Use of minor borehole obstructions as seismic sources

[0002] The invention relates to the downhole generation and recording of seismic waves for use in investigation and monitoring of earth formation reservoir characteristics surrounding a wellbore. In particular, the invention relates to a method and system for monitoring seismic energy radiated from wellbores into surrounding earth formations. The seismic body waves radiated into the surrounding earth formation, which may be converted from borehole tube waves, are used for cross well type projects and reverse VSP type projects to investigate and monitor hydrocarbon or other mineral deposits over the productive lifetime of a producing reservoir.

[0003] This is a system for monitoring the distribution of the contents of a subsurface mineral deposit over its economic life span for long-term resource management. The system employs acoustic waves generated when borehole tube waves impinge upon minor borehole obstructions. Time varying changes of selected attributes of those acoustic waves that have transited the deposit between boreholes, may be indicative of the temporal changes in the mineral content.

[0004] Geophysical surveys are used to discover earth structure, mineral deposits, and the subsurface extent of mineral deposits such as oil, natural gas, water, sulphur, etc. Geophysical methods may also be used to monitor changes in the deposit, such as depletion resulting from production of the mineral over the economic lifetime of the deposit. The usefulness of a geophysical study depends on the ability to quantitatively measure and evaluate some geophysical analogue of petrophysical parameters related to the presence of the mineral under consideration.

[0005] Seismic methods may be applied to production-management monitoring as well as to exploration. As is well known to geophysicists, an acoustic source at or near the surface of the earth is caused periodically to inject an acoustic wavefield into the earth at each of a plurality of source survey stations. The wavefield radiates in all directions to insonify the subsurface earth formations. The radiated wavefield energy is reflected back to be received by seismic sensors (receivers) located at designated stations also usually located at or near the surface of the earth, but which may also be in the subsurface. The seismic sensors convert the mechanical earth motions, due to the reflected wavefield, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing.

[0006] The travel-time lapse between the emission of a wavefield by a source and the reception of the resulting sequence of reflected wavefields by a receiver is a measure of the depths of the respective earth formations from which the wavefield was reflected. The relative amplitudes of the reflected wavefields may be a function (an analogue) of the density and porosity of the respective earth formations from which the wavefields were reflected as well as the formations through which the wavefields propagated. The phase angle and frequency content of returned signals in the reflected wavefields may be influenced by formation fluids, the sought-for minerals or other formation characteristics.

[0007] The processed seismic data associated with a single receiver are customarily presented as a one-dimensional time scale recording displaying rock layer reflection amplitudes as a function of two-way wavefield travel time. A plurality of seismic traces from a plurality of receivers sequentially distributed along a line of survey at intervals, such as 25 meters, may be formatted side by side to form a two dimensional (2-D) analog model of a cross section of the earth. Seismic sections from a plurality of intersecting lines of survey distributed over an area of interest provide three-dimensional (3-D) imaging. A series of 3-D surveys of the same region made at successive time intervals, such as every six months, would constitute a 4-D, time-lapse study of the subsurface that would be useful to monitor, for example, the fluid-depletion rate of an oil field.

[0008] The term “signature” as used herein, means the variations in amplitude, frequency an phase of an acoustic waveform (for example, a Ricker wavelet) expressed in the time domain as displayed on a time scale recording. As used herein the term “coda” means the acoustic body wave seismic energy imparted to the adjacent earth formation at a particular location. The coda associated with a particular seismic energy source point or minor well bore obstruction in this invention will be the seismic signature for that seismic energy source point. The term “minor borehole obstruction” or “borehole discontinuity” or “discontinuity” means an irregularity of any shape or character in the borehole such that tube wave energy transiting the wellbore will impart some energy to the irregularity in the borehole and thus radiate body wave energy into the surrounding earth formation while also transmitting and reflecting some the tube wave energy as well. The term “impulse response” means the response of the instrumentation (seismic sensors and signal processing equipment) to a spike-like Dirac function or impulse. The signal energy of an acoustic wavefield received by seismic sensors depends upon the texture of the rock layers through which the wavefield propagated, from which it was reflected or with which it is otherwise associated, whether along vertical or along lateral trajectories. The term “texture” includes petrophysical parameters such as rock type, composition, porosity, permeability, density, fluid content, fluid type and inter-granular cementation by way of example but not by way of limitation.

[0009] From the above considerations, it is reasonable to expect that time-lapse seismic monitoring, that is, the act of monitoring the time-varying characteristics of seismic data associated with a mineral deposit such as an oil field over a long period of time, would allow monitoring the depletion of the fluid or mineral content, or the mapping of time-varying attributes such the advance of a thermal front in a steam-flooding operation.

