Method and apparatus for demagnetizing a borehole

A demagnetizing sub having an electromagnet or a rotating magnet is used for demagnetizing magnetized material in a wellbore. By gradually reducing the magnetic field, the magnetized material within the wellbore is demagnetized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a downhole formation analysis and in particular to demagnetizing a borehole.

2. Summary of the Related Art

It is well known that drilling mud may contain magnetic particles which may influence surveys taken by monitoring while drilling (MWD) directional sensors. This effect is described in IADC/SPE 87169 and SPE 71400. The particles are mainly from casing and drill-string wear. The particles collect especially in synthetic oil-based mud which is recycled and used for a long period of time and even at different locations. Magnetic filters (ditch magnets) can not filter the magnetic particles out of the mud because they are too small to be attracted to the magnetic filter. The mud contaminate with magnetic steel particles can invade the formation and can also collect in the filter cake. This collection of magnetic particles creates a zone close to the borehole wall containing magnetic material which can not circulate with the rest of the drilling mud. When permanent magnets from nuclear magnetic resonance (NMR) tools, magnetic fishing tools or casing collar locaters are moved through the borehole, these collected particles in the filter cake or the invaded zone can become magnetized. Because of the magnetic remanance the particles maintain their magnetic influence and emit a magnetic field for a long time. The magnetic field created by these magnetically aligned particles can influence the performance of magnetic tools deployed from a wireline or drill string such as magnetic azimuth measurements or other magnetic measurements associated with the borehole. A similar problem occurs in cased borehole where the casing is made of a magnetic material such as steel. The resulting magnetization of the casing may affect the performance of sensors such as magnetometers that are subsequently conveyed through the casing.

SUMMARY OF THE INVENTION

One embodiment of the invention is an apparatus for reducing the effect of magnetized material in a wellbore in an earth formation. The apparatus includes a downhole assembly conveyed in the borehole and a device included in the downhole assembly which produces an alternating magnetic field in the wellbore, wherein the device is operated to gradually diminish an amplitude of the alternating magnetic field at at least one depth in the wellbore. The device may be an electromagnet coupled to a source of alternating current and a processor which controls the source to reduce the amplitude of the field produced by the electromagnet. The device may be a spinning magnet, permanent or a DC-powered electromagnet. The downhole assembly may be a bottomhole assembly conveyed on a drilling tubular or it may be a component of a string of wireline conveyed instruments. When an electromagnet is used, it includes a coil and a core. The electromagnet may further include a yoke made of a soft magnetic material. The core may have a laminated structure. The processor may select an initial amplitude of the alternating current from the current source based on a saturation field of the magnetized material and/or a magnetic field intensity which produced magnetization of the magnetized material. The axis of the electromagnet may be parallel to or transverse to the longitudinal axis of the downhole assembly. The electromagnet may have a two-pole structure or a four-pole structure. The core may have a helical structure. The borehole may be a cased borehole.

Another embodiment of the invention is a method of demagnetizing magnetized material in a wellbore in the earth formation. Use is made of a magnet included in a downhole assembly conveyed in the wellbore to produce an alternating magnetic field therein. The amplitude of the alternating magnetic field is then reduced. The magnet may be an electromagnet, in which case, the method includes coupling a source of alternating current to the electromagnet, and controlling the source of the alternating current to reduce the amplitude of the alternating magnetic field. The magnet may be a rotating permanent magnet or a rotating DC-powered electromagnet moved through the borehole. The method may include conveying the downhole assembly into the borehole on a drilling tubular or a wireline. The electromagnet may be provided with a coil through which the alternating current is passed and a core which concentrates magnetic flux provide by the magnetic field. The borehole may be a cased borehole.

DETAILED DESCRIPTION OF THE INVENTION

In view of the above, the present invention through one or more of its various aspects and/or embodiments is presented to provide one or more advantages, such as those noted below.FIG. 1illustrates a schematic diagram of a MWD drilling system10with a drill string20carrying a drilling assembly90(also referred to as the bottom hole assembly, or “BHA”) conveyed in a “wellbore” or “borehole”26for drilling the wellbore. The demagnetizing sub100is positioned on the drill string20below NMR tool79. The drilling system10includes a conventional derrick11erected on a floor12which supports a rotary table14that is rotated by a prime mover such as an electric motor (not shown) at a desired rotational speed. The drill string20includes tubing such as a drill pipe22or a coiled-tubing extending downward from the surface into the borehole26. The drill string20is pushed into the wellbore26when a drill pipe22is used as the tubing. For coiled-tubing applications, a tubing injector (not shown), however, is used to move the tubing from a source thereof, such as a reel (not shown), to the wellbore26. The drill bit50attached to the end of the drill string breaks up the geological formations when it is rotated to drill the borehole26. If a drill pipe22is used, the drill string20is coupled to a drawworks30via a Kelly joint21, swivel28and line29through a pulley23. During drilling operations, the drawworks30is operated to control the weight on bit, which is an important parameter that affects the rate of penetration. The operation of the drawworks is well known in the art and is thus not described in detail herein.

