Device and method for measurement of resistivity outside of a wellpipe

[0002] This invention concerns an invention to measure resistivity in the geological formations surrounding a well in a petroleum reservoir. More particularly, a measuring device is described, consisting of a transmitter antenna and a series of receiver antennas placed outside the lining pipe in an injection well or a production well.

[0003] During injection of water through an injection well in a petroleum reservoir it may be very useful with monitoring of the state of the reservoir. Of particular importance is to perform monitoring of the so-called oil/water contact (OWC) being the boundary surface between the usually overlying oil and the underlying water in the permeable rocks constituting the reservoir, e.g. sandstone or limestone. If water under large pressure is injected below OWC this may result in pressure increase in the oil- and gas reservoir above OWC, and result in increased outflow of oil and gas from production wells being in hydraulic communication with the injection well. A device and a method according to the invention will be used both in injection wells and production wells in order to measure and perform monitoring of the electrical properties in the reservoir to indicate, among other things, the position of the oil/water contact and its movement. It is very difficult to perform measurements of the resistivity in the geological formations if one has to measure through the wall of a metallic lining (casing) pipe, an injection pipe or production tubing. For observation of the oil/water contact's level changes it is therefore highly uncertain to perform such observations through the wall of the wellpipe. Further, in order to care for the flow capacity, it is not convenient to arrange measuring devices inside a wellpipe during normal operation of the well.

[0004] Below will be given a simplified summary of some of the factors which affect the propagation of electromagnetic waves in a rock. According to the invention there will be emitted, from the transmitter antenna, electromagnetic waves in the form of continuous, sweep or pulsed waves. These pulses are refracted in the rock strata relatively shallowly in the geological formation so that a part of the energy is picked up in the receiver antennas.

[0005] The attenuation or reduction of the energy of the electromagnetic signal happens essentially due to three main factors:

[0006] I. geometrical, approximately spherical dispersion,

[0007] II: electrical properties (resistivity and dielectric) and

[0008] III: backscattering (homogenous backscattering from reflecting geological electrically conductive more or less homogenous horizons, and occasional backscattering due to reflecting mineral particles).

[0009] AboutI: The geometrical, approximately spherical dispersion follows approximately 1/r³with r being the distance between transmitter and receiver.

[0010] AboutII: The electrical properties is the resistivity and the varying dielectric (called the “dielectric constant”.) The relative dielectric constant varies from 6 for 20%-porous oil saturated quartz sand to ca. 13 for water saturated 20%-porous quartz sand. The resistivity in the rocks also determines the attenuation of the electromagnetic pulses. In FIG. 5 the attenuation in dB/m is displayed as function of frequencies between 1 and 16 MHz, for resistivities between 5 Ωm and 30 Ωm. The resistivity of oil sand in the reservoir may be between 20 and 200 Ωm. The resistivity of rocks containing formation water, below the oil/water contact (OWC) is between 0.5 and 5 Ωm. Thus the electromagnetic pulses will be attenuated much more while the transmitter- or receiver antennas are situated below the oil/water contact OWC than while the receiver antennas find themselves surrounded by oil-saturated sandstone.

[0011] AboutIII: Backscattering or reflection occurs from geological or fluid surfaces being homogenously continuous at an extent of comparable with the wavelength of the electromagnetic waves. For the actual rocks this is for wavelengths between 2 and 8 MHz crudely estimated from 10 to 2 meters. In this invention's connection this reflection a pure loss of signal.

[0012] AboutIII: Occasional backscattering happens especially by point reflection from electrically conductive mineral grains in the rocks, e.g. pyrite, haematite and magnetite.

[0013] Examples of the Known Art.

[0014] An apparatus for measurement of formation resistivity through casing pipes is given in U.S. Pat. No. 5,680,049. The U.S. patent has electrodes being pressed against the casing pipe from a logging sonde by means of hydraulics. The logging thus occurs through an electrically conducting casing pipe which will mask the much lower conductivity (i.e. higher resistivity) in the rocks outside the casing pipe.

[0015] One method for more direct measurement of formation resistivity and reservoir monitoring outside a casing pipe is given in U.S. Pat. No. 5,642,051. Electrodes are cemented fixed in the well outside the casing pipe in hydraulically isolated zones of the reservoir. A current is sent between an electrode in the ground outside the reservoir and the electrodes in the well. In column 2 in U.S. Pat. No. 5,642,051 is described that an electrical isolation is required on the outside of the casing pipe. In this way one may regard the method for less actual for most purposes, as one must take into account that smaller and larger rifts in the isolation around the casing pipe must be expected during the installment, especially for petroleum wells below the seabed. It is also very difficult to imagine communication with electrodes in a production or injection well by means of electrically conducting in the seabed, as the wellstring from a floating platform will be impossible to isolate electrically.

