Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Well logging
Reexamination Certificate
2001-09-18
2003-07-22
Moskowitz, Nelson (Department: 3663)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Well logging
C324S353000, C073S632000
Reexamination Certificate
active
06597633
ABSTRACT:
The present invention relates to a method and equipment for obtaining information concerning the rock and soil surrounding a borehole.
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.
Downhole logging is known in which equipment is lowered downhole, a seismic or sonic signal is generated by a seismic or sonic shock and is sent out from the borehole, this seismic signal generates an electric field and an electromagnetic signal is received and can then be analysed to obtain information concerning the rock or soil surrounding the borehole. The generation of an electric signal by this means is called an electrokinetic signal (EKS).
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 0043769 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.
Patent Application PCT/GB96/02542 discloses a method of measuring the properties of rock surrounding a borehole in which a seismic pulse is generated downhole which propagates outwards from the borehole to produce electrokinetic signals which are detected within the borehole and used to measure the properties of the surrounding rock.
In these methods the borehole is not cased i.e. there is no metal casing surrounding the borehole, the seismic or sonic signal is propagated directly into the surrounding formation and the electrokinetic signal generated is received from the formation. If the borehole were cased the metal casing would prevent the reception of electromagnetic signals within the borehole as the casing would act as an electromagnetic shield or cage; this means that these methods can only be used before the well is cased.
In use, operational wells are cased and so the prior art methods cannot be used for downhole logging in operational wells and this is a limitation on the application of EKS downhole logging. It EKS downhole logging could be used in cased wells this would greatly expand the application of such methods and would enable there to be regular monitoring of the rock structure surrounding a hole or a number of holes whilst in production. Such information can be generated as required, with minimal interference of drilling operations and can facilitate the selection of the location of step out wells in a structure and detect discontinuities and other information concerning the rock formation. In addition, after a production well is no longer in production, EKS downhole logging could be used to monitor the surrounding rock and to generate further knowledge about the structure and its features without requiring the drilling of extra holes.
We have surprisingly found that it is possible to produce and detect electrokinetic signals returned from the surrounding rock formation from the metal casing in a cased well.
When a well is cased and surrounded by a metal casing, usually made of steel, the returned electromagnetic signals, which are weak would be expected to be shorted out by the casing and so no signal would be received between electrodes in contact with the casing inside the borehole.
We have surprisingly found that it is possible to detect such signals and we have now devised a method for EKS logging in a cased well.
According to the invention there is provided a method for measuring the properties of an earth formation traversed by a borehole which is cased by a metallic casing in which a seismic or sonic shock is generated downhole within the borehole and is propagated into the surrounding formation and an electrokinetic signal generated by the seismic or sonic shock is detected by at least two spaced apart electrodes in contact with the casing.
The invention also comprises apparatus for detecting electrokinetic signals generated by a seismic or sonic shock generated downhole in a borehole in which apparatus there are at least two spaced apart electrodes adapted to make contact with the well casing in the cased borehole and a means connected to the electrodes which is able, in conjunction with the electrodes, to detect the electrokinetic signals.
It has been surprisingly found that, by contacting the casing with at least two spaced apart conductors, an electrical signal can be detected which has been generated by the seismic shock. Although the casing acts as a low-value resistor in parallel with the conductors it has been found, in practice, that not all the signal is shorted out.
The electrodes which are in contact with the casing are spaced apart so that the signal is generated between them and fed to the amplifier. The conductors can be in the form of a dipole and suitable separation of the conductors is from 0.05 m to 2 m.
The conductors preferably make good contact with the casing and can be in the form of spring loaded brushes as in conventional contacts, or rolling wheels which can cut through debris to make contact. Conductors can make contact in a localised area or can be in the form of a ring which fits inside the borehole and makes contact along its circumference.
Alternatively a plurality of pairs of electrodes can be positioned circumferentially around the borehole.
In one embodiment of the invention the electrical receiver preferably consists of at least one pair of electrodes forming a short dipole antenna with electrically isolated ends. For each pair the ends are 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.
The amplifier chosen is one which can be used with very low impedance sources. For the preferred results the amplifier can deliver an amplified signal at the frequency of the received signal, and the amplifier preferably has sufficient open loop gain at this frequency to give a detectable and measurable signal. The frequency is preferably in the range 1 Hz to 100 KHz and a gain of at least 25 decibels is preferred.
An amplifier which can be used which includes operational amplifiers of the OP37 type which can deliver a signal from a 0.1 ohm signal which can be used in the present invention. Use of and the AD849 type is also possible.
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 th
Clarke Richard Hedley
Millar John William Aidan
Bartlett & Sherer
Groundflow Limited
Moskowitz Nelson
Sherer Ronald B.
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