ELECTROSEISMIC TECHNIQUE FOR MEASURING THE PROPERTIES OF ROCKS SURROUNDING A BOREHOLE

Abstract: Apparatus and method for measuring the properties of rocks such as permeability, porosity and fluid properties downhole around a borehole (3) in which a seismic source (6) which radiates a seismic signal substantially radially within the borehole is lowered down the borehole (3) and the electrical signal generated by the seismic signal within the surrounding rock is detected by detectors (4, 5) mounted above and below the seismic source (6) so that signals are received from substantially all radial directions. Preferably a continuous wave signal is generated and the signal detected whilst the apparatus (1) is raised or lowered. The apparatus (1) gives a much better and more accurate measurement than previous methods.

Description: ELECTROSEISMIC TECHNIQUE FOR MEASURING THE PROPERTIES OF ROCKS SURROUNDING A BOREHOLE The present invention relates to a method and equipment for measuring the properties of subsurface rock from a tool lowered down a borehole. In particular it relates to a method and equipment for measuring such properties as the response time and amplitude of the electrokinetic coefficient, porosity and permeability of fluid-bearing porous rocks. The measurement of permeability of rocks surrounding a borehole is important in assessing the location of water or oil reserves, including the quality and quantity of the reservoir rock. Existing methods are unable to measure the permeability of a porous rock directly with any accuracy from a downhole tool In addition to its value in the assessment of the quality and quantity of water or oil reservoirs, rock permeability is very important in determining at what rate and at what cost these fluids can be produced from boreholes. US Patent 3,599,085 describes a method in which a sonic source is lowered down a borehole and used to emit low frequency sound waves. Electrokinetic effects in the surrounding fluid-bearing rock cause an oscillating electric field in this and is measured at least two locations close to the source by contact pads touching the borehole wall. The electromagnetic skin depth is calculated from the ratio of electrical potentials and the permeability of the rock deduced US Patent 4,427,944 and the equivalent European Patent 0043768 describe a method which injects fluid at high pressure from a downhole tool to generate electrokinetic potentials; these are measured by contact electrodes against the borehole wall The risetime of the electrical response is measured and from this the permeability of the porous rock is determined. UK Patent 2,226,886A and the equivalent US Patent 4,904,942 describe several arrangements for recording electrokinetic signals from subsurface rocks mainly with the electrodes for measuring the signals at or close to the earth's suiface but including use of an acoustic source mounted on a downhole tool. There is no indication of permeability being deduced or of inferring porosity A further related (inverse) method is described in European Patent 0512756A 1 , which contains several arrangements for setting out electrical sources and acoustic receivers (geophones) in order to measure electro-osmotic signals induced in subsurface rocks PCT Patent WO 94/28441 describes a method whereby sound waves of fixed frequency are emitted from a downhole source and the resulting electrokinetic potentials measured An electrical source of fixed frequency is then used to produce electro-osmotic signals and the acoustic response measured. Using both responses together, the permeability is then deduced, provided the electrical conductivity of the rock is also separately measured. In these methods the seismic shock is generated at intervals, which propagate out in one direction only from a tool in contact with the borehole wall and the electric signal generated by the electro kinetic effect is received from substantially that one direction. This and other defects in their construction has resulted in their only having a limited practicality We have now devised an improved method for measuring the properties of surrounding porous rocks from a tool located down a borehole which reduces these difficulties making the measurements quicker to carry out and more accurate and representative According to the invention there is provided a method of measuring properties of rocks surrounding a borehole which method comprises generating a seismic signal from a location in the borehole, which signal propagates within the borehole substantially radially in all directions, passing on through the borehole wall and surrounding rock detecting electrical signals arising in the rock surrounding the borehole and receiving and processing the electrical signals to measure the rock properties. Properties which can be measured by the method of the present invention include permeability, porosity and fluid properties The source of the seismic signal is preferably not in contact with the borehole wall but positioned substantially centrally within the borehole The seismic signal is propagated radially outwards in all directions through the borehole fluid (the fluid in the