During drilling operations, a suitable drilling fluid31from a mud pit (source)32is circulated under pressure through a channel in the drill string20by a mud pump34. The drilling fluid passes from the mud pump34into the drill string20via a desurger36, fluid line38and Kelly joint21. The drilling fluid31is discharged at the borehole bottom51through an opening in the drill bit50. The drilling fluid31circulates uphole through the annular space27between the drill string20and the borehole26and returns to the mud pit32via a return line35. The drilling fluid acts to lubricate the drill bit50and to carry borehole cutting or chips away from the drill bit50. A sensor S1preferably placed in the line38provides information about the fluid flow rate. A surface torque sensor S2and a sensor S3associated with the drill string20respectively provide information about the torque and rotational speed of the drill string. Additionally, a sensor (not shown) associated with line29is used to provide the hook load of the drill string20.

The rotating the drill pipe22only rotates the drill bit50. In another embodiment of the invention, a downhole motor55(mud motor) is disposed in the drilling assembly90to rotate the drill bit50and the drill pipe22is rotated usually to supplement the rotational power, if required, and to effect changes in the drilling direction.

In the embodiment ofFIG. 1, the mud motor55is coupled to the drill bit50via a drive shaft (not shown) disposed in a bearing assembly57. The mud motor rotates the drill bit50when the drilling fluid31passes through the mud motor55under pressure. The bearing assembly57supports the radial and axial forces of the drill bit. A stabilizer58coupled to the bearing assembly57acts as a centralizer for the lowermost portion of the mud motor assembly.

A drilling sensor module59is placed near the drill bit50. The drilling sensor module contains sensors, circuitry and processing software and algorithms relating to the dynamic drilling parameters. Such parameters preferably include bit bounce, stick-slip of the drilling assembly, backward rotation, torque, shocks, borehole and annulus pressure, acceleration measurements and other measurements of the drill bit condition. A suitable telemetry or communication sub72using, for example, two-way telemetry, is also provided as illustrated in the drilling assembly90. The drilling sensor module processes the sensor information and transmits it to the surface control unit40via the telemetry system72.

The communication sub72, a power unit78and an NMR tool79are all connected in tandem with the drill string20. Flex subs, for example, are used in connecting the MWD tool79in the drilling assembly90. Such subs and tools form the bottom hole drilling assembly90between the drill string20and the drill bit50. The drilling assembly90makes various measurements including the pulsed nuclear magnetic resonance measurements while the borehole26is being drilled. The communication sub72obtains the signals and measurements and transfers the signals, using two-way telemetry, for example, to be processed on the surface. Alternatively, the signals can be processed using a downhole processor in the drilling assembly90.

The surface control unit or processor40also receives signals from other downhole sensors and devices and signals from sensors S1-S3and other sensors used in the system10and processes such signals according to programmed instructions provided to the surface control unit40. The surface control unit40displays desired drilling parameters and other information on a display/monitor42utilized by an operator to control the drilling operations. The surface control unit40preferably includes a computer or a microprocessor-based processing system, memory for storing programs or models and data, a recorder for recording data, and other peripherals. The control unit40is preferably adapted to activate alarms44when certain unsafe or undesirable operating conditions occur.

Turning now toFIG. 2, as shown inFIG. 2the present invention is suitable for deployment in a borehole from a drill string. In this case the demagnetizing sub100of the present invention is deployed from a wireline beneath a multi-component resistivity tool9including a NMR tool79.FIG. 1schematically shows a wellbore1extending into a laminated earth formation, into which wellbore an induction logging tool as used according to the present invention has been lowered. The wellbore inFIG. 1extends into an earth formation which includes a hydrocarbon-bearing sand layer3located between an upper shale layer5and a higher conductivity than the hydrocarbon bearing sand layer3. An induction logging tool9and/or an NMR tool79including the demagnetizing sub100of the present invention have been lowered into the wellbore1via a wire line11extending through a blowout preventor13(shown schematically) located at the earth surface15. The surface equipment22includes an electric power supply to provide electric power to the set of coils18and a signal processor to receive and process electric signals from the receiver coils19. Alternatively, the power supply and/or signal processors are located in the logging tool.

