1. Field of the Disclosure
The present disclosure relates to well logging. In particular, the present disclosure is an apparatus and method for determining the property of subsurface formations using contact devices.
2. Background of the Art
The disclosure is first described with reference to the use of contact devices in resistivity measurements. In conventional galvanic resistivity measurement tools using a focusing technique, a guard electrode emits current in order to lead the current beam of a measurement electrode deeper into a conductive material. The resistivity of the material is determined by means of measurement electrode's voltage and current registration. The driving potential on guard and measurement electrode must be exactly the same to avoid disturbances of the ideal electrical field, which makes sure that the focusing effect takes place. Higher driving potential differences may lead to currents from guard to measurement electrode or vice versa passing the borehole fluid around the tool, which would completely destroy the focusing effect and lead to high measurement errors if not considered. In general, the focusing effect will lead to an electrical current with a higher penetration depth compared to that without focusing.
One of the problems in making resistivity measurements while drilling (MWD) is that the drilled borehole has a larger diameter than the sensor module. The difference in diameter results in different standoffs of the sensor electrodes from the borehole wall. In water-based mud, the varying standoff results in current flows that are not radially directed from the sensor electrodes to the wall, resulting in smearing of the resistivity image. In oil-based mud, the varying standoff result in different gap impedances in the flow of electric current from the electrode to the formation, so that the value of the current is not indicative of the formation resistivity near the electrode.
Prior art methods have attempted to address this problem with varying degrees of success by using focusing and guard electrodes (adding to the complexity of the hardware) and by processing methods. The present disclosure provides a simple hardware solution to the problem of variable standoff.
One embodiment of the disclosure is an apparatus configured to evaluate an earth formation. The apparatus includes: a carrier configured to be conveyed in a borehole; a torsion bar coupled to the carrier; a contact assembly coupled to the torsion bar; and an actuator associated with the torsion bar, the actuator configured to provide a torsion force to the torsion bar, the torsion force being used to maintain a contact assembly in a position proximate to a wall of the borehole.
Another embodiment of the disclosure is a method of evaluating an earth formation. The method includes: conveying a carrier including a torsion bar into a borehole; and using an actuator associated with the torsion bar to provide a torsion force that maintains a contact assembly on the carrier proximate to a wall of the borehole.
The novel features that are believed to be characteristic of the disclosure, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the disclosure is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the disclosure:
During drilling operations, a suitable drilling fluid 31 from a mud pit (source) 32 is circulated under pressure through a channel in the drillstring 20 by a mud pump 34. The drilling fluid passes from the mud pump 34 into the drillstring 20 via a desurger 36, fluid line 28 and Kelly joint 21. The drilling fluid 31 is discharged at the borehole bottom 51 through an opening in the drill bit 50. The drilling fluid 31 circulates uphole through the annular space 27 between the drillstring 20 and the borehole 26 and returns to the mud pit 32 via a return line 35. The drilling fluid acts to lubricate the drill bit 50 and to carry borehole cutting or chips away from the drill bit 50. A sensor S1 preferably placed in the line 38 provides information about the fluid flow rate. A surface torque sensor S2 and a sensor S3 associated with the drillstring 20 respectively provide information about the torque and rotational speed of the drillstring. Additionally, a sensor (not shown) associated with line 29 is used to provide the hook load of the drillstring 20.
In one embodiment of the disclosure, the drill bit 50 is rotated by only rotating the drill pipe 22. In another embodiment of the disclosure, a downhole motor 55 (mud motor) is disposed in the drilling assembly 90 to rotate the drill bit 50 and the drill pipe 22 is rotated usually to supplement the rotational power, if required, and to effect changes in the drilling direction.
In the embodiment of
Turning now to
The stabilizer shown at 107 serves several functions. Like conventional stabilizers, one function is to reduce oscillations and vibrations of the sensor assembly. However, in the context of the present disclosure, it also serves another important function, viz, centralizing the portion of the bottomhole assembly (BHA) including a sensor assembly, and also maintaining the sensors with a specified standoff from the borehole wall. This is not visible in
The equivalent circuit for the flow of current through a sensor electrode is shown in
To avoid the problems associated with variable standoff, a novel subassembly is used. This is illustrated by 400 in
As shown in
The torsion bar 401 may be provided with bearings 503. The rotation of the torsion bar 401 is indicated by the arrow 505 while the motion of the actuator is indicated by 507.
When the contact assembly is provided with a sensor pad having sensor, the pad can follow the contour of the borehole The force on the contact assembly 407 against the borehole will be proportional to the displacement of the lever 403 which, in turn, is proportional to the hydraulic pressure in the hydraulic unit 405.
With such a configuration, it is possible to have the sensor pad with a very small offset from the borehole wall. The currents through the electrodes are then indicative of the resistivity property of the earth formation as the gap impedance is very small. A resistivity image of the borehole wall can be produced from the currents in the electrodes using prior art methods and recorded on a tangible medium. To protect the sensor pad 407 from wear, it may be provided with a hardfacing such as polycrystalline diamond (PCD).
The displacement of the sensor pad 407 can be measured by measuring the torsion angle of the torsion bar 401 indicated by the displacement of the lever 403 or the piston. For the purposes of the present disclosure, the displacement of the lever 403 and of the piston are considered to be equivalent. The displacement of the contact assembly 407 thus provides a caliper measurement of the borehole. Assuming that the side of the sub 400 opposite to the sensor pad 407 is also in contact with the borehole wall, a continuous measurement of borehole diameter is obtained. When combined with the orientation measurement from the orientation sensor 111, it is possible to obtain a continuous image of the size of the borehole in addition to the resistivity image obtained from the resistivity measurements.
With the apparatus and method of the present disclosure, a resistivity image can be obtained in a MWD environment using orientation measurements by a suitable orientation sensor 111 such as a magnetometer. Methods of producing such images are discussed, for example, in U.S. Pat. No. 6,173,793 to Thompson et al, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. The method of the present disclosure may also be used to produce a resistivity image of an earth formation using a plurality of pads conveyed on a wireline, each of the pads containing a plurality of measure electrodes, guard electrodes and bridge-coupling circuits.
The processing of the data may be done by a downhole processor to give corrected measurements substantially in real time. Alternatively, the measurements could be recorded downhole, retrieved when the drillstring is tripped, and processed using a surface processor. Implicit in the control and processing of the data is the use of a computer program on a suitable machine readable medium that enables the processor to perform the control and processing. The machine readable medium may include ROMs, EPROMs, EEPROMs, Flash Memories and Optical disks.
This applications claims priority from U.S. Provisional Patent Application Ser. No. 61/172,942 filed on Apr. 27, 2009.
Number | Date | Country | |
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61172942 | Apr 2009 | US |