[0016] An electromagnetic pulse transmitter is described in U.S. Pat. No. 4,849,699. In the actual U.S. patent the pulser is designed into a logging tool which by definition is arranged to be displaced through a borehole or a lined well.

[0017] Another pulse induction logging tool is described in U.S. Pat. No. 4,481,472.

[0018] One aspect of the present invention is

[0019] Solution to the problem.

[0020] A device and a method to measure resistivity in the geological formations outside a wellpipe, e.g. in the form of a production-, injection- or casing tube is given by the following patent claims defining this invention. The invention is a device for measurement and monitoring of resistivity in a petroleum reservoir in a geological formation with an injection-, observation- or production well comprising an electrically conductive metallic wellpipe, with the new and inventive being characterized by the following features:

[0021] a) an electrically energy source;

[0022] b) at least one inductive transmitter antenna for electromagnetic waves, fixedly arranged in the well, outside the wellpipe's outer metallic surface;

[0023] c) at least one sensor series comprising n inductive or possibly magnetostrictive or electrostrictive resistivity sensors (5a,5b,. . . , 5n) arranged for receiving the electromagnetic waves and generation of measurement signals, fixedly arranged in the well, by the petroleum reservoir and outside of the wellpipe's outer metallic surface; and

[0024] d) a signal conductor for the measurement signals.

[0025] The invention also comprises a method for measurement of resistivity in a petroleum reservoir in a geological formation with an observation- or production well by means of a device being defined above, characterized in that it comprises the following steps:

[0026] i) emission of electromagnetic waves from the inductive transmitter antenna to preferably the upper part of the petroleum reservoir,

[0027] ii) reception of electromagnetic waves by a sensor series consisting of n inductive or possibly magnetostrictive or electrostrictive resistivity sensors (5a,5b,. . . , 5n) arranged by the petroleum reservoir;

[0028] iii) generation of signals representing the electromagnetic waves sensed by the resistivity sensors (5a,5b,. . . , 5n); and

[0029] iv) registration of signal representations of the signals.

[0030] Additional features by the invention is given in the dependent claims.

[0031] Below a description of the invention will be given, with reference to the following drawing figures, with:

[0032] FIG. 1 illustrating in principle an injection well at an oilfield at sea. The injection well is attached to a well frame or wellhead and has a pipe connection with a production vessel at the surface.

[0033] FIG. 2adisplays schematically an embodiment of the invention with a transmitter antenna and a sensor series in a petroleum well, with the sensor series possibly consisting of electric coils or antennas.

[0034] FIG. 2bdisplays schematically an alternative embodiment of the invention with a transmitter antenna and a sensor series in a petroleum well, with the sensor series possibly consisting of optical sensors arranged to sense electromagnetic signals.

[0035] FIG. 3 displays an addressed control unit on a so-called communication bus for use in a preferred embodiment of the invention. Each control unit may have connected one transmitter antenna and at least one sensor.

[0036] FIG. 4 shows a first preferred embodiment of the invention, a module of a wellpipe comprising a transmitter antenna and two receiver antennas in the form of coils around the wellpipe.

[0037] FIG. 5 is a diagram of attenuation of electromagnetic waves in a conductive medium, e.g. a rock, with resistivities in the range between 30 Ωm and 5 Ωm, by a relative dielectric constant of 6, for frequencies between 1 MHz and 16 MHz.

[0038] FIG. 6 shows an illustration of a possible embodiment according to the invention, of the energy supply and the signal transport from the resistivity sensors to the surface.

[0039] FIG. 7 displays an illustrated imagined log of resistivity measured according to the invention.

[0040] FIG. 8 displays an illustration of an imagined injection well in a geological formation with a schist barrier above a gas-, oil- and water-bearing formation, with a wellpipe pumping water into the water-bearing part of the formation. A series of modules of an embodiment according to the invention is arranged along the wellpipe. Imagined logs of resistivity are displayed by the graphs (a) early in the injection phase and (b) later in the injection phase, and a graph (c) indicating remaining oil removal.

[0041] FIG. 9 displays an illustration of the main features in a communication device for application with the invention.

[0042] FIGS. 10a,10b,and 10cshow a laboratory model of a preferred modular embodiment according to the invention. The laboratory model is constructed in scale 1:3.

