US 20080002522 A1
Methods are disclosed employing seismic waves having infrasonic frequencies in the range of about 0.1-20 Hz for generating electromagnetic waves of similar infrasonic frequency in hydrocarbon bearing subterranean formations located at depths up to about 5000 meters, said electromagnetic waves having sufficient voltage amplitudes for detection at the earth's surface. Also disclosed are seismic sources capable of generating infrasonic seismic waves of sufficient amplitude for generating electromagnetic waves in hydrocarbon bearing formations at depths up to 5000 meters, said electromagnetic waves having voltage amplitudes sufficient for detection at the earth's surface.
1. A method for geophysical prospecting a selected geographical area to determine the location, areal extent and depth to hydrcarbonaceous bearing strata under the earth's surface on land or beneath a body of water, comprising the steps of:
a) deploying an acoustical energy source and an acoustic wave detector in a lineal array;
b) deploying an electromagnetic wave generator and one or more electromagnetic wave receivers in said linear array;
c) generating an infrasonic frequency acoustic wave front which propagates downwardly from the earth's surface and is reflected, refracted and scattered by the earth's media and generates an acoustic signal train which returns to the earth's surface and is detected by said acoustic wave detector, said infrasonic acoustic wave front also generating and induced electrokinetic effect electromagnetic wave in the infrasonic frequency range which propagates upwardly when said infrasonic acoustic wave front impinges upon a hyrcarbonaceous stratum beneath the earth's surface;
d) generating an energy burst of a low frequency electromagnetic sounding wave from said electromagnetic wave generator which propagates downwardly in the earth media and is reflected, refracted and scattered, thereby returning a portion of its energy as a sounding electromagnetic wave toward the surface of the earth;
e) detecting and measuring said returning acoustic wave energy, said induced electrokinetic electromagnetic wave and said sounding electromagnetic wave at said receiver and said electromagnetic detector in said lineal array; and
f) combining said detected and measured signals from said three sources together in a manner to provide location, areal extent and depth to hydrocarbonaceous strata beneath the earth's surface.
Claiming priority of Provisional Application 60/541643 filed Feb. 4, 2004
1. Field of the Invention
This invention relates to the acquisition of geophysical data including that produced by induced electromagnetic effect, seismic, and coupled phenomena thereto, such as electromagnetic propagation velocity dispersion effects in lossy media.
2. Description of the Prior Art
It is a well known phenomena that electromagnetic fields impinge on the earth's surface at all times and create both electric fields and magnetic fields that are detectable utilizing proper techniques and apparatus. Such fields, for example, are generated by solar activity and ionospheric activity in the earth's own atmosphere. It has been known since the 1930's that seismic waves generated in seismic exploration techniques can generate Induced Electrokinetic (IEK) effects in the form of electromagnetic waves emanating from the earth formation structures impinged upon by the seismic waves. Prior art patents, such as U.S. Pat. No. 3,975,674 to McEuen, have taught that the electrokinetic effect radiation may be used for determining, remotely, resistivity and equivalent salinity of fluids saturating pore space in subterranean formations.
Although the McEuen patent and other more recent patents have indicated an interest in prospecting for geophysical data using induced electrokinetic (IEK) effect electromagnetic radiation, most researchers have conceded that the depth of investigation of such techniques is limited. Papers as lately as the mid 1990's have indicated that a depth of approximately 500 meters from the earth's surface was about the limit from which useful data concerning induced electrokinetic (IEK) effect electromagnetic radiation.
During the interim since the 1930's, (and the discovery of the electrokinetic effect in its first publication), seismic data processing has undergone remarkable advances. More sensitive detectors, two dimensional arrays of detectors on land, and offshore exploration by seismic streamer cable towing ships together with the advancement of computer technology have enabled seismic data to be processed and corrected for the time migration or “move out” of such data. This correction is needed to convert time line arrival data onto actual depth due to acoustic wave velocity dispersion and velocity differences in subsurface media. The earth's true acoustic velocity profile in its layers is usually unknown at the time of seismic exploration.