[0010] Successful time-lapse monitoring requires that differences among the processed data sets must be attributable to physical changes in the petrophysical characteristics of the deposit. This criterion is severe because changes in the data-acquisition equipment and changes in the processing algorithms, inevitable over many years may introduce differences among the separate, individual data sets from surveys that are due to instrumentation, not the result of dynamic reservoir changes.

[0011] In particular, using conventional surface exploration techniques, long-term environmental changes in field conditions such as weather and culture may affect the outcome. If time-lapse tomography or seismic monitoring is to be useful for quantitative oil-field reservoir monitoring, instrumentation and environmental influences that are not due to changes in reservoir characteristics must be transparent to the before and after seismic data sets. Successful time-lapse tomography requires careful preliminary planning.

[0012] One way to avoid many time-dependent environmental changes and updated state of-the-art instrumental changes is to permanently install seismic sources and seismic detectors in one or more boreholes in and around the area of economic interest. Identical processing methods are applied to the data throughout the monitoring period using cross-well (cross-borehole) tomography rather than conventional surface type operations. One such method is disclosed in patent application Ser. No. 08/949,748, filed Oct. 14, 1997 now U.S. Pat No. 5,886,255 and assigned to the assignee of this invention and which is incorporated herein by reference as a teaching of cross-well tomography.

[0013] U.S. Pat. No. 5,406,530, issued Apr. 11, 1995 to Tokuo Yamamoto, teaches a non-destructive method of measuring physical characteristics of sediments to obtain a cross sectional distribution of porosity and permeability values and variations and of shear modulus and shear strength. A pair of boreholes has borehole entries spaced apart from each other at a predetermined distance and a plurality of hydrophones is spaced at predetermined known locations. A pseudo random binary sequence code generator as a source of seismic energy is place in another borehole and activated to transmit pseudo random wave energy from the source to the hydrophones. Seismic wave characteristics are measured in a multiplicity of paths emanating from the source to the hydrophones using cross-well tomography.

[0014] The Yamamoto teaching is primarily directed to use in shallow boreholes for engineering studies. Such holes are less than 100 meters deep, as opposed to oil-field boreholes, which may be two to five kilometers deep. The requirement for an active source to be placed at various levels in the borehole is problematic because the source can damage the hole and interfere with production. Since the seismic equipment must be moved up and down the boreholes, it is impossible to maintain identical recording conditions over an extended time period.

[0015] G. W. Winbow in U.S. Pat No. 4,993,001 issued Feb. 12, 1991, describes a method and apparatus for converting tube waves into downhole body waves for seismic exploration. The equipment comprises a rotary-valve tube wave source for producing swept-frequency tube waves that are injected into tubing or well bore fluid. The tube waves are converted into body waves by an elongate tube wave converter located at a selected position downhole. The tube wave converter comprises an elongate body that preferably substantially fills the well bore or tubing and has a preferred shape in order to convert efficiently the tube waves to body waves at the selected position downhole. This patent is directed primarily to reverse VSP. Winbow acknowledges that it is well known in the art that “nonuniformities in the borehole” cause seismic-wave mode conversions that cause secondary acoustic radiation and associated multiples.

[0016] Winbow employs a single tube-wave converter to serve as a single source of direct and reflected seismic waves but he must repeatedly reposition the device at spaced-apart intervals down the length of the borehole to get extended vertical coverage as in cross-well tomography. That system thus is difficult to implement for the fixed permanent instrumental installation required for 4-D seismic monitoring operation.

[0017] There is a need for a system of seismic sources fixed permanently in boreholes that may be used for monitoring time-varying reservoir attributes such as the distribution of the contents of a mineral deposit. Additionally, there is a need for a system that not only would be used with intentionally generated seismic energy, but could also take advantage of naturally occurring or ambient energy in boreholes, energy that creates tube waves that may be converted to seismic body waves radiated into earth formations around boreholes.

