WO2007104103A1

WO2007104103A1 - Method of orientating a core sample

Info

Publication number: WO2007104103A1
Application number: PCT/AU2007/000315
Authority: WO
Inventors: Andrew Beach; Gavin Mcleod
Original assignee: 2Ic Australia Pty Ltd
Priority date: 2006-03-14
Filing date: 2007-03-13
Publication date: 2007-09-20

Abstract

A method for orientating a geological core sample extracted from a borehole comprising: providing a first indication of a directional characteristic of the core or the borehole at the commencement of a core run; providing a second indication of a directional characteristic of the core or the borehole at the end of the core run; and, determining orientation of a core sample extracted from the borehole on the basis of both the first and second directional characteristics.

Description

METHOD OF ORIENTATING A CORE SAMPLE

Field of the Invention

The present invention relates to the orientation of a geological core sample to provide an indication of the orientation of the core sample relative to the strata from which it is cut.

Background of the Invention

Geological core sampling is used to enable geological surveying of the ground for various purposes including exploration, mine development and civil construction. Analysis of the core sample provides valuable information including information relating to the composition of the strata from which the core is cut as well as the physical structure of the strata. However, unless one is able to correctly and accurately determine the orientation of the core sample relative to a known value (e.g., gravity, buoyancy and/or the azimuth and inclination of the borehole) , the value of this information is substantially diminished.

Many different types of systems have been developed to enable orientation of a core sample relative to gravity or buoyancy, and separate systems have been developed to provide the azimuth and inclination of the bore hole itself, as well as other potentially relevant structural and environmental data. Some systems operate by taking a reading, or marking a core, to determine the gravitational bottom side of the core sample at the commencement of the core run (i.e. just prior to commencement of the cutting of a core sample) , and rely on physical contact between an orientation tool or marker and a toe of the hole from which the core is cut. Such systems include, for example, the use of a scribe or a pencil for marking the core, a VAN RUTH type system that uses slidable pins to provide a profile record of the core face, or a resilient pad that takes an impression of the core face.

Other systems operate at the end of a core run; that is, once the core has been physically detached from the surrounding strata. Such systems include, for example, the BALLMARK system described in International Publication No. WO 00/75480.

Other systems rely on electronic tools to record the gravitational top or bottom side of a device located in an inner core tube prior to or just after a core run and then relate that reading back to a core.

Yet another system digitally scans the entire length of an open borehole after it has been completed and the drill pipe removed from the borehole . This digital scan is related to a known point set by the operator at the commencement of the scan and orientation of the strata is achieved. This can be related back to the core sample removed by correlating and matching the core sample with the hole depth. This method, however, is time-consuming and unreliable due to the difficulty of matching hole depth registered by the scanner to the core sample.

Systems have also been developed that will either magnetically, or by the use of accelerometers, gyroscopes, a compass or even optics, determine the azimuth and inclination of a borehole independent of the coring process. However, each of these products stop or significantly interfere with the core production process .

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any- other country.

In the claims of this application and in the description of the invention, except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as

"comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Summary of the Invention

According to the present invention, there is provided a method for orientating a geological core sample extracted from a borehole comprising: providing a first indication of a directional characteristic of the core or the borehole at the commencement of a core run; providing a second indication of a directional characteristic of the core or the borehole at the end of the core run; and, determining orientation of a core sample extracted from the borehole on the basis of both the first and second directional characteristics.

In one embodiment, one of the first and second directional characteristics may comprise (a) an indication of core orientation, or (b) an indication of borehole azimuth and/or inclination, as well as other relevant structural and environmental data of the core sample or borehole . In any particular embodiment the first and second directional characteristics need not be the same . For example both could be indications or data relating to core orientation or both could be an indication or data relating to the bore hole, or one may relate to core orientation and one to borehole azimuth and/or inclination.

The method may further comprise correlating borehole azimuth and/or inclination, as well as the other relevant structural and environmental data of the core sample or the borehole with orientation data collected to the physical core sample.

An embodiment of the method allows the provision of the correlated borehole survey data, as well as core orientation data every core run to facilitate correction and/or deviation of a borehole while it is being drilled.

The method may further comprise providing a first directional characteristic measuring, recording or marking device; providing a second directional characteristic measuring, recording or marking device; and operating each device to provide respective indications of directional characteristics of either a core sample or bore hole at substantially the same time. Thus, two separate first indications and/or two separate second indications can be obtained. A comparison may then be made between the two separate indications taken at substantially the same time to provide an audit on the accuracy of the tools or devices . The tools or devices may either be both mechanical, both electronic or one mechanical and one electronic. For example, if the directional indications provided by the separate devices are within an acceptable tolerance range (e.g. up to 5°), a high level of confidence is provided that each of the mechanical and electronic devices is providing reliable indications of directional characteristics. However, if the devices give readings that differ by more than an acceptable range, potential errors in the indications of orientation can be flagged on site to enable corrective action. This method is particularly useful to be employed, say, at the beginning of each day to ensure that mechanical and/or electronic tools or devices used in the system are providing reliable readings. For example, looking specifically at core orientation, one may initially use both a mechanical and electronic device to obtain two separate core orientation readings and then make a comparison of the readings . Assuming there is relative concordance between the readings, the mechanical device may then be decoupled and the orientation method for the rest of the day completed with either or both of the devices.