borehole e g. drilling mud etc ) and, subject to distortion by the borehole wall and variations in the rock, the seismic signal propagates outwards substantially uniformly in all directions The electrical signal generated within the surrounding rock is received and detected at the tool within the borehole from substantially all directions This invention also provides apparatus for measuring the properties of rocks surrounding a borehole, which apparatus comprises a casing adapted to be lowered down a bore hole in which casing there is a seismic means for generating seismic signals in substantially all radial directions and having associated therewith, a means adapted to detect electrical signals generated by the effect of a seismic shock generated by seismic means The means for generating the seismic signals preferably generates a series of pressure pulses or, more preferably, a continuous pressure oscillation, at one or more finite frequencies It may consist of a mechanical vibrational device, an electromagnetic device, a sparker source, an explosive source, an airgun operated hydraulically or electrically or any other such conventional sonic source designed for use on a downhole tool but preferably it should be a magnetostrictive or piezoelectric transducer whose signal is controllable electrically The term "seismic pulse" can include a pulse which can be referred to as a sonic or acoustic pulse A preferred means for enabling the seismic signal to be generated radially comprises a cylindrical chamber having holes in its side, which when downhole will be full of drilling fluid with the sides of the chamber being close to the sides of the borehole, there being a means to transmit a shock or applied force to the fluid in the chamber so as to cause the shock to be transmitted through the fluid in the chamber through the holes into the surrounding rock The holes should be distributed substantially uniformly around the casing so that the shock is transmitted in all directions The shock or force can be applied by any ofthe means referred to above The seismic signal can be generated whilst the apparatus is lowered or raised up from the borehole, thus providing a continuous or semi-continuous measurement of rock along the borehole The electrical signals can be detected by means of a pair of electrodes positioned within the borehole close to the borehole wall or, alternatively, a coil receiver mounted on the tool or, preferably, an electrode pair or short dipole antenna mounted on the tool aligned centrally within the borehole. In the equipment of the present invention it is convenient to use one or two electrical receivers placed above and below the acoustic source, the case of the dipole antennae preferably aligned vertically or horizontally above and below the source and in the case of the coils with the plane of the coil aligned vertically or horizontally at the centre o the borehole. The electrical receiver preferably consists of one or two pairs of electrodes forming a short dipole antenna with electrically isolated ends or two coils with electrically isolated lines. For each pair the ends are preferably connected to an amplifier which amplifies the signals whilst keeping them electrically isolated; this is carried out by referring the potential of each end independently to a floating reference potential. The signals are preferably amplified and converted to digital form before being communicated (e.g. by wire) to the surface for recording and processing. Preferably the means for detecting the electrical signals compares the potential at the ends, in the case of the dipole antenna, or measures the electrical field strength in the case of the coil The potential at the ends of dipole antenna in the one case or of the coil in the other, are compared by connecting them to an amplifier in which the potentials are preferably referred to a non-earthed reference (a virtual earth) and these new potentials are amplified and compared Such a procedure allows amplification with very little distortion of the potential to be measured and with a high degree of common-mode noise rejection and is superior to other conventional methods of amplification Preferably the non-earthed reference potential is that of a common line in the amplification and data acquisition circuitry ofthe receiver and is not connected directly to earth Preferably there is provision for isolating and balancing the signals from each of the electrodes or coils before they reach the amplifier circuit in order to giver the maximum common-mode rejection of electromagnetic noise This balancing can be achieved manually before running in a given borehole to compensate for variations in electrode performance in a given hole or by means of a suitable electronic circuit giving continuous feedback whereby continual adjustment can be made The seismic source preferably continuously emits sound simultaneously on at least two finite frequencies with the resultant oscillation the sum of the various sinusoidal pressure oscillations Preferably if two frequencies are used these frequencies are from 50 to 200 Hz and 0 1 to 5 KHz, e g about 100 Hz and 1 KHz Preferably the