level, it is known that oil may be present in porous limestone reefs formed at the edge of marine basins. These reefs once rose'steeply from the oceans bottoms and then were buried by later deposited materials. Recent geological evidence indicates that this sedimentary material about reefs is not necessarily the type of impervious formations usually associated with sedimentary oil traps, such as shale, but may be formed of chemically deposited materials, such as rock salt, anhydrite, or dense limestone. These chemically deposited materials have extremely small pore space so that oil or gas can be trapped within the reef and accumulate by gravity separation from the formation fluids.

While the general location and form of a salt dome or a reef surrounded by these chemically deposited materials may be found at the earths surface by gravity or seismic prospecting techniques, it has been found that the exact location of the oil reservoirs associated with these structures can often only be determined by actually drilling a well. This is because the shapes of the salt doom or reefoften irregular by the depths of interest-are not clearly defined by the surface seismic data. Even when actually drilling, should the borehole enter, by accident or the like, the salt dome or the rock formations surrounding the reef, there is still considerable doubt in the operators mind as to the proper direction the well bore should take to encounter the oil reservoir. For example, if he has drilled into a salt dome, he must consider whether he should continue drilling along the same direction of the well bore, assuming the well bore is in a salt ledge or overhang, or Whether he should whipstock away from the center of the dome toward the interface of the sedimentary beds and the salt dome, assuming the dome is shaped more like a pyramid. Accordingly, it is the primary purpose of this invention to accurately delineate from a borehole the position of an interface of the massive rock formation of chemically deposited material adjacent to an oil-bearing structure or reservoir after the well bore has entered chemically deposited rock formations by mistake, accident, or the like, so that the well bore can then be redirected with enough accuracy to penetrate the reservoir.

In a previous patent application (Method for Mapping a Salt Dome at Depth by Measuring the Travel Time of Electromagnetic Energy Emitted From a Borehole Drilled Within the Salt Dome, William T. Holser, Robert R. Unterberger and Stanley B. Jones, US. Patent No. 3,286,163 issued Nov. 15, 1966) we proposed to use electromagnetic radiation from a borehole deliberately drilled into a salt dome for mapping its side walls to accurately guide the subsequent drilling of the wells to penetrate the sedimentary beds adjacent the side walls of the dome. In the present method, we extend the use of the electromag netic ranging technique for use in massive rock formations of chemically deposited materials, including salt domes and the chemically deposited rock formations surrounding reefs, to guide the redirection of the well bore to encounter oil reservoirs associated with these types of structures after the well bore has, by accident, mistake, or the like, entered such massive rock formations.

Further objects and advantages of the invention will become apparent from the following detailed description, taken in conjunction With the accompanying drawings, which form a part of this specification.

In the drawings:

FIGURE 1 is a sectional view of a borehole penetrating a salt dome and illustrates the position of an electromagnetic energy transmitting and receiving sonde within the well bore along with associated equipment at the earths surface to detect and record the distance to the interface between the salt dome and the oil-bearing sedimentary formations, both to the side and below the borehole, after the borehole has entered the salt dome by accident, mistake, or the like;

FIGURE 1A illustrates in greater detail a transmitting and receiving antenna useful in the system of FIGURE 1;

FIGURE 2 is a sectional view of a well bore penetrat ing a chemically deposited rock formation formed about a reef containing oil and also illustrates the position of an electromagnetic energy transmitting and receiving sonde within the well bore along with associated equipment at the earths surface for measuring and detecting the distance to the reef from the well bore, both to the side and below the borehole;

FIGURE 3 is a waveform diagram useful in understanding a frequency modulating radiating system for ranging to an interface where the interface to be mapped from the well bore is close to the well bore;

FIGURE 4 is a schematic diagram of a transmitterreceiver and associated circuitry for determining lateral distance in the frequency modulating ranging system;

FIGURE 5 illustrates an alternative antenna system useful in mapping an interface located below the bottom of the well bore;

FIGURE 6 illustrates in greater detail a logging sonde including an electromagnetic pulsed antenna system useful in determining the lateral distance to an interface from a borehole in accordance with the method of the present invention.