According to another aspect of the present invention, there is provided a method of orientating a geological core sample comprising: providing a mechanical gravity based orientation device and an electronic based orientation device; disposing both devices down a borehole; and, operating both devices to take separate orientation readings at substantially the same time .

According to a further aspect of the present invention, there is provided a method of orientating a core sample comprising: obtaining first and second separate indications of a directional characteristic of a core sample ^• or corresponding borehole from which the core sample is cut, the first and second separate indications either (a) captured at substantially the same time at the commencement of a core run or at the end of a core run by respective separate tools or devices or (b) captured at different times, one at the commencement of a core run and a second at the end of the core run, using either separate or the same tool or device .

In any embodiment the method comprises providing the first and/or second indications during a core run. Brief Description of the Drawings

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a standard core barrel assembly and overshot used for core drilling; and,

Figure 2 is a table representing different combinations of orientation and borehole survey devices that may be used in various embodiments of the present method.

Detailed Description of Preferred Embodiments

Figure 1 is a schematic representation of a outer core barrel 10 and inner core barrel assembly 12 for a drill string 13 together with an overshot 14, which is used for lowering and retrieving the inner core barrel assembly 12 through the drill string 13. While specific features of the outer core barrel 10, inner core barrel assembly 12 and overshot 14 vary from manufacturer to manufacturer, the general form of these components is the same. Further, the specific form of these components is not critical to embodiments of the invention.

The outer core barrel 10 comprises a outer tube 24 screwed to a downhole end of the drill string 13. A core bit 18 is attached to the downhole end of the outer tube 24. The inner core barrel assembly 12 is typically latched to the outer core barrel 10 and includes an inner core tube 20 that receives a core being cut by the core drill. A core lifter case 22 is located at a downhole end of the inner core tube 20 which is seated within the outer tube 24 of the core barrel assembly 10. An uphole end of the inner core barrel assembly 12 comprises a head assembly/back end and a spear point 26 that can be engaged by the overshot 14 to allow lowering of the core barrel assembly 12 through, and retrieving the core barrel assembly 12 from, the drill string.

Many different types of core orientation devices and borehole survey tools are already known. These include devices that mark a core face or record a core face profile. The simplest form of a core orientator is a pencil or scribe that extends through the bit 18 and physically marks the toe of the hole into which the drill is lowered. The toe forms the core face of a core sample being cut by the drill.

A well known device for recording the profile of a core face is the VAN RUTH profiler, which comprises a plurality of pins that are able to slide along their length. The VAN RUTH orientator initially extends through the core bit 18. When the drill is lowered onto the toe of the hole, the pins slide axially upon contact with the toe to provide a profile record of the core face. Typically, such a device has a weighted bearing to provide a gravitational reference point or a metal ball enclosed in a race that is clamped in a bottom side position when contact with the toe of the hole is enacted.

Another type of known device is the BALLMARK system, which incorporates a washer and free rolling ball in the core barrel assembly. Prior to a core break, the ball is able to move freely under the influence of gravity and will therefore lie at the bottom or low side of an angled hole. During a core break, the ball and washer are pressed together so that the ball indents the washer. The washer is keyed to the core barrel. Thus, the indentation made by the ball on the washer can be transposed to the core to provide an indication of the orientation of the core. The above device may be broadly referred to as a mechanical device as they rely upon moving mechanical part or mechanical contact to record or log directional or orientation characteristics.

Electronic core orientation devices are also known that do not rely upon the weight of the drill string on the toe of the hole nor the force required during a core break to provide indications of core orientation. Typically in these systems, electronic devices provide a constant read out or record of the gravitational bottom side of the device . A drill rig operator then logs the time the core run is finished and the reading taken at that time is then transposed to the core inside the inner core tube once it has been removed from the borehole by the drill rig operator .

A variety of borehole survey tools are also known that can record the azimuth, inclination and other structural and environmental data, either in the open borehole a sufficient distance from the influence of interference that can be created by the magnetic nature of the drill string or, for example, within the drill string while it is suspended in the completed borehole by the lowering and raising on a wireline of an optically based system known as a Maxibore, which is not affected by either the magnetics of the surrounding rock strata or the metal of the drill string.

Irrespective of which core orientation or borehole survey system is used, ultimately a Geologist or other suitably trained and skilled personnel will use the information provided by each system to physically mark a core sample with a line indicative of the orientation of the core sample, and/or graphically or mathematically represent the borehole path that the sample was obtained from and correlate all other structural and environmental