amplified electπcal signals are demodulated with respect to the source frequencies and the amplitude and phase relative to the source sampled at a frequency of about 1-100 Hz per channel and converted from analogue to digital form, of 12 or 16 bit accuracy The digital data transmitted to surface is recorded as a data file and can then be processed The amplitude and response of the electrokinetic response to an acoustic pulse have been shown to be closely related to the electrokinetic coefficient and the permeability of the target porous rock respectively For a sonic oscillation of a known frequency the amplitude and phase of the electrical response with response to the source is a function of both electrokinetic coefficient and permeability, however, measurement of amplitude of response on two frequencies allows each of these properties of the rock to be determined independently. After processing a log of rock permeability, electrokinetic coefficient, electrical conductivity and porosity can be produced. Alternatively, if the amplitude and phase of the electrokinetic response at a single frequency are measured are measured, the permeability and porosity may be derived from these It is believed that the method of the present invention makes use of an electrokinetic effect in which the seismic wave generated by the seismic source and, passing through the interface of the borehole with the surrounding porous rock and through interfaces within the rock where the fluid properties change, stimulates electrical signals detected at the receiving electrodes or coils. The seismic oscillations within the porous rock give rise to fluid flow within the rock and as cations and anions adhere with differing strengths to capillary walls, a resulting electric dipole is generated within the rock. This electric dipole distorts the quasi-static electric field within the slightly conducting medium of the rock and this distortion propagates back to the tool, where it is measured. The invention will now be described with reference to the accompanying drawings in which - Fig 1 is a schematic view of the tool of the invention operating downhole. Fig. 2 is an enlarged view of part ofthe tool Fig. 3 is a circuit diagram showing amplifier connections; and Fig. 4 is a circuit diagram ofthe connection of electrodes to amplifier Referring to Figs 1 and 2, the downhole tool ( 1 ) is connected to a cable (2) so that it can be raised and lowered down borehole (3 ) In the tool are two antennae comprising two pairs of electrodes (4a, 4b) and (5a, 5b) There is a seismic source (6) which consists of a hammer (23) and piston (7) and a cylindrical chamber (8) with holes disposed uniformly about it. In use, the tool ( 1 ) is lowered down the borehole (3) until it is in position The hammer and piston (23) and (7) are activated to produce a seismic signal - 1 - comprising continuous acoustic oscillations by compression of the drilling fluid which fills the borehole and the amplitude and response time of the electrokinetic signal generated measured The seismic signal propagates through the holes in cylindrical chamber (8) via the drilling fluid Referring to Fig 1 , the seismic signal generates an electrical signal in fluid bearing rocks (9) which is detected by antennae (4) and (5) Referring to Fig 4, the sonic source (6) is controlled by means of a controller/signal generator ( 10) and generates the seismic signal in the surrounding rock which generates an electrical signal picked up by antennae (4,5) These signals are passed to amplifier ( 18) where they are amplified and passed to converter ( 19), which converts then from analogue to digital form These signals are then passed via data connectors (20/21 ) to computer (22) which controls, samples and records the data and finally processes and displays them Referring to Fig. 3, the signal from antennae (4,5) pass to a pair of low-noise operational amplifiers (28) which provide some gain, a low-pass filter and efficient common-mode noise rejection to (CMNR) at low frequencies whilst keeping the two channels isolated from each other Independent balancing controls (29) control the balancing of CMNR in each line before they are compared and amplified by a final stage amplifier ( 14)

Claims: Claims 1. A method of measuring properties of rocks surrounding a borehole which method comprises generating a seismic signal from a substantially central location in the borehole, which signal propagates substantially radially into the borehole wall and surrounding rock, detecting electrical signals arising in the rock surrounding the borehole and receiving and processing the electrical signals in order to measure the rock properties 2 A method as claimed in claim 1 in which the means for generating the seismic signal is not in contact with the wall ofthe borehole 3 A method as claimed in claim I or 2 in which the amplitude and response time ofthe electrical signals are measured 4 A method as claimed in any one of claims 1-3 in which the seismic signals are generated as a series of pressure pulses and the amplitude and phase ofthe response at each frequency is measured 5 A method as claimed in any one of claims 1 -4 in which the seismic signals are generated as a continuous pressure oscillation, at one or more finite frequencies and the amplitude of the response at each frequency is measured 6. A method as claimed in claim 5 in which the phase of the response at each frequency is measured. 