The present invention incorporates techniques for acquisition and processing of seismic data when necessary in order to verify or enhance induced electrokinetic electromagnetic radiation effects which are measured simultaneously therewith or in timed sequence therewith at the earth's surface. Experimental data gathered with the system of the present invention and practicing the methods thereof have successfully differentiated between dry hole prospects and producing wells in known hydrocarbon producing formations. Further these techniques have explained the discrepancies between the initial seismic survey data and the later induced electrokinetic electromagnetic radiation survey data over the same area.
Moreover, the present invention provides a means for directly detecting hydrocarbon interfaces at depth down to 5000 meters. This sharply contrasts with the previous published results as late as the mid 1990's and indicates that the invention is a significant advance in the art of geophysical prospecting.
Now, according to the present invention, I have discovered improved methods for direct prospecting for hydrocarbon bearing formations in subsurface earth strata, which methods comprise:
In a geographical area of interest:
The present invention relates to improved methods and apparatus for detecting subsurface deposits of hydrocarbons employing electrokinetic phenomenon. It is well known that seismic waves, generated by acoustic sources, generate electromagnetic dipoles within subterranean earth formations with which the seismic waves interact. These electromagnetic dipoles generate electromagnetic waves which propagate through the earth and may be detected at the earth's surface. These electromagnetic waves provide information concerning the subterranean formations within which the electromagnetic waves were generated. As with seismic or acoustic (mechanical) wave velocities, the electromagnetic (EM) waves can also suffer attenuation and velocity dispersion due to the electric and magnetic properties of the earth's materials. In some instances, these may be substantial enough to require taking into account in field data interpretation.
Seismic waves 105 propagating through the earth 106 encounter sub-surface formations, represented by formation 103 in
Referring now to
Electromagnetic waves 107 resulting from induced electrokinetic effect (
Arrival time of IEK signal after activation of acoustic source 102 can be estimated by the equation:
As stated above electromagnetic waves 107 have the same frequency spectrum as the seismic waves 105, from which electromagnetic waves 107 are generated. Seismic waves 105 are amplitude attenuated as they propagate through earth formations comprising the body of the earth 106, thereby having less amplitude as they penetrate deeper into the earth 106. Consequently, the amplitude of seismic waves 105 which interact with subsurface formations, such as formation 103, become less as the depth 108 to formations 103 increases. Additionally, the higher frequency components of seismic waves 105 are attenuated to a greater extent than lower frequency components of seismic waves 105 as the seismic waves 105 are propagated deeper into the earth 106. Therefore, as seismic waves 105 penetrate to greater depths in the earth 106, the amplitudes of seismic waves 105 are attenuated at an uneven rate according to frequency. That is, the higher frequency components of seismic waves 105 are attenuated more than the lower frequency components of seismic waves 105. The amplitude of seismic waves 105 at any depth determines, in part, the voltage amplitude of electromagnetic (EM) waves 107 generated by interaction of seismic waves 105 with formation 103. Thus, as the depth 108 to formation 103 increases, the voltage amplitude of electromagnetic waves 107 generated, in general, decreases as a result of the attenuation of seismic waves 105.
As shown above, the voltage amplitude of electromagnetic waves 107 are also attenuated as electromagnetic waves 107 propagate through the earth 106. Attenuation of the voltage amplitudes of electromagnetic waves 107 increases with the distance the electromagnetic waves 107 travel through earth 106. Thus, as the depth 108 to formation 103 increases, attenuation of electromagnetic waves 107 reaching receiver 104 increases. Also, the higher frequency components of the electromagnetic waves 107 are more highly attenuated than the lower frequency components as the electromagnetic waves 107 propagate through the earth 106. The EM wave propagation velocity also varies as a function of multiple properties of the rock matrix and fluid content of earth media 106.
