Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1998-04-17
2001-05-01
Williams, Hezron (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C367S035000, C324S323000, C324S351000
Reexamination Certificate
active
06225806
ABSTRACT:
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.
U.S. Pat. No 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. U.S. Pat. No 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 U.S. Pat. No 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 surface 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 0512756A1, 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 of the 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 of 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
Clarke Richard Hedley
Millar John William Aidan
Bartlett & Sherer
Court Services Limited
Jolly Anthony
Sherer Ronald B.
Williams Hezron
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