7 A method as claimed in any one of claims 1 -6 in which the electrical signals are detected by means of a pair of electrodes positioned within the borehole close to the borehole wall. 8. A method as claimed in claim 7 in which the electrodes are not in contact with the borehole wall. 9 A method as claimed in any one of claims 1 -8 in which the electrical signals are detected by means of a coil receiver mounted on the tool 10 A method as claimed in any one of claims 1 to 9 in which the electrical signals are detected one or more electrical receivers placed above and below the seismic source 1 1 A method as claimed in claim 10 in which each signal is referred to a virtual earth 12 A method as claimed in claim 10 or 1 1 in which the signals from each ofthe electrical receivers are isolated and balanced 13 A method as claimed in any one of the preceding claims in which the seismic source continuously emits sound simultaneously on at least two finite frequencies with the resultant oscillation being the sum ofthe various sinusoidal pressure oscillations 14 A method as claimed in claim 13 in which two frequencies are used and these frequencies are from 50 to 200 Hz and 0 1 to 5 KHz 15 A method as claimed in any one of claims 1 to 14 in which the seismic signal is generated whilst the means for generating the seismic signal is lowered or raised up the borehole 16 A method as claimed in any one of claims 1 to 15 in which the electrical signals are amplified and the amplified electrical signals are demodulated with respect to the source frequencies and are converted from analogue to digital form 17 Apparatus for measuring the properties of rocks surrounding a borehole, which apparatus comprises a casing adapted to be lowered down a bore hole in which casing there is a seismic for generating seismic signals in substantially ail radial directions and having associated therewith, a means adapted to detect electrical signals generated by the effect of a seismic shock generated by seismic means 18 Apparatus as claimed in claim 17 in which the means for generating the seismic signals generates a series of pressure pulses 19 Apparatus as claimed in claim 17 in which the means for generating the seismic signals generates a continuous pressure oscillation, at one or more finite frequencies. 20 Apparatus as claimed in any one of claims 17 to 19 in which the means for transmitting the seismic signal to the surrounding rock comprises a cylindrical chamber having holes in its side, which when downhole will be full of drilling fluid with the sides ofthe chamber being close to the sides of the borehole, there being a means to transmit a shock or applied pressure to the fluid in the chamber so as to cause the shock or pressure oscillation to be transmitted through the fluid in the chamber through the holes into the surrounding rock 21 Apparatus as claimed in any one of claims 17 to 20 in which the means for generating the seismic signal comprises a mechanical vibrational device, an electromagnetic device, a sparker source, an explosive source or an airgun operated hydraulically or electrically 22. Apparatus as claimed in claim 21 in which the means for generating the seismic signal comprises a magnetostrictive or piezoelectric transducer whose signal is controllable electrically 23 Apparatus as claimed in any one of claims 17 to 22 in which the means adapted to detect the electrical signals comprises a pair of electrodes positioned within the borehole 24 Apparatus as claimed in claim 23 in which the pair of electrodes are adapted not to be in contact with the borehole wall 25. Apparatus as claimed in any one of claims 17 to 22 in which the means adapted to detect the electrical signals comprises a coil receiver. 26. Apparatus as claimed in any one of claims 17 to 24 in which the means adapted to detect the electrical signals comprises two pairs of electrodes forming a short dipole antenna with electrically isolated ends. 27 Apparatus as claimed in claim 26 in which for each pair of electrodes the ends are connected to an amplifier which amplifies the signals whilst keeping them electrically isolated. 28. Apparatus as claimed in any one of claims 17 to 27 in which the means adapted to detect the electrical signals comprises two coils with electrically isolated lines. 29. Apparatus as claimed in any one of claim 12 to 28 in which there are one or two electrical receivers placed above and below the means for generating the seismic signal. 30 Apparatus for measuring the properties of rocks surrounding a borehole as hereinbefore described with reference to the drawings.

Inventors: CLARKE RICHARD HEDLEY, MILLAR JOHN WILLIAM AIDAN, CLARKE, RICHARD, HEDLEY, MILLAR, JOHN, WILLIAM, AIDAN
Applicants: CLARKE RICHARD HEDLEY, MILLAR JOHN WILLIAM AIDAN, CLARKE, RICHARD, HEDLEY, MILLAR, JOHN, WILLIAM, AIDAN
Application year: 1996
Main classification: G01V3
IPC codes: G01V3