Turning now toFIG. 3, a schematic of the demagnetizing sub100of the present invention is illustrated showing demagnetizing sub100with a magnetic flux, B field310parallel to the borehole306axis. As shown inFIG. 3, in this geometry the axis of the coil308of the electromagnet is parallel with the cylinder axis of the demagnetizing sub100. The sub may be part of a bottomhole assembly (BHA) conveyed on a drilling tubular or may be part of a string of wireline conveyed subs. For the purposes of the present invention, the term “downhole assembly” is used to designate a BHA or a string of wireline tools. Smart alternating current supply302includes a processor for providing a controlled alternating current to the electromagnet coil308. A vertical coil with an iron core would produce a magnetic field similar to an NMR tool, but weaker unless an electric input of many kW is provided. The solution for producing a strong magnetic field with low power is the use of a yoke to concentrate the field outside the borehole in a ring around the tool.

Turning now toFIG. 4, a schematic of the invention is illustrated showing a demagnetizer sub100with B field311orthogonal to the borehole306axis. The sub may be part of a bottomhole assembly conveyed on a drilling tubular or may be part of a string of wireline conveyed subs. In this configuration the field direction is in a plane orthogonal to the borehole axis and the demagnetizer sub100axis. The electric current runs parallel to the axis. The electromagnet can be similar an anchor of a direct current (DC) electromotor. The anchor can have one pole or more, for example, 2 pole and 4 pole configurations.

The present invention provides a demagnetizer sub100that can demagnetize the borehole wall306that has been magnetized by a previous magnetic tool run. It is assumed that the magnetizable particles are in the drilling mud and hence in the mud cake, lining the borehole wall. Magnetization of the borehole environment can also happen if ferromagnetic components such as magnetite exist in the geologic formation. The demagnetizing is achieved by providing an AC (alternating current) electromagnet and moving it along the axis of the borehole. The present invention is also useful with a wireline logging run.

To demagnetize hard magnetic material the standard method is the application of an alternating magnetic field. The AC amplitude should reach a relatively high level, ensuring that the magnetic hysteresis loop of the component to be demagnetized becomes symmetrical to the origin of the BH diagram. Subsequently the AC amplitude is decreased slowly in order that the run-through hysteresis loop becomes smaller and smaller and disappears eventually.

The idea of the demagnetizer sub100described herein is that of an AC electromagnet, which provides the high magnetic field amplitude. The continuous and slow decrease of the field amplitude at the place of the magnetized mud particles is achieved by moving the electromagnet along the borehole axis. Ideally the demagnetizing field should initially reach at least the saturation field of the magnetized component, for iron this is of the order of 2 Tesla. To produce fields of this magnitude in the borehole wall, however, may be impractical, assuming that the electric power is limited to a couple of hundred Watts. Given the power limitations, it presently should be sufficient to produce a magnetic field substantially as high as the magnetic tool field which magnetized the particles. For example, if an NMR tool generates a magnetic field of 2000 Gauss having a particular spatial distribution, the present invention demagnetizing sub generates a demagnetizing field of 2000 Gauss having a similar spatial distribution.

Due to the cylindrical geometry of the borehole there exist essentially two geometries demonstrated herein for such an electromagnet. The electromagnet can apply a field that is either essentially parallel or orthogonal to the borehole axis as shown above inFIG. 3andFIG. 4.

FIG. 5illustrates the magnetic flux density510of a nuclear magnetic resonance (NMR) tool. The maximum flux density at the tool surface (radius 92 mm) is about 2200 Gauss. For the centered tool the flux density at the nominal borehole wall (radius 108 mm) is about 1200 Gauss.FIG. 5illustrates the radial dependence of the flux density at Z=230 mm, i.e. at the lower end of the permanent magnet.

FIGS. 6-8and10-16are finite element models showing a top right quarter section of a symmetrical illustration.FIG. 6illustrates one example of geometry for the electromagnet of the demagnetizing sub100for generating a magnetic flux parallel to the borehole axis. Areas105and106are transformer iron. Area107is a yoke made of soft magnetic material to facilitate manufacturing and concentrate the flux. Area101is a copper (coil) and area108is a gap where the erasing magnetic flux density is concentrated.FIG. 7illustrates a contour plot of the magnetic flux density for the geometry ofFIG. 6. The plot shows flux density that it is mainly 15,000 Gauss in the transformer iron.