[0043] FIG. 1 and FIGS. 2aand 2bdisplay an injection-, observation- or production well 2. The well may comprise an electrically conductive metallic wellpipe 3, e.g. an injection- or production tubing 3 and possibly comprising a perforated or permeable liner pipe 6. These electrically conductive wellpipes may constitute the mechanical basis for the transmitter antenna 4 and a sensor series 5 according to the invention. A metallic pipe outside the transmitter antenna 4 or sensors 5 will attenuate the signals to a high degree. Essential in this connection is that if a liner or casing pipe 6 is arranged outside the production tubing, the transmitter antennas 4 and the resistivity sensors (5a,5b,. . . , 5n) constituting the sensor series 5 are arranged and fixed also outside of this outer metallic surface in the well. In a preferred embodiment of the invention the antennas are arranged such that no electrically disturbing fluid flow pass between the antenna and the borehole wall. In a preferred embodiment of the invention the transmitter antennas 4 and the sensor series 5 is cemented fixed together with the wellpipe 3,6. The invention is in the preferred embodiment especially arranged for being installed in an injection well, but also possible to being installed in production- or observation wells. Water may be injected in a perforated zone of the wellpipe into a water-bearing part of a reservoir 1 in a geological formation 9. The resistivity in the water-bearing zone typically is 0.5 to 5 Ωm such as indicated in FIG. 1. The resistivity in the oil-bearing part of the reservoir 1 typically is between 20 and 200 Ωm, with little change by the transition to the gas-bearing zone.

[0044] General Embodiment.

[0045] An electronic energy source or accumulator 10′ is arranged preferably near the at least one inductive transmitter antenna 4 being arranged for emission of electromagnetic waves 40. The transmitter antenna 4 is fixedly arranged in the well 2, outside the wellpipe's 3,6 outer metallic surface, and in a preferred embodiment generally such that no electrically disturbing fluid flow may pass between the antenna and the borehole wall. At least one sensor series 5 comprising n inductive resistivity sensors (5a,5b,. . . , 5n) arranged for reception of the electromagnetic waves 40 and generation of measurement signals 42, are fixedly arranged in the well 2, 2′, by the petroleum reservoir 1 and outside the wellpipe's 3, 6 outer metallic surface. The present invention gives in a preferred embodiment a volumetrically measured resistivity, not by means of electrodes arranged for measurement of resistivity by direct electrical contact with the formation. The fixed arrangement in the well 2 of the antennas is preferably performed by fixation cementing by means of cement. Fixation by means of hardening mass as glue etc. may be imagined. A signal conductor 7 for the measurement signals 42 leads to a registration unit 70 for signal representations 44 of the signals 42, which further represent the electromagnetic waves 40 sensed by the resistivity sensors (5a,5b,. . . , 5n).

[0046] By each resistivity sensor may be connected an addressable control unit 52 displayed in FIG. 3 and FIG. 9. Resistivity sensors (5a,5b,. . . , 5n) may be connected in pairs to each of their control units 52, possibly together with one transmitter antenna 4. The control unit 52 may comprise at least one memory 54, and address unit 55, and energy/charging unit 56 and a rest mode unit 57, and may be connected via the signal conductor 7 with e.g. the registration unit 70 which may be situated down by the reservoir, by the wellhead at the seabed, on a platform or at a ship having signal communication with the wellhead and further down to the sensor series 5. In one embodiment each resistivity sensor (5a,5b,. . . , 5n) may be connected to a respective control unit 52. In another embodiment all the resistivity sensor may be connected to one single control unit 52. Due to effect- and signal capacity limitations between the particular components and the registration unit 70, and further between the registration unit and the surface, each control unit may be set into or out of rest mode by a command to the rest mode unit 57. In such manner each sensor may be controlled to give measurement signals upon command.

[0047] Modular Embodiment.

[0048] In one preferred embodiment one inductive transmitter antenna 4 and preferably one sensor series 5 comprising n=2 inductive resistivity sensors (5a,5b) are arranged in a module 8 arranged for being combined in selected combinations of modules 8 and other wellpipe parts together constituting a wellpipe 3. Because one usually knows the position of the reservoir very well in injection- and production wells, modules 8 are entered into the well completion string 3 before the entire string is installed in the well. The signal conductor 7 and the control units 52 including the registration unit 70 may be arranged in ready milled tracks on the wellpipe 3 and its modules 8, or such tracks may be prefabricated on the wellpipe 3 and its modules 8. When the wellpipe 3 is arranged in the well 2 it is fixedly cemented to the wall of the borehole in the ordinary way.

[0049] Electrical Embodiment.

[0050] The sensor series with the resistivity sensors (5a,5b,. . . , 5n) consists in a preferred embodiment preferably of electrical coil antennas, preferably pairs of resistivity sensors (5a,5b) with n=2.