Reference is now made to the drawings. In particular, FIGURE 1 schematically indicates use of a method of this invention to map, at depth, the location of the wall of the salt dome 10 after Well bore 11 has entered the salt dome by mistake, accident, or the like. The purpose of such mapping is to aid the operator in determining the direction the well bore should take to encounter sedimentary beds 12, 13 and 14 adjacent to the salt dome. As shown, these beds are normally tilted upward by the intrusion of the salt dome 10 through the beds after they have been laid down horizontally. The salt dome in such cases may be formed to have an overhang 15 so that, after the well bore has penetrated the overhang, there is considerable doubt as to the direction the well bore should take to encounter oil-bearing sedimentary beds 12 and 14. (The initial location and direction of well bore 11 to encounter beds 12, 13 and 14 are based on data obtained from top-surface seismic operations, often inconclusive at the depths of interest.) Obviously, if the exact horizontal and vertical locations of the wall of the dome are known-Le, to the side and directly below the well borethe operator can redirect the drilling of the well in the most economical manner to encounter the oil reservoirs 16 and 17. In some cases, the most economical manner may be to abandon the borehole and drill a new well at a new location on the earths surface. In this regard, the costs already expended in drilling the borehole versus the costs of developing a new well at a different site at the earths surface are used for the comparison. Among the factors which affect the decision are depth of present well, distance to the interface, extent and location of causing in place, ease of sidetracking the well, etc.

FIGURE 2 schematically illustrates the use of the method of the present invention in another application. As shown, a borehole 20 penetrates an earth formation 21 formed of a chemically deposited material-such as rock salt.(halite), anhydrite, or dense limestoneadjacent to a reef 22, say of limestone containing oil reservoirs (not shown). The purpose of borehole 20 is to penetrate reef 22 to develop such reservoirs. The reef 22, together with the surrounding formation 21, being formed of impervious material forms traps for gravity accumulation of the oil from other formation fluids associated with the forma- 4 tion of reef 22. When the borehole has entered the adjacent formation 21 by accident, mistake, or the like, there may be considerable doubt in the drillers mind as to the proper direction the borehole should take to penetrate the reef 22 for the same reason previously mentioned. The method of the present invention is useful in mapping, at depth, the horizontal and vertical location of the interface of the reef 22, both to the side and below the borehole, to guide the driller in selecting a direction for the borehole 20 that will penetrate the reef and allow development of its oil. If the exact horizontal or vertical location of the interface of the reef 22 is known, the borehole can be redirected by the driller in the most economical manner to encounter the reef.

In carrying out the purpose of the present invention, the logging sonde 24 is supported by cable 25 within the well bore penetrating the formation of interest, for example, well bore 11 within salt dome 10 (FIGURE 1), to map the horizontal and vertical location of the interface of the salt dome, or as in another example, within borehole 20 of a formation 21 to map the interface of reef 22 (FIGURE 2). The distance to the interface of the salt dome or to the reef from the borehole is determined by transmitting pulsed or frequency modulated electromagnetic energy through the adjacent formation surrounding the borehole and detecting the portion of the energy reflected from the interface. By measuring the time between transmission and reception of the electromagnetic energy as measured by analysis of outgoing and incoming pulses or analysis of their differences in frequencies, the interface of dome 10 or reef 22 can be indicated and displayed at the earths surface inasmuch as the velocity of the energy in the formation is known.

In carrying out the method of the present invention, the electromagnetic energy has a frequency in the range of at least 10 hertz but not greater than 10 hertz so as to better propagate within the adjacent formation surrounding the borehole without undue dispersion or attenuation. It has been found in loss-tangent measurements in samples of halite taken from actual salt domes that the method of this invention operates with maximum efficiency at frequencies within the aforementioned frequency range. It is also known in crystallization of salt to form a salt dome or in the formation of sedimentary beds of halite, anhydrite, or dense limestone about a reef that frequently small pockets of the original brine are left. These pockets of saturated salt water will have a dimension of a few millimeters but seldom include large amounts or large pockets because of the small pore size of these formations. Accordingly, electromagnetic waves will travel through these relatively homogeneous formations and return by reflection from the interface remote from the borehole without undue attenuation or dispersion of the waves.