[0018] This is an acoustic system and method for monitoring the radiation of a seismic wavefield into an earth formation surrounding at least a first source borehole containing a column of wellbore fluid. The energy source for the radiated seismic waves may be naturally occurring or intentionally provided energy that produces borehole tube waves that are converted to body waves are discontinuities in borehole tubulars. A plurality of arbitrarily spaced borehole discontinuities or minor obstructions or irregularities is present axially in a borehole fluid column. Each borehole discontinuity or minor borehole obstruction forms a point source for diffractively radiating an acoustic wavefield into the surrounding formation whenever any one of said discontinuities or minor obstructions intercepts the tube wave as the tube wave transits the column of well bore fluid. The resulting plurality of acoustic wavefields, each exhibiting a unique waveform coda, is thereby radiated into the formation. An acoustic sensor in association with one or more said discontinuities or minor borehole obstructions detects the unique waveform coda characteristic of each of the respective wavefields to provide a plurality of uniquely encoded pilot signals. A plurality of sensors for detecting seismic energy separated from the source borehole by an intervening earth formation is provided. The plurality of seismic receivers for receiving the plurality of acoustic wavefields radiated from the source borehole after the wavefields have propagated through the intervening earth formation may be in another borehole or on the surface of the earth. A cross correlator is provided for cross-correlating the respective uniquely encoded pilot signals with a corresponding received acoustic wavefield to furnish a cross correlogram that is indicative of the current value of the pre-selected attribute. A time-lapse profile is created that is representative of the temporal changes in the petrophysical characteristics and mineral content or distribution of the intervening earth formation by repeating the process at subsequent time intervals.

[0019] The novel features which are believed to be characteristic of the invention, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the invention is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the invention:

[0020] FIG. 1 is a step-by-step diagram of tube wave propagation impinging on a discontinuity or minor borehole obstruction to become a body wave.

[0021] FIG. 2 is a time-lapse sequence as tube wave energy reverberation impinges on two minor borehole obstructions and the well bottom hole to create source signatures at the discontinuity locations.

[0022] FIG. 3 is a schematic simulation of a tube wave creating codas or source signatures.

[0023] FIG. 4 is a diagram of minor borehole obstructions as they are placed in a well bore.

[0024] FIG. 5 is a diagram of ‘enlargement’type borehole discontinuities placed in a well bore.

[0025] FIG. 6 is design for a complex borehole discontinuity.

[0026] FIG. 7 is a schematic of a Reverse VSP type project utilizing the invention.

[0027] FIG. 8 is a schematic of a Cross-Well application type project utilizing the invention.

[0028] FIG. 9 is a schematic of a Multi-Well Cross-Well application type project utilizing the invention.

[0029] For the purpose of clarity and explanation, the method of this invention will be described by way of example, but not by way of limitation, with respect to cross-well tomography with receivers in boreholes as well as the earth surface. It is to be clearly understood that the method may be applied to any multichannel data recording geometry or acquisition regime whether seismic sensors are in the subsurface, in well bores, or at or near the surface of the earth. Additionally, when the terms “borehole obstruction” or “borehole discontinuity” are used, they refer to any shape or character of location in a borehole where tube wave energy is reflected, transmitted or converted to body wave energy.

[0030] Borehole tube waves occur the fluid column of well bores. The energy source that excites tube waves may be naturally occurring or intentionally generated by any known method. This invention may be practiced using tube wave generated entirely by naturally occurring or ambient energy sources, as would be the case in a completely passive system of recording. This invention may be practiced using tube waves generated by an acoustic driver, which is a seismic source of any known type intentionally exciting borehole tube waves. The invention may be practiced using a combination of passive and active borehole tube wave energy source methods.

[0031] The tube wave energy transits along the fluid column. Part of the tube wave energy will be converted to body waves at well bore discontinuities of any type, for instance, a change in the character of borehole tubulars such as a wellbore packer. Body waves travel through earth formations and may be received at seismic receivers.

[0032] The conversion of tube wave energy to body waves as tube wave energy transits axially in a well bore is illustrated in FIG. 1A. Tube wave energy 103 transits directionally along a well bore 101 as shown in cross-section in FIG. 1A. As the tube wave energy 103 impinges on a discontinuity or minor borehole obstruction 105 of any type, some of the energy is imparted to the discontinuity at an arbitrary time T1. Some tube wave energy will be radiated out of the borehole and into the surrounding earth formation as body wave energy 106 at the location in the borehole where the borehole discontinuity 105 is present. Additionally, as shown in FIG. 1B at arbitrary time T2, some energy will be reflected back in the opposite direction the tube wave energy has just transited 107. Part of the energy 109 will also be transmitted and so continue transiting along the well bore along the same direction as before the tube wave energy impinged on the obstruction.

[0033] FIG. 2 is a schematic time lapse sequence of tube wave energy reverberating in a well bore 101 with two borehole discontinuities or minor borehole obstructions and the well bore bottom converting tube waves to body waves. Body waves will be radiated into the surrounding earth formation at each location an obstruction exists in a well bore. Each location body waves radiate from will produce a unique source signature or coda over time. In FIG. 2, there are three locations that will produce unique coda, the upper discontinuity 105, the lower discontinuity 111, and the bottom of the well bore 113.