[0051] The inductive transmitter antenna 4 for electromagnetic waves 40 is in a preferred embodiment arranged for transmission of coherent continuous electromagnetic waves. A preferred frequency range for the electromagnetic waves is from 1 to 20 MHz, but other frequency ranges are not excluded. In an additionally preferred embodiment of the invention the inductive transmitter antenna 4 is arranged for emission of electromagnetic waves in the frequency range 2-8 MHz. In alternative embodiments the emission of the signals may take place by means of sweep signals inside the preferred frequency range. An additional alternative embodiment of the invention may be arranged to emit electromagnetic pulses, e.g. by means of a pulse transmitter such as disclosed in U.S. Pat. No. 4,849,699, which is hereby incorporated by reference in its entirety. In the mentioned U.S. patent the pulse transmitter is designed into a logging tool being by definition arranged to be displaced through a borehole or a lined well.

[0052] A ceramic isolator 43 is arranged between the metallic wellpipe 3 and the inductive transmitter antenna 4. This is displayed in FIG. 4, with the module 8 having milling-cut cut-outs in the wellpipe 3 in a cylindrical volume under the transmitter antenna 4. A corresponding ceramic isolator 45 is arranged between the metallic wellpipe 3 and each of the resistivity sensors (5a,5b,. . . , 5n). In order to protect and isolate the antennas and the sensors, an isolating sleeve 46 is arranged outside on the inductive transmitter antenna 4 and the resistivity sensors (5a,5b,. . . , 5n). This isolating sleeve 46 does not necessarily need to be continuous between the transmitter antenna 4 and the resistivity sensors (5a,5b,. . . , 5n) internally. A hydraulically balancing cavity 48, filled preferrably with silicone oil, is arranged between the metallic wellpipe 3 with the inductive transmitter antenna 4 and the resistivity sensors (5a,5b,. . . , 5n), and the electrically isolating sleeve 46. This cavity's liquid distributes pressure and reduces mechanical shear forces so that it protects the module's electrical components.

[0053] On the modules 8 are found in a preferred embodiment of the invention elastic centralizers 49 arranged near each inductive transmitter antenna or sensor series 5, preferably in at least one end of each module 8.

[0054] Energy Supply and Signal Transport.

[0055] The energy supply may happen from the outside of the well, but in a preferred embodiment of the invention is applied a local energy source or accumulator 10′ and an energy supply conductor 10 by the inductive transmitter antenna 4. The energy source or accumulator 10 may be charged via energy supply conductors (not shown) from the surface. In a preferred embodiment of the invention, illustrated in FIG. 9, all signal transmission and energy supply between e.g. a well frame at the seabed or generally on the surface and the well via a so-called EXAL-bus having a highest signal speed of 300 Baud and up to 56 V voltage and 24 mA maximum current. In the well frame may be arranged a communication card of the type DHI-103 using this EXAL-bus. An alternative to the communication card DHI-103 is a corresponding one named DIH-107 having five times higher effect.

[0056] Alternative Embodiment.

[0057] In an alternative embodiment of the invention illustrated in FIG. 2b,the transmitter antenna 4 may consist of a high-effect pulse emitting capacitive coil, preferably wound around the wellpipe 3 in a way corresponding to the description in MPI's U.S. Pat. No. 4,849,699 for a logging tool. This capacitive coil is arranged for slow charging with the capacity permitted by the energy supply and signal conductors, and arranged for time-controlled discharge via a spark gap. Such a transmitter antenna 4 will have a very large effect and will give an electromagnetic pulse reaching far down into the geological formation.

[0058] In an alternative embodiment there may be arranged an optical fiber 7 constituting a signal conductor 7 for communication between the resistivity sensors (5a,5b,. . . , 5n) and a registration unit 70 for signals 42 which represent the electromagnetic waves 40 sensed by the resistivity sensors (5a,5b,. . . , 5n). Each resistivity sensor (5a,5b,. . . , 5n) may be constituted by a magnetic coating on Bragg-gratings 50 arranged to couple changes of the inducing electromagnetic field to changes in the optical Bragg-grating 50. Each Bragg-grating constituting a part of each resistivity sensor (5a,5b,. . . , 5n) on the optical cable has each its discrete internal grating separation different from the other resistivity sensors (5a,5b,. . . , 5n). Mechanical vibrations will be induced on the Bragg-grating so that the internal grating separation will vibrate about the initial separation. In this way a broad-spectered optical pulse from e.g. registration unit 70. Each resistivity sensor (5a,5b,. . . , 5n) then will “respond” on the optical pulse by reflecting the locally sensed electromagnetic frequency and amplitude and phase as signals 42 modulated in each of their frequency bands on the broad-spectered optical pulse. The signals 42 represent the electromagnetic waves 40 sensed by the resistivity sensors (5a,5b,. . . , 5n). This is possible because the broad-spectered optical pulse has a very rapidly vibrating frequency in the nanometer range, while the electromagnetic pules as mentioned above reside between 2 to 10 meters. The signals 44 may be transformed e.g. to digital signal representations 44. Thus it is in the alternative embodiment of the invention possible to perform several readings by many resistivity sensors (5a,5b,. . . , 5n) simultaneously via one single optical cable. This optical cable may alternatively lead the entire way from the sensors 5 to the surface.