Surface-recording equipment for indicating the distance to the interface of the salt dome or the reef is indicated at 18 in FIGURES 1 and 2 and includes three indicators: for depth, 26; for distance, 27; and for azimuth, 28. Depth indicator 26 shows the mapping depth of the sonde 24 within boreholes 11 and 20. The mapping depth is measured by pulley 29; in turn, the pulley 29 is shown on indicator 26. The distance from the borehole to the mapped interface at the mapping depth is indicated by the time between transmission and reception of the electromagnetic energy at the sonde 24 and the known velocity of the energy in the formation. The time may be indicated in two Ways: by analyzing the time elapsed between the emission and reception of pulses of the energy, or by determining the differences in frequency of the transmitted and received energy as the output frequency is varied. Azimuthal direction of the radiated energy, if directive emission is used, may be indicated by position indicator 28, here shown as an oscilloscope. By physically associating depth indicator 26, distance indicator 27 and azimuthal indicator 28, the information on all three units can be assimilated to indicate the distance and direction of the mapped interface relative to the borehole whether to the side or below the logging sonde. With this information, the driller redirects the well bore in the most economical manner to encounter oil reservoirs associated with salt domes and reefs.

In mapping an interface, in accordance with the present invention, the sonde 24 is preferably held stationary at a location closely adjacent to or in contact with the bottom wall 23 of the borehole. The preferred mapping depth will thus allow the operator to take greatest advantage of the already drilled extent of the borehole. Antennas within the sonde at the preferred mapping depth may also have an azimuthally omnidirectional radiation pattern, say as provided by a dipole antenna. On such applications, the first received signal at the distance indicator 27 represents the nearest interface of the mapped salt dome or reef. The azimuthal direction of the interface may be approximated with reference to surface sonic data.

Where the surface sonic data is inconclusive, however, it may be desirable to utilize antennas having a more directive antenna pattern in azimuth as in FIG. 1A. As indicated, transmitting and receiving antennas 30 and 31 are illustrated as supported within housing 32 on bearings 33 having their ends flared as illustrated. They are called horn antennas and can be dielectrically loaded to reduce their dimensions at the frequencies of interest. These antennas are most useful when their principal axes of radiation are substantially normal to the interface of the salt dome or reef to be mapped. Accordingly, since the location of such interfaces vary, in azimuth, relative to the well bore, an antenna rotor 35 may be connected to the antennas through gears 36 and 37 for their controlled rotation about the axis of the well bore. The rotor 35 includes a sensor suitably connected by leads forming a portion of the cable 25 to the indicator 28 at the earths surface for azimuthally indicating the direction of the launched and received energy. The rotation of the rotor 35 is initiated by associated circuitry Within the sonde and at the earths surface well known in the control art.

FIGURE 5 illustrates an alternative directive antenna system within sonde 24 useful in directing electromagnetic energy in a downward direction through bottom wall 23 of the well bore. As indicated, electromagnetic energy in the aforementioned frequency range is emitted from a transmitting horn antenna 40 and can be either frequency modulated or pulsed. Although the antenna 40 is illustrated as stationary in elevation relative to the sonde 30 so as to transmit energy into the formation below the well bore in only a single downward direction, it can be provided with suitable mechanisms, such as gears energized by an antenna rotor through appropriate circuitry, to rotate it about an axis normal to that of the well bore. This may be desirable when the operator desires to have a more extensive picture of the width of the salt dome or reef below the well bore. After the energy is reflected from the interface of the salt dome or the reef, the returning electromagnetic energy is then detected at the sonde by receiving horn antenna 41 pointed in the same direction as transmitting antenna 40.