As a consequence of the increased attenuation of seismic waves 105 and increased attenuation of the electromagnetic waves 107 as the distance such waves travel through the earth 106 increases, the depth 108 of a formation 103 from which meaningful information may be recovered is limited. That is, the voltage amplitude of the electromagnetic waves 107 reaching receiver 104 must be sufficient to be detected and recorded, and just so with the seismic waves reaching seismic detector.
According to the present invention, I have discovered that by employing a seismic source 102 which produces a substantial proportion of its seismic energy in the form of seismic waves 105 having frequencies in the infrasonic range, (that is, from about 0.1 to about 20 Hz), allows generation of electromagnetic waves 107, having a similar frequency range in hydrocarbon bearing formation 103 at a depth 108 of up to about 5,000 meters or more, which electromagnetic waves 107 have sufficient voltage amplitude to be recorded and processed in receiver 104. The voltage amplitudes of such electromagnetic waves 107 are in part determined by the relatively low frequencies, in the infrasonic range, and the presence of hydrocarbons, particularly oil, in the formation 103.
Seismic waves 105 and electromagnetic waves 107, having frequencies in the infrasonic range, are attenuated less than seismic and electromagnetic waves having higher frequencies. Hydrocarbons, particularly oil, present in formation 103, have a high electrical resistivity and consequently contribute to generation of electromagnetic waves 107 having higher voltage amplitudes than electromagnetic waves generated in formations which do not contain hydrocarbons. In common cases, specific electrical resistivity of natural hydrocarbons is 8-10 orders of magnitude higher than that of contact brines which also occur. The reduced attenuation at infrasonic frequencies of seismic waves 105 leads to generation of higher amplitude electromagnetic waves 107 due to the higher resistivity of hydrocarbons in formation 103. These larger amplitude (at infrasonic frequencies) waves 107 allows detection of the electromagnetic waves 107 generated at greater depth 108 from hydrocarbon bearing rock matrices than from non-hydrocarbon bearing rock matrices.
A seismic wave generator which produces a substantial portion of its seismic energy in the infrasonic range (about 0.1 to about 20 Hz) is preferably used as sonic source 102. For example, explosives, thumpers, vibrators, and other seismic generators which convert substantial portions of their seismic energy into infrasonic seismic waves may be employed in the present invention. A preferred seismic generator which may be used as seismic source 102 is shown in
Referring now to
Generally speaking, the metering and formatting circuitry 202 comprises an analog to digital converting system and computer firmware, hardware and software to provide inputs to a computing system. The computing system comprises computers 203 and 204 which are interconnected to each other as indicated by line 206. The system may be thought of as two separate computing components 203 and 204 which are interconnected by line 206 as previously stated. However, from a practical standpoint a single multi programmable general purpose digital computer can serve to provide the function of the separate individual computers 203 and 204. Further, it will be appreciated by those skilled in the art that it is understood that the computing system used in this data acquisition and control may comprise anything from personal computers or PC's, to mainframe high speed digital processing systems.
Referring now to
It will be observed in
Referring again to
In general, the distances between A, B and C will be chosen for an individual survey for geophysical data. These distances will depend on the depth and size of hydrocarbon accumulation suspected to be present. They will also be affected by the depth of the ocean (in offshore exploration), and the physical properties of the subterranean earth media (such as conductivity and resistivity). High conductivity tends to attenuate electromagnetic return signals. These distances, of course, will be customized on any individual survey made with the system of the present invention.
Barrel 501 is comprised of electrically conductive material, such as steel or copper, and is of sufficient strength to maintain it's shape in use. Electrode 503 is electrically conductive and preferably comprises copper. Plasma injector-electrode 503 is connected via an electrical supply line 504 to the negative pole of a DC electric power source, not shown. Barrel 501 is electrically connected via line 505 to the positive pole of the DC electric power source or to electrical ground, not shown. Electrical supply line 504 is electrically insulated down to its connection with electrode 503.
