DRILLING UNIT ENERGY SOURCE FOR PIEZOELECTRIC EXPLORATION

Abstract

In addition to drilling operations a hard rock drill rig is used to induce seismic energy into subsurface rock formations. In quartz rich rock the seismic energy will partially convert to electrical energy thereby allowing for the prospecting of ore-bearing quartz veins. Seismic energy and electric energy are detected by a sensor array. The electric energy is used to infer the presence of piezoelectric materials in the rock formations, and such materials are mapped using the detected seismic energy and electric energy.

Claims

1. A method for determining piezoelectric properties of a subsurface rock mass, comprising: drilling the rock mass using a rotary drill bit or hammer drill capable of generating piezoelectric signals by applying stress to the rock mass at the drill bit; detecting electrical signals at one or more locations; and determining the piezoelectric properties of the rock mass using the detected electrical signals.

2. The method of claim 1 wherein the electrical signals comprise voltages induced in at least one wire coil.

3. The method of claim 1 wherein the electrical signals comprise voltages imparted across at least one pair of electrodes.

4. The method of claim 2 further comprising using a geophone or accelerometer disposed proximate to the wire coil to detect vibration signals, and using the vibration signals to correct the detected electrical signals for motion of sensors used to detect the electrical signals.

5. The method of claim 1 comprising an accelerometer attached to the drill string detecting seismic signals from an accelerometer correlating the detected seismic signals and electrical signals to determine the piezoelectric properties of the rock mass.

6. The method of claim 1 comprising detecting seismic signals from the rock mass and correlating the detected seismic signals and electrical signals to determine seismic or elastic properties of the rock mass.

7. The method of claim 1 further comprising using the detected seismic signals and the detected electrical signals to determine background electrical noise and background seismic noise in zones of the rock mass having substantially no piezoelectric minerals, and using the background electrical noise and background seismic noise to correct the detected electrical signals and detected seismic signals noise in zones of the rock mass having substantially the piezoelectric minerals.

8. The method of claim 1 further comprising choosing sensor frequency to detect frequencies for which the conductive drill string length string acts as an antenna

9. The method of claim 1 further comprising detecting electrical signals induced in a drill string, and using the detected current electrical signals to infer the presence of piezoelectric minerals in the rock mass during drilling thereof.

10. The method of claim 1 further comprising using the identified piezoelectric minerals and a measurement related to crystal structure thereof to determine presence or concentration of at least one precious metal in the rock mass.

11. An apparatus for determining piezoelectric properties of a rock mass during drilling, comprising: an acceleration sensor operable coupled to a drill string operated by a drilling rig to drill a wellbore in the rock mass; an electric signal sensor disposed at the surface of the rock mass; and a processor in signal communication with the acceleration sensor, and the electric signal sensor, the processor having instructions thereon to determine the piezoelectric properties, wherein the processor further has instructions thereon to correlate signals from the electric signal sensor with measurements from the acceleration sensor.

12. The apparatus of claim 11 further comprising a toroid coil sensor disposed about the drill string and in signal communication with the processor, the processor having instructions thereon to indicate presence of piezoelectric materials in the rock mass from signals generated by the toroid coil sensor.

13. The apparatus of claim 11 further comprising an electric current sensor or voltage sensor disposed along the drill string and in signal communication with the processor, the processor having instructions thereon to indicate presence of piezoelectric materials in the rock mass from signals generated by the electric current sensor or voltage sensor.

14. An apparatus for determining piezoelectric properties of a rock mass during drilling, comprising: an acceleration sensor operable coupled to a drill string operated by a drilling rig to drill a wellbore in the rock mass; a seismic sensor and an electric signal sensor disposed at the surface of the rock mass; and a processor in signal communication with the acceleration sensor, the seismic sensor and the electric signal sensor, the processor having instructions thereon to determine the piezoelectric properties, wherein the processor further has instructions thereon to correct signals from the electric signal sensor for motion and background noise using measurements from the acceleration sensor and the seismic sensor.

15. The apparatus of claim 15 further comprising a toroid coil sensor disposed about the drill string and in signal communication with the processor, the processor having instructions thereon to indicate presence of piezoelectric materials in the rock mass from signals generated by the toroid coil sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 shows schematically how mechanical stress generates electrical potential in a piezoelectric material.

[0039] FIG. 2 illustrates a direct piezoelectric field induced by an impact.

[0040] FIG. 3 shows a radial distribution of the electric field in FIG. 2.

[0041] FIG. 4 shows a graph of an electrical field from a vertically oriented piezoelectric source with a moment of 1

[0042] FIG. 5 shows a graphic display of electric field measurements from a piezoelectric source.

[0043] FIG. 6 shows an example embodiment of a drilling unit and a measurement array.

DETAILED DESCRIPTION

[0044] This disclosure sets forth a method for characterizing the piezoelectric properties of materials being drilled by a drilling system including a rig, drill string and drill bit, where drill bit interactions with the formation being drilled generate an electromagnetic field. This field is detected by one or more sensors located in the vicinity of the drilling system.