In mapping a segment of a salt dome from within a borehole penetrating the salt dome or in mapping a reef from a borehole exterior of the reef, a borehole may be spaced a relatively short distance from the interface to be mapped, say from one inch to several hundreds of feet. It is, therefore, proposed in such cases that a frequency modulated (FM) ranging system, operating within the aforementioned frequency range, be employed to measure these small distances.

FIGURE 3 illustrates a principle of operation of an FM ranging system. The transmitter of the FM system has a central frequency i equal to at least 10 hertz but less than 10 hertz (cycles per second). The frequency of the transmitter is varied from f to and f, as shown, in a linear fashion, but such that ;f+ is within the above frequency range. This variation can be sinusoidal, however, as it can be shown that the average frequency difference over a cycle of sinusoidal modulation is equivalent to that obtained from a linear variation within the same modulating period. One cycle of this variation is accomplished. at a rate of f hertz so that the time required to vary the energy through the full range of frequencies (one full cycle) is 1/ f seconds. In the length of time that'it has taken to transmit energy out to the interface andfor that energy to be reflected back to the sonde, the frequency then being transmitted by the transmitting antenna has changed in frequency by a certain finite amount determined by the rate at which the transmitter frequency is being varied.

In FIGURE 3, travel time of the wave is illustrated as a time delay and is represented by the quantity Zd/ v, where d is the distance to the interface and v is the velocity of transmission of the energy through the transmitting formation and is given by v=c/n=c/ /E/E where c is the speed of light and n and E'/E are the index of refraction and the real part of the dielectric constant of the formation normalized by that of free space, respectively. The difference in frequency of the transmitted energy and the reflected energy represents the distance to the interface and back; and, if these two signals are beat one against the other, in a suitable mixer, the resulting difference frequency may be employed to determine the distance to the salt dome or reef. This determination is based on a knowledge of the index of refraction of the intervening formation, as determined by an analysis of cores taken from that formation during drilling of the well bore.

The relationship of the difference in frequency to distance is found in the following equation:

Difference in frequency=rate of change of the changing frequencyxtime between transmission and the reflection Af -R X T which can be written as:

I E. where =modulation rate. B =band width of the frequency modulation d=lateral distance to the interface, and v=the velocity of transmission in the formation which, for measurement purposes, is equal to:

v=c EL 0 where c=velocity of light in air E=the real part of the complex dielectric constant of the formation traversed by the energy at the frequency center E =the real part of the complex dielectric constant of free space.

To improve the near range resolution of the system, the rate of change of frequency (R can be increased by increasing the band width (B) of frequency modulation. In this regard, it has been found that the rate of change of the changing frequency (R can be equal to about 10 to 10 hertz (cycles per second) for interfaces spaced at distancev from a few inches to much longer distances from the ranging system.

FIGURE 4 illustrates a schematic diagram of a ranging system for performing the method of the present invention. In this figure, an oscillator 50 is energized by power supply 51 to generate the basic frequency for transmission into the adjacent earth formation surrounding the borehole. The oscillator may be a magnetron or klystron capable of operating at the desired frequencies and power output. A sweep generator 52 is synchronized with the oscillator and generates a varying potential at the frequency f to cause variation of the transmitted frequency about its center frequency f The output of the oscillator is supplied through an isolator 53 to a transmission line 54 carrying the energy to transmitting antenna 55. Between the isolator 53 and the transmitting antenna is a directional coupler 58 for sampling the frequency of the oscillator 50. The sampled signal is supplied through attenuator 59 to balanced mixer detector 61.

As shown in FIGURE 4, receiving antenna 62 is located adjacent to the transmitting antenna 55 and connected through a transmission line 63 to an attenuator 64. The output of attenuator 64 is supplied as a second input to balanced mixer detector 61 where the transmitted and received signals are mixed to develop a difference frequency. This difference frequency is fed into amplifier 65. A frequency meter 66 measures the frequency of the signal from the balanced mixer detector and supplies that information to the distance indicating device 27 at the earths surface. A camera (not shown) can be utilized to photograph the distance information on distance indicator 27 from which the distance to a salt dome or reef from the present location of the well bore can be determined. The given distance on indicator 27 is associated with the depth on digital indicator 26 and the azimuthal information on indicator 28.