The dimensions of sparker gun 500 are determined by the electrical energy to be discharged in operation. For example, for the discharge of electrical energy of approximately 1000 amps at a voltage from about 600-2500 volts, barrel 501 may have a diameter 510 in the range of about 7 inches to 25 inches. Electrode 503 is of a diameter sufficient to carry the electric current without overheating when sparker gun 500 is fired. For the electrical load above, (600-2500 volts; 1000 amps), a copper electrode 503 may have a diameter in the range of about 0.2-1.0 inches. Preferably, electrode 503 forms a sharpened tip 51 1, such that electrical energy will discharge from electrode tip 511 at a high energy density.
In operation, sparker gun 500 is aligned such that barrel bottom 508 is oriented toward a earth strata which is of interest for study according to the method of the present invention. The interior of barrel 501 is filled with an electrically conductive fluid 507, such as salt water or brine. When employed in an offshore situation within a body of water, as will be hereinafter be described, bottom 508 of barrel 501 may be open such that barrel 501 is filled with incident water from the body of water. When employed in an onshore operation, as will be hereinafter described, barrel open bottom 508 may be sealed with a resilient member 506, and barrel 501 filled with an electrically conductive fluid 507, such as salt water or brine. Resilient member 506 will have sufficient strength to contain the electrically conductive fluid 507 within barrel 501 without substantial leakage, but not be so rigid as to unduly attenuate the acoustic waves generated upon firing of sparker gun 500. Resilient member 506 may conveniently comprise a resilient rubber or plastic membrane attached to the open bottom 508 of barrel 501.
In operation, sparker gun 500, with barrel 501 filled with electrically conductive fluid 507 and having bottom 508 directed toward the subsurface formations of interest, is discharged by passing a DC electric current from a power source, not shown, through line 504, to electrode 503. From electrode 503, electric current is discharged from electrode point 51 1 through the electrically conductive fluid 507 to the inner wall 513 of barrel 501. From barrel 501 the electric current flows via line 505 to the positive pole of the electrical power source or to an electrical ground, not shown In an offshore situation, barrel 501, in contact with the incident body of water, ay serve as electrical ground.. Discharge of electrical energy from electrode point 511 into conductive fluid 507 generates a volume of plasma and vapor, the plasma sock 512, in conductive fluid 507. The generation of plasma sock 512 creates a mechanical energy impulse which is discharged from the interior of barrel 501 through barrel bottom 508, generating acoustic waves which pass into a body of water, (for offshore operations), or into the earth, (for onshore operations), with which sparker gun 500 is in contact. Sparker gun 500, as shown and described herein above, advantageously generates acoustic waves having about 50% or more of their energy concentrated in acoustic waves having a frequency in the infrasonic range of about 0.1-20 Hz.
A time delay of less than about 0.5 seconds after firing the seismic source was imposed before the recording of the voltage amplitudes received was initiated. At a time of about 0.5 seconds, the detected voltage amplitudes from the three receivers reach peaks 604, 605 and 606. These voltage amplitude peaks 604, 605 and 606 correspond to a known hydrocarbon bearing formation at a depth of about 500 meters in the Golitzino field. Further, at a time of about 1.4 seconds, the voltage amplitudes detected by the three receivers reach peaks 607, 608 and 609 which correspond to a known hydrocarbon bearing formation at a depth of about 2200 meters in the Golitzino field. The voltage amplitude peaks of electromagnetic waves received from the known hydrocarbon bearing formations, compared to the lower voltage amplitudes of electromagnetic waves received from non-hydrocarbon bearing formations, demonstrate the utility of the methods and apparatus of the present invention for detecting the presence of hydrocarbon bearing formations.
In addition to detecting the presence of hydrocarbon bearing formations, application of the method of the present invention provides information which allows determination of the depths to the hydrocarbon bearing formations and of the relative amounts of hydrocarbon in the formations.