[0045] The drill bit impacts on the formation crack the rock. The cracking creates an electric dipole impulse-like signal at the bit/rock interface, where the amplitude is in the millivolt range and the frequency is roughly centered at 1 Mhz. The properties of the impulse-like signals wavelet are affected by the electromagnetic properties of the rock.

[0046] The crack-generated electric dipole impulse-like signal travels away from the interface into the formation, up the wellbore in the air or fluid in the wellbore, and through the drill string. 1 Mhz is in the low frequency radar regime, where the wavenumber is complex. The wave propagation component is attenuated by the properties of the propagation medium. Air or fresh water, having a low electrical conductivity, are less attenuating than a conductive formation or the steel drill string. Consequently, the signal traveling along the air or fluid path up the wellbore is the most energetic signal received at the surface. One or more electromagnetic or electric field sensors, e.g., radar antennas, in some embodiments tuned to this frequency range. can be used to detect the electric dipole impulse-like signals at the surface. These antennas can be attached at selected locations on and around the drill rig to improve the signal quality.

[0047] The sensor response may be affected by noise from seismic signals propagating from the drill bit or from other acoustic/seismic sources in the vicinity of the drilling system. Signals from collocated seismic sensors may be used with suitable data processing to remove this noise from the signals detected by the electromagnetic sensor(s).

[0048] An Example Implementation

[0049] A method according to the present disclosure includes an operating drilling system including a rig, drill string and drill bit, and using the drill bit impacts on the formation as a piezoelectric energy source and collecting EM data during drilling. A sensor or sensors is located in the vicinity of the drilling system to detect the piezoelectrically induced signals resulting from the bit impacts on the formation.

[0050] As shown above, in background formations (rock) that do not contain significant amounts of PZ material such as quartz, the PZ coefficients are very small and there will be a minimal electrical field associated with drilling induced seismic energy. In quartz or other high coefficient materials, however, the induced electric field can be substantial and can be measured with the sensor(s).

[0051] FIG. 6 illustrates an example embodiment of an apparatus and method. A rotary or hammer based rig 10 is shown drilling a wellbore 12 into a rock mass 14. The rig 10 may be powered by a diesel motor that converts fuel energy into mechanical hammer or rotary drill bit 15 motion; in some embodiments an electrically powered rig can also be used. The rotary bit or hammer 15 motion may be recorded by an accelerometer 16 located adjacent to a drill string 18 (a length of pipe and tools used to operate the hammer or drill bit 15) used to drill the rock mass 14.

[0052] Within the wellbore 1212 during drilling, the drill bit or hammer 15 is applying mechanical stress to the rock mass 14 at the rock/bit interface, point B, which will (eventually) fracture and/or pulverize the rock mass 14 in contact with the hammer or drill bit 15. The mechanical stress imparted by the hammer or drill bit 15 generates shock or seismic waves C near the hammer or drill bit 15, which shock waves C propagate within the adjacent rock mass 14. These shock waves may C be measured by sensors 21 in a horizontal sensor array DH disposed in a selected pattern on the ground surface and/or in a vertical sensor array DV disposed in an adjacent well 12A. At point A the shock waves C are converted to electrical current in the presence of a PZ material 20, e.g., a quartz vein.

[0053] If the shock (seismic) waves C encounter a PZ material 20, e.g., a quartz vein, then some of the mechanical energy will be converted to electrical energy at the material interface. This electrical energy will travel at the propagation velocity of an electromagnetic wave in the rock mass 14 and will arrive at either sensor array DH, DV much sooner than the corresponding shock or seismic waves C (generated by the same mechanical energy imparted by the hammer or drill bit 15), although the seismic waves C will have similar amplitude with respect to time characteristics to the electromagnetic wave. Note that only the mechanical energy (in the form of the seismic waves C) that encounters the PZ material 20 (e.g., the quartz vein) will induce electrical signals in addition to seismic signals. As used herein to describe the PZ signals generated by mechanical stress applied to the rock mass 14 “electrical” means an impressed voltage, electromagnetic wave or both. Such signals may be detected by either galvanic sensors (spaced apart pairs of electrodes), electromagnetic sensors, magnetometers or combinations of the foregoing. Examples of such sensors will be described further below.

[0054] During the drilling operation, electrical and seismic signals are measured by either sensor array DH, DV. These signals comprise drilling and background electrical and mechanical noise in addition to seismic and EM signals indicative of PZ material targets. Sensors in the sensor arrays DH, DV may comprise seismic sensors 21, e.g., geophones, and/or accelerometers. The accelerometer 16 provides signals corresponding to the drilling stress, which is directly related to seismic and electrical signals transmitted into the rock mass 14 from the hammer or drill bit 15. Electrical sensors 23, which may be wire coils or electrode pairs, detect the electrical signals generated at the interface and propagated through the rock mass 14. Wire coils or magnetic field sensors, shown generally at 28 may be included in the sensor array(s). An electric current sensor 26, for example a toroidal coil, may be disposed about the drill string 18 to detect current flowing along the drill string 18. A voltage sensor 27 may be connected across an electrical isolator 29 disposed in the drill string 18 to detect voltages induced in the drill string 18.