Another form of the transmitter-receiver circuit for the FM ranging system of the present invention is shown in phantom line in FIGURE 4 and employs a single antenna for both transmitting and receiving the electromagnetic energy to reduce both the size of the downhole components of the equipment and the near range resolution of the system. In accordance with this embodiment of the invention, a single antenna, say antenna 55 of FIGURE 4, can be adapted for this purpose by connecting a directional coupler 70 (shown in phantom line) in series between the antenna and attenuator 64 to supply the second input to balanced mixer detector 61. A the mixer detector 61, the transmitted and received signals are then beat together to produce a difference frequency into amplifier 65 and, ultimately, to give an indication of distance to the interface, as previously described.

Another modification of the system not shown herein is the use of a horn antenna with modifications to develop circularly polarized electromagnetic energy, such as placing quarter wave plates within the body of the transmitting horn antenna. In situations where the present invention is useful, under certain conditions only circularly polarized energy can be transmitted successfully through formations having rather high water content.

FIGURE 6 illustrates an alternative antenna system employing pulsed electromagnetic energy. In this embodiment, the sonde includes an instrument housing 80 which preferably includes a high-frequency transmitter 81 and suitable coupling and timing circuits 82 to send electromagnetic pulses to slot antenna 83. The slot antenna 83 includes a cylindrical housing supported on bearings 84. The pulses of electromagnetic energy are radiated from the antenna in an almost omnidirectional, azimuthal pattern normal to the longitudinal axis of the antenna but, because of slot 92, have a cusp or null in one azimuthal direction. The timing circuits 82 control the switching of the antenna 83 periodically from transmitter 81 to receiver 85 by means of a TR switch 86. The output of receiver 85 is transmitted to the earths surface for indication of the travel time of the wave to and returning from the mapped interface of the salt dome or reef. Power supply 87 is shown positioned within the housing 80 but, of course, may be located at the earths surface if permitted by the electrical characteristics of cable 88. At the earths surface, surface-recording equipment includes indicators for depth, for azimuth, and for distance. The distance from the borehole to the nearest reflected side of the salt dome or reef is indicated by the two-way travel time of a pulse of energy and the velocity of the energy in the formation. The azimuthal direction of the interface is indicated by rotating the antenna by means of antenna rotor 90 and gears 91 as the pulses of electromagnetic energy are transmitted and received. The first returning echo signal received at the transmitter represents the closest interface of the salt dome or reef. The slot antenna 83 is rotated until the first returning echo disappears either from an indicator within the sonde or at the earths surface. By associating the null in the returning signal with the azimuthal direction of the slot 92 of the antenna, say by a sensor within the rotor 90, the azimuthal direction of the near interface is determined and displayed.

While certain preferred embodiments of the invention have been specifically disclosed, it should be understood that the invention is not limited thereto as many variations will be readily apparent to those skilled in the art, and the invention is to be given its broadest possible interpretation of the following claims.

We claim:

1. A method for guiding the drilling of a well bore for penetrating oil reservoirs of at least a first earth formation in locational association with an interface formed between said first earth formation and a second rock formation of chemically deposited salt, limestone, anhydrite, or the like, capable of passing electromagnetic energy without undue attenuation or dispersion, after said borehole has entered that said second formation by accident mistake, or the like, comprising: positioning in said well bore within said penetrated second formation a logging sonde at a known logging depth including an electromagnetic generator and an electromagnetic receiver, said generator having an output in a frequency range of 10 to 10 hertz; irradiating said formation with electromagnetic energy from said electromagnetic generator in said logging sonde toward said interface; detecting a portion of said irradiated electromagnetic energy that is reflected back from said interface to said receiver in said logging sonde; comparing the time of travel of said transmitted and received energy to derive data indicating the distance to said interface at said known logging depth; and thereafter directin the drilling of the well bore based on said data to encounter said first formation.