As to depth of the formation, seismic waves generated by the seismic source are propagated through the overlying water and the intervening earth formations at sonic velocities and the induced electromagnetic waves are propagated back at the speed of EM wave propagation. Depth determination, however, is not a straight forward calculation because the speed at which the seismic waves and electromagnetic waves traverse the water and each intervening formation may vary. Also, the seismic and electromagnetic waves may be refracted in some of the formations traversed. Thus the speed of sound and degree of refraction of seismic waves must be determined for each formation through which the seismic waves pass, and a “migrated” path and velocity of seismic waves from the seismic source to the formation of interest must be determined. This determination of the time migrated path and velocity of seismic waves is a well known and widely practiced art in the field of seismic exploration of subterranean formations and will not be further described herein. Upon determination of the migrated path and sonic velocities of the seismic waves, the depth to a formation of interest may be calculated, provided the EM wave propagation velocity can be determined. Electromagnetic sounding waves can be used to determine the EM propagation velocity.
An indication of the amount of hydrocarbon in a formation may be obtained from the voltage amplitude of the induced IEK electromagnetic waves received from the formation. The greater the voltage amplitude of induced electromagnetic waves received from a formation of interest compared to the voltage amplitudes of electromagnetic waves received from other formations at similar depths and distances from the receivers, then, the greater the amount of hydrocarbon present in the formation of interest.
Pure seismic data is reliant on a knowledge or assumption concerning the acoustic wave propagation rate in the rock media beneath the earths surface. This is necessarily so since the speed of acoustic wave propagation is so slow that it may take several seconds for a single energy impulse of acoustic energy to travel to deeper rock layers and return to the surface. In contrast to this, the return of the Induced Electrokinetic Effect (IEK) electromagnetic EM wave field occurs generally faster. This is essentially the same in all subsurface media. It will be noted, of course, that attenuation of the amplitude of electromagnetic energy in subsurface media varies more than does attenuation of acoustic energy through the same media. In some instances, particularly in cases where the subsurface formation media are more electrically conductive than insulative, such media tend to rapidly attenuate electromagnetic wave propagation.
In spite of these variations, however, the propagation of the electromagnetic EM wave energy through earth formations for geophysical prospecting may be assumed to be a determinable function of time from the time it is generated and transmitted until the time it is received and impinges upon the surface or near surface receivers. A distance versus time plot of IEK return signals can require some “move out” or wave migration processing corrections. Such corrections are known to be necessary with seismic waves in order to correct for velocity dispersion and to migrate the seismic waves to true depth. Similarily, the technique of the present invention includes those for correcting the EM wave field to true depth.
Referring now to
The first striking feature observed in
It will also be observed, however, that large positive going peaks over the same geographical area are indicated for well number 2 and well number 4 which are known producing wells having successful hydrocarbon production therefrom. The contrast shown in
Referring now to
The results obtained in this pilot IEK survey have shown as follows:
Dry prospects confirmed by drilling have no IEK anomalies.
Referring now to
Referring now to
Referring now to
It may be stated, from the above descriptions of
The effect of EM wave velocity dispersion in earth formations near the surface is addressed in an experimental paper by A. H. Thompson and G. A. Gist at page 1169 of the December 1993 issue of “The Leading Edge”. This experimental paper, however, only deals with relatively near surface phenomenon down to a depth in the earth of 20 meters or so.