[0059] Signal Processing.

[0060] The transmitter antenna will radiate energy in all directions out into the formation. Depending on the wavelength of the emitted signal there will be one corresponding critical angle leading to refraction of the electromagnetic wavelength so that the energy propagation takes place in the geological formation 9 near the wall of the borehole. This refracted wave will radiate out and back again from the borehole wall and be sensed by the sensors (5a,5b,. . . , 5n). Reflections along the refracted path will be a pure loss in this measurement process. This refracted wave will be the first arrival to the sensors, and in a preferred embodiment this first arrival constitutes the electromagnetic radiation being most suitable for the purpose for the invention. Reflections from more remotely situated parts of the formation will arrive later and will thus be disregarded. Further, reflections will, in most actual considerations, be several dB lower than the refracted wave, so that the reflected waves will have little effect on the part of the measurement signal which according to the invention will be transformed to registered signals which should represent the resistivity.

[0061] Method of Resistivity Measurement.

[0062] A preferred embodiment of the invention consists basically of a method for measurement of resistivity in a petroleum reservoir 1 in a geological formation 9 with an injection-, observation- or production well 2 by means of a device as described in earlier embodiments. The method comprises the following steps:

[0063] i) emission of electromagnetic waves 40 from the inductive transmitter antenna 4 to preferably the upper part of the petroleum reservoir 1,

[0064] ii) reception of electromagnetic waves 40 by a sensor series 5 of n inductive resistivity sensors (5a,5b,. . . , 5n) arranged by the petroleum reservoir 1;

[0065] iii) generation of signals 42 representing the electromagnetic waves 40 sensed by the resistivity sensors (5a,5b,. . . , 5n); and

[0066] iv) registration of signal representations 44 of the signals 42.

[0067] Primarily the refracted part of the electromagnetic waves 40 will be applied for generating signals 42, further being transformed to signal representations 44. The resistivity may be calculated on the basis of the amplitude and the phase of the received waves 40. The signal representations 44 will according to a preferred embodiment of the invention be drafted as a log of resistivity in the reservoir. In order to trace the development in the reservoir during time, the operation may utilize a method comprising the following steps:

[0068] a) a first performance of the steps (i-iv) according to the method described above;

[0069] b) storage of the first signal representations 44₁;

[0070] c) a second performance of the steps (i-iv) according to the method mentioned above;

[0071] d) storage of the second signal representations 44₂;

[0072] e) formation of a difference between the second signal representations 44₂and the first signal representations 44₁;

[0073] f) interpretation of the difference with regard to changes in liquid surfaces, preferably those of an oil/water contact's (OWC), level in the reservoir 1.

[0074] With one transmitter antenna 4 and a long series of resistivity sensors (5a,5b,. . . , 5n) corrections should be made for geometrical, approximately spherical or cylindrical dispersion of the electromagnetic wave before the log is drafted on the basis of the measurements or the signal representations, such as illustrated in FIG. 7. By interpretation of the difference it is however in one basic embodiment only the relative changes of the signal between the signal representations 44₁, 44₂, . . . , 44_tas a development through time, measured at each particular sensor of the sensors (5a,5b,. . . , 5n) being necessary to monitor the movements of the oil/water contact. With a series of modules 8 arranged along the wellpipe 3 it will preferably be equal separation between each pair of transmitter antenna and resistivity sensor 5_iin the sensor series 5. Thus an observation log may be drafted on the basis of the pair by pair measurement of resistivity without need to make geometrical correction of the measurements.

[0075] FIGS. 10a,10b,and 10cillustrate a laboratory model of a preferred modular embodiment according to the invention. The laboratory model is designed in scale 1:3.

[0076] The above mentioned embodiments are to be considered as non-limiting examples of possible embodiments of the invention. The invention is only limited by the attached patent claims.