[0034] In FIG. 2 at arbitrary time T1 tube wave energy 103 impinges on the upper discontinuity 105 and body wave energy 106 emanates from the well bore. At time T2 the tube wave energy reverberates up 107 and down 109 along the well bore away from the upper discontinuity. At time T3 the downgoing tube wave reverberation energy 115 impinges on the lower discontinuity 111 and part of the energy is converted to body wave energy 117 radiating into the surrounding earth formation. At time T4 the tube wave energy reverberates up 119 and down 121 along the well bore from the lower discontinuity 111. At time T5 the downgoing tube wave energy 123 impinges on the bottom of the well bore 113 where part of the energy is converted to body wave energy 125. The upgoing energy 127 impinges on the upper obstruction 105 where some of the energy is converted to body waves 129. At time T6 tube wave energy 131 is reflected or diffracted off of the well bore bottom 113 and moves up the well bore fluid column. Also at T6 energy has likewise been separated into downgoing energy 133 and upgoing energy 135 by discontinuity 105.

[0035] Each discontinuity produces its own source signal coda based on the time body waves emanate from the well bore at each discontinuity or bottom hole location. For instance, the coda produced at discontinuity 105 in FIG. 2 contains body wave energy produced at arbitrary time steps T1 and T5. The coda produced at discontinuity 111 will contain body wave energy transmitted at arbitrary time step T3. The coda generated by body waves transmitted from the bottom of the well bore 113 will contain energy at time step T5. These coda, or source signature patterns, are cross correlated with the received acoustic wavefield to form an image of the subsurface as is well known in the art, and to determine parameters of interest in the earth formation the body wave energy has transited through.

[0036] FIG. 3 is a schematic tube wave simulation demonstrating how waveform coda are produced over time. Each discontinuity will have a unique waveform coda that will be the source signature for the seismic energy emanating into the surrounding earth from that discontinuity location. FIG. 3 contains two discontinuities and a bottom hole. The discontinuities shown vertically in depth are labeled in 201, 202 and the bottom hole is 203. The horizontal axis represents time. A tube wave 205 travels down to discontinuity 201 where some of the energy is converted to body waves at the point in time represented by the dot 206. At 206 the energy not converted into body wave energy splits into upgoing tube wave energy 207 and downgoing tube wave energy which travels to the first dot 209 on the next lower discontinuity 202. At 209 some energy is converted to body wave energy imparted to the surrounding formation, other energy remains as tube wave energy, some traveling up to the discontinuity 201 and other energy going toward the bottom hole 203.

[0037] Codas for discontinuity 201 and 202 are shown, and the bottom hole 203 is included to show how reverberation from other discontinuities influence coda generation. The coda for discontinuity 201 is shown below the simulation model at 211. Dot 206, the first body wave conversion is shown, followed in time by all the other body wave generation time points. The vertical bars through the dots of coda 211 represent the time body waves are generated at the discontinuity 201. The coda for discontinuity 202 is represented by 212, again with 209 representing the first body wave conversion from tube wave energy occurring for discontinuity 202. The codas are unique to each discontinuity, although each coda is affected by the output of every other discontinuity in the borehole. The coda for body wave energy converted at the well bottom 203 is not shown.

[0038] In FIG. 3 the initial tube wave 205 may be generated by naturally occurring or ambient energy inside of or in the area of a well bore. For instance production equipment in or associated with a well may create tube waves. The natural flow of fluids in a well bore may generate tube waves. Alternatively, tube waves may be intentionally generated with an impulsive source such as an airgun or with a swept frequency source, and any seismic or acoustic source may be used. Tube wave energy generated intentionally by many different methods or generated by naturally occurring or incidental phenomena may be used with this invention.

[0039] In the preferred embodiment a receiver is placed near a discontinuity to record the coda associated with the discontinuity. However, there are other methods of determining or measuring coda, and receivers are not required to directly measure the coda for every discontinuity.

[0040] The discontinuities or minor borehole obstructions may be of any type found in a well bore whether placed intentionally for use with this invention or not. As shown in FIG. 4, the discontinuities may be spaced arbitrarily along the well bore. In FIG. 4, the lengths between discontinuities 401, 403, 405 and bottom hole 407 are not necessarily equal. So, in the FIG. 4 example, length L411≠L413≠L415≠L417. Closely spaced discontinuities may appear as a point source in the far field. For example the energy received by receivers from discontinuity 405 and the bottom hole 407, as shown in FIG. 4, can appear as a point source in the far field.

[0041] The discontinuities or minor borehole obstructions may be restrictions, as shown by discontinuities 401, 403 and 405 in FIG. 4, or enlargements in the well bore, as shown in FIG. 5 for discontinuities 501, 503, 505 and bottom hole 507. Again, distances between enlargements may be irregular so that, for instance, in FIG. 5, L511≠L513≠L515≠L517.