A more recent experimental and theoretical paper by J. Garambois and M. Dietrich, published Jun. 11, 2001 in “Geophysics Online” and in “Geophysics”, VOL. 66, NO. 5, pp 1417-1430 (September-October 2001) delves into this thorny subject in more depth. This paper suggests the use of separate transducers sensitive to the electric and to the magnetic components of the induced EM waves, when coupled with theoretically derived filter functions to suppress noise, etc., can result in acquisition-of useful data from greater depth under the earth's surface. The paper suggests, however, that the near surface material electrical and magnetic properties may tend to mask or hide valid IEK electromagnetic signals generated at greater depth if not properly accounted for. The technology of the present invention, however, belies such fears and complications as agreement and enhancement of IEK EM wave data as well as seismic data are consistent with each other as given previously in the examples shown in
Another more recent article appearing in “First Break” Vol. 20, pp 142-143. Mar. 3, 2002 discusses how an EM wave “sounding” technique can be combined with seismic data taken over the same geographical area to provide an indication of whether liquids imaged by the seismic data are hydrocarbon or water. In this paper the authors (T. Eialesmo and S. Ellingswal) used an EM. wave source located on the sea floor to import EM waves at low frequencies into the earth's layers below the sea floor. A horizontal electric dipole source and an array of electric field receivers are used to emit an EM wave signal in the frequency range from a few tenths of one hertz to a few tens of hertz into the underlying earth layers. The signal is rapidly attenuated in the conductive sediments, but in a resistive layer (hydrocarbon bearing) the propagation of the EM wave is more efficient and the layer can act as a “duct” or wave guide for propagation along the layer. Energy from the ducted EM wave leaks back upward toward the seafloor deployed electric field receivers where it is detected. When the source to receiver offset (ie distance) is comparable to, or greater than, the depth to the ducting layer, energy leaking from the layer will dominate over the energy reflected back to the surface from shallower or deeper EM wave reflectors such as brine or salt water bearing layers.
U.S. Pat. No. 6,717,411B2 issued Apr. 6, 2004 and is directed to the technique in the “First Break” article discussed immediately above. It is noted, however, that this technique is viewed by the inventors as a stand alone EM wave exploration method and system which does not contemplate, or even discuss, the IEK signals generated by contemporaneous use of seismic energy to excite such IEK signals. Instead the patented technique is viewed as a tool to discover and monitor reservoir fluid movement over a period of time or to locate and determine the depth of the ducting layers in the earth. Such a use for an EM wave sounding system is also disclosed in another research article appearing in “First Break” Vol. 20, pp 224-225, Apr. 4, 2002 by A. Ziolkowski, B. Hobbs, and D. Wright This article describes experimental data acquired over an underground gas storage reservoir in France. Again, however, the use of seismic induced IEK signals is not discussed or even contemplated in the paper.
The methods and systems of the present invention are directed to using a combination of electrokinetic (IEK), electromagnetic (EM wave) and seismic data to get a much more reliable appraisal of evaluation of a geographic area being investigated than has heretofore been possible. The present invention enables location and depth to a hydrocarbon bearing earth strata to be determined along with an estimate of the nature of the fluids contained therein.
In practicing the invention techniques offshore the seismic source and detectors of seismic returns, the EM wave transmitter and receiver and the IEK response sensing detectors may all be deployed along a streamer cable towed by a conventional exploration vessel. Using the seismic source an infrasonic burst of seismic energy is applied via the seismic source and a seismic reflection response from the seismic detectors (hydrophones) is detected. An EM wave source such as a low frequency generator and capacitor bank is used to generate a powerful EM wave burst of energy in the 0.1 hertz to 50 hertz frequency range which propagates downwardly and is reflected and refracted back toward the EM wave detectors in the streamer array. The EM wave response (or sounding) is used to determine EM wave propagation velocity by timing and recording the return EM wave signals. The range of power used, the length of the EM wave antenna on the streamer and any orientation angle of polarization which can be detected are determined by the geological conditions of the area being explored. The IEK signal induced in the subsurface interface from deep hydrocarbonaceous strata is also detected and recorded These data may then be processed together using the concepts of the present invention to determine the presence or absence of a hydrocarbon containing reservoir, an areal outlining of the reservoir and an appraisal of its depth. In this processing the firing rate of the seismic source, the firing time of the EM wave source and other parameters are treated as independent variables which can be adjusted in real time during the survey based on data quality being gathered. Thus a feedback adjustment is available for optimizing the survey as it is being made.