2. A method in accordance with claim 1 in which said electromagnetic energy is continuously emitted but its central frequency within said frequency range is varied between a frequency above and below said central frequency and said distance to said interface is determined by comparing the instantaneous frequency of the transmitted energy with that of the received energy.

3. A method in accordance with claim 1 in which said electromagnetic energy is a pulse of radiation and said distance to said interface is determined by measuring the two-way travel time of said pulse between said interface and back to said sonde.

4. A method in accordance with claim 1 in which said electromagnetic energy propagates omnidirectionally in a plane through said sonde and said distance to said interface is the nearest distance relative to said well bore in said plane of propagation.

5. A method in accordance with claim 1 in which said energy is directed into a confined path and said distance is determined in accordance with the lateral and azimuthal orientation of said path relative to said well bore.

6. A method in accordance with claim 5 in which said path is substantially parallel to the axis of the well bore and said energy passes through the bottom of said well bore.

7. A method in accordance with claim 5 in which said path is substantially horizontal relative to said well bore and is of a known azimuthal direction relative to said well bore. t

8. A method in accordance with claim 1 in which said energy is directed in azimuth in a modified omnidirectional pattern having a known null direction therein, and said distance to said interface is the closest distance relative to said well bore in said azimuthal direction of energy propagation.

9. A method in accordance with claim 8 in which the azimuthal direction of said closest interface is determined by sequentially irradiating said formation as said null direction is varied in azimuth, said azimuthal direction being indicated when the null direction in azimuth coincides with that of said closest interface.

10. A method in accordance with claim 1 in which said last-mentioned step of directing the drilling of the well bore to encounter said first earth formation includes sidetracking said well bore in a new direction relative to its existing direction.

11. A method in accordance with claim 1 in which said last-mentioned step of directing the drilling of the well bore to encounter said first earth formation includes continuing drilling along the existing direction of the well bore.

12. A method for guiding the drilling of a well bore for penetrating oil reservoirs in at least a sedimentary formation in locational association with an interface formed between said sedimentary formation and a salt dome after said borehole has entered said dome by accident, mistake, or the like, comprisingz' positioning in said well bore within said penetrated dome a logging sonde at a known logging depth including an electromagnetic energy generator and receiver, said generator having an output in a frequency range of 10 to 10 hertz; irradiating said dome with electromagnetic energy from said electromagnetic generator in said logging sonde toward said interface; detecting a portion of said irradiated electromagnetic energy that is reflected back from an interface of said dome to said receiver in said logging sonde; comparing the time of travel of said transmitted and received energy to derive data indicating the distance to said interface at said known logging depth; removing said logging sonde from said well bore; and directing the drilling of the well bore based on said data to encounter said sedimentary formation.

13. A method for guiding the drilling of a well bore for penetrating oil reservoirs associated with a reef within a rock formation formed of limestone, anhydrite, or the like, after said borehole has entered that said formation by accident, mistake, or the like, comprising: positioning in said well bore Within said penetrated formation a logging sonde at a known logging depth includin an electromagnetic energy generator and receiver, said generator having an output in a frequency range of 10 to 10 hertz, irradiating said formation with electromagnetic energy from said electromagnetic generator in said logging sonde; detecting a portion of said irradiated electromagnetic energy that is reflected back to said receiver in said logging sonde from an interface formed between said reef and said formation; comparing the time of travel of said transmitted and received energy to derive data indicating the distance to said interface at said known logging depth; removing said logging sonde from said well bore; and directing the drilling of the well bore based on said data to encounter said reef.

References Cited UNITED STATES PATENTS 3,286,163 11/1916 Holser et. al 324-6 3,350,634 10/1967 Hoehn 324-6 RUDOLPH V. ROLINEC, Primary Examiner.

GERARD R. STRECKER, Assistant Examiner.

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