[0055] A processor/recorder 40, which may be any form of microcomputer, field programmable gate array, controller or similar signal processing and recording device may be in signal communication with all of the foregoing sensors and may have programmed thereon instructions to carry out signal processing to be further described below.

[0056] Data processing workflow to be performed on the processor/recorder 40 or any other processor or computer may be designed to provide a quartz/no quartz indicator on depth-related segments of the wellbore 15 as it is being drilled.

[0057] The measured seismic signals (detected by electrical sensors 21) from the accelerometer 16 may be convolved, e.g., in the processor/recorder 40, with the seismic and electrical signals (detected, respectively by seismic sensors 21 and 23/28) in order to isolate drilling-induced PZ signals from background noise. The isolated electrical and seismic signals are related to the PZ and seismoelectric characteristics of the rock mass 14 being drilled. The time signature of these isolated signals may then be used to map the structure of a PZ mineral body such as the quartz vein 20, if desired, using simple straight-ray tomography or other imaging techniques known in the art

[0058] Piezoelectric signals may be created by distinctly different mechanisms. Rock fracture will generate and propagate high frequency PZ signals, of the order of megahertz and strain waves from the hammer or drill bit 15 will generate and propagate PZ signals at lower frequencies, of the order of tens to thousands of hertz.

[0059] The drill string 18 will act as an antenna for EM signals that have a wavelength equivalent to the wavelength of the propagated PZ signals, or a factor or fraction of such wavelength. For example for a 3 MHz PZ signal will excite a resonant signal that will be amplified in a 10 m drill string as this length is equal to ¼ wavelength of the PZ signal. The properties of the drills string will enhance the transmission of certain frequencies and improve signal to noise ratio at the sensors. In some embodiments, the electromagnetic sensors, e.g., sensors 23 and 28, may be tuned to have increased sensitivity to signals that are whole number multiples of ¼ wavelengths of the drill string length.

[0060] To enhance the electrical signals measured relating to the drill pipe apparatus may be used to effectively electrically insulate the drill pipe from electrical grounding to the rig, such that the voltage signal induced in the drill pipe is maximized. This signal can be measured directly as a potential difference, or via a radio frequency signal or capacitive sensor.

[0061] In other embodiments, to enhance the electrical signals measured relating to the drill pipe apparatus may be used to effectively electrically connect the drill pipe to an electrical ground at the top of the drill pipe to the rig, such that the current signal induced passing up the drill pipe is maximized. This signal can be measured directly as a current, or remotely sensed as a coil.

[0062] To those practiced in mining geology, there are known correlations and relationships between quartz content, distribution and crystal structure and the presence of precious minerals and metals. The resulting signals relating primarily to quartz content and crystal structure may be used to derive relationships between rock types and the presence of valuable minerals in the rock mass 14, including but not limited to gold, silver, copper and platinum using empirical relationships derived from measurements of ore grade from analysis of drill cuttings or other measurements. These may be generated by machine learning methods and algorithms such as artificial neural networks. In this case the combination of drill cuttings geochemical measurements and the piezoelectric measurements would provide a training dataset for said machine learning methods. Subsequently the trained machine learning algorithms would be used to estimate the ore grade from the piezoelectric measurements alone.

[0063] Piezoelectric signals, as explained above, travel near electromagnetic propagation speed in the rock mass (14 in FIG. 1) and can be considered to arrive without delay compared to seismic signals which at the , e.g., 28 and 23, travel at the velocity dependent on rock mass elastic properties. A seismic signal recorded, in a preferred embodiment, and will arrive at the top of the drill string could be correlated with seismic sensors 21 with the EM signals recorded by corresponding time delay relative to the sensor or sensors to identify and enhance the EM signals created by the bit impacts.

[0064] Under ordinary drilling operation within normal background (e.g., no PZ minerals in the rock mass 14) the drill string 18 will carry electrical currents related to grounding currents from drilling operations. If, however the drill bit or hammer 15 encounters a quartz vein or other PZ mineral body, then the measured current along the drill string 18 will be different, likely somewhat larger, due to the connection to piezoelectric material in the wellbore 12. In some embodiments, a toroid coil 26,may be disposed around the drill string 18, to measure electrical currents within the drill string 18. Such measurements may be used as a quick indicator for the presence of PZ minerals within the wellbore 12.

[0065] A galvanic voltage or current sensor 27 may be disposed across the shock sub (e.g., isolator 29) to measure a parameter related to current flow along the drill string. In this example, the drill string would be electrically connected to the rig mast by the sensor 27. It may also be possible to measure the potential difference between the drill string pipe and the drill itself (mast/chassis) if the shock sub is an electrical insulator (e.g., isolator 29). In this case the drill string 18 would be insulated from the rig mast. Voltage and/or current measurements made by the foregoing sensors 27, 26 may be conducted to the processor/recorder 40 for analysis as to presence of PZ minerals in the rock mass 14.

[0066] Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the examples. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.