FIG. 8illustrates a contour plot of the magnetic flux density for the geometry ofFIG. 6, showing the distribution outside the demagnetizer.FIG. 9illustrates a magnetic flux density, B over radius, R in the center plane of the demagnetizer sub for the geometry ofFIG. 6. The Z-axis represents the longitudinal axis and center of the borehole and of the demagnetizing sub100. Further increasing the electrical power would not gain much more magnetic flux density as it is limited by the beginning of saturating the iron. An advantage of this geometry is the iron can easily formed from layers of transformer sheet (a laminated structure). The 2-D finite element models show a cross section orthogonal to the tool axis. The calculated field profiles are only correct for a tool that extends to infinity in the direction of the axis. For this reason the power dissipation results are stated in Watts/mm.

FIGS. 10-13show finite element modeling for one-quarter of a full cross section of a two-pole electromagnet generating a magnetic flux field perpendicular to the borehole axis. Shown inFIG. 10is the geometry. A horizontal section of the two-pole demagnetizer is shown inFIG. 10a.FIG. 11shows the magnetic flux density in the demagnetizer.FIG. 12shows the magnetic flux density outside the demagnetizer.FIG. 13shows the decay of the flux density1301with radius.

FIGS. 14-17show finite element modeling for one-eighth of a full cross section of a four-pole electromagnet generating a magnetic flux field perpendicular to the borehole axis. The Z-axis is parallel to the longitudinal axis of the borehole and the demagnetizing sub100, which is perpendicular to the X, Y plane ofFIGS. 10-17.FIGS. 14 and 14ashow the geometry of the four-pole demagnetizer. The magnetic flux is concentrated about the gaps108.FIGS. 15 and 16are contour plots of the flux density.FIG. 17is a plot of the radial decay of the flux density.

In one embodiment the iron sheets are not be stacked directly on top of each other but slightly shifted by a small angle so that the finished iron core forms a helical structure. For a two-pole core a helix is provided with half a turn to ensure that the entire borehole wall gets demagnetized when we run the (non-rotating) demagnetizer tool through the borehole. This is illustrated inFIG. 18. For a four-pole tool a quarter helix is provided to achieve a complete demagnetizing. In another embodiment, the two-pole configuration, two demagnetizing fields separated by 180° are provided and the demagnetizing sub rotates at least 180° or one-half turn to expose the entire 360° angular section of the borehole at a particular depth to the demagnetizing field generated by the demagnetizing sub electromagnet. Similarly, for the four-pole configuration, four demagnetizing fields separated by 90° are provided and the demagnetizing sub rotates at least 90° or one-quarter turn to expose the entire 360° angular section of the borehole at a particular depth to the demagnetizing field generated by the demagnetizing sub electromagnet.

The pitch of the helix should not be too small to ensure a good demagnetizing effect further away from the tool. Eventually the length of the tool and its power dissipation depends on the minimum pitch that can be tolerated. An iron yoke is provided with a reasonably small gap109to produce a magnetic field of high enough strength with a limited electric power input. The transverse field geometry provides less problems with iron saturation. It is also better suited to the use of laminated transformer iron. The field decay quickly away from the demagnetizer tool.

It is desirable to ensure in the design that induced eddy current losses are minimized as they can consume a lot of power. If the power source is DC, it is useful to use at least two or more poles. By giving the different poles an appropriate phase shift to each other we can ensure that, at least theoretically, at every point in time we consume the same power.

Examples: Two poles: The phase shift should be 90°. Three poles: The phase shift should be 60° or 120° between poles. It may be that in the case of 120° makes better use of the iron, as in the case of a 3-phase main transformer.

Another embodiment of the present invention uses a spinning permanent magnet on the downhole assembly. The spinning magnet can be either a permanent magnet or a DC-powered electromagnet. This is depicted schematically inFIG. 19. Shown in a borehole having a wall501is a downhole assembly503that includes a permanent magnet505. The permanent magnet spins within the borehole while the assembly is being moved through the borehole.FIG. 20illustrates the magnetic551field that would be observed at an specific point on the borehole wall if the downhole assembly is moved through the borehole. When the downhole assembly is conveyed on a drilling tubular, the spinning may be accomplished by rotation of the drilling tubular. When the downhole assembly is conveyed on a wireline, then a suitable motor (not shown) would be provided on the downhole assembly to accomplish the rotation.

While an embodiment of the invention has been shown by the above invention, it is for purposes of example only and not intended to limit the scope of the invention, which is defined by the following claims.