[0042] The enlargements may be arbitrarily shaped, uniformity is not required, and non-uniformity for enlargements or restriction type discontinuities may be desired to enhance the diversity of body wave signal characteristics. Any combination of arbitrarily shaped minor borehole obstructions, whether the discontinuities are enlargements, restrictions or combinations, may be used.

[0043] Complex and arbitrary shapes, as shown by discontinuity 601 in FIG. 6, may be designed to increase the variability of the source signature as well as to change the effective bandwidth of the radiated signal. Special shapes of discontinuities may be used, for instance an inverse Venturi configuration. The distances separating the minor borehole obstructions along the well bore may be spaced equally or they may be unequal distances apart. All well bores have discontinuities or irregularities along the borehole that will diffractively transmit acoustic body wave energy converted from borehole tube waves into the earth at the discontinuity locations.

[0044] Each discontinuity is a separate point source of body wave energy and each discontinuity has a unique source coda or signature. For this example, we assume a seismic sensor for measuring the coda has been placed at or near the well bore discontinuity locations of interest, but this is not a limitation on the invention, as methods other than direct near-source measurement for detecting source signatures are known.

[0045] Each discontinuity or minor borehole obstruction is an energy radiation source point with a unique waveform coda for acoustic energy radiated into the adjacent earth formation. A sensor is placed to record the waveform coda as it is generated. Each coda is then cross correlated with the acoustic wave field recorded away from the well bore, for instance with seismic sensors in another well bore or on the surface of the earth or the ocean floor.

[0046] The invention may be used with a reverse VSP type project as shown schematically in FIG. 7. A well 761 has discontinuities 701, 703, 705 and well bottom 707 from which body waves may radiate into the surrounding earth formation. As shown by lines originating at discontinuity 703, body wave energy may travel directly to earth surface 740 where sensors 711 receive the energy. Body wave energy originating at, for instance, discontinuity 703 may reflect or refract off of geologic interfaces 750 in the earth. Body wave energy may emanate from any of the discontinuities, 701, 703, 705 or 707, which also may have associated sensors to directly measure the associated source waveform coda or seismic energy received from any other source point.

[0047] Cross-well type projects may be undertaken with this invention as shown in FIG. 8. A source well 861 will contain source point discontinuities 801, 803, 805 and 807 or combinations of discontinuities appearing as source points in the far field. Body waves will be converted from tube waves at these discontinuities. A cross-well project will have at least one receiver well 863 containing receivers or receiver with source-point discontinuity combinations 811, 813, 815, 817, 819 and 820. A few possible wave paths are shown by the lines originating at discontinuity 803. The body wave energy may travel directly to a receivers or receiver-discontinuity combinations 811, 813, 815 and 817 in another well 863. The body wave energy may reflect or refract off of geologic interfaces 850 in the subsurface before being received by receivers or receiver-discontinuity combinations 815, 817 and 819. As is well understood in the art, many other body wave paths traversing intervening earth formations containing useful information will be received at sensors in a cross-well type project.

[0048] The cross-well project may be a multi-well project so that many boreholes may be utilized to simultaneously record body wave energy emanating from a source well 963 as shown in FIG. 9. The source well 963 will have source points 906, 907, 908, 909 and 910 for example. Receiver wells 961, 965 will contain receivers along the well bores 901 through 905, and 911 through 915 respectively. As in the FIG. 8 example, some or each of the receivers may be associated with minor borehole obstructions in the receiver wells, since the receiver wells may also be utilized as source wells. Example body wave raypaths are shown originating at source point 907. The raypaths may be direct as shown between the source well 963 and receiver well 961. The raypaths may be combinations of reflected energy off of a geologic interface 950 with direct raypaths as shown by energy emanating at source well 963 and being received by receiver well 965.

[0049] As the foregoing examples demonstrate, this method may be used for hydrocarbon or other mineral deposit assessment at any point in time, over several time periods, and may be utilized as an ongoing part of the management and monitoring of a producing reservoir. Time varying changes of selected attributes of acoustic waves that have transited the earth formation between boreholes are indicative of the temporal changes in the mineral content. This invention allows for constant or periodic assessment of production and resource management during the productive life of oil, gas or other mineral deposits. Hydrocarbon migration during production, hydrocarbon migration during operations such as secondary recovery, steam flood progression, water invasion into oil and gas reservoirs are all examples of reservoir processes that may be monitored over time with this invention. Analysis methods include tomographic reconstruction and mapping a position of a fluid interface within the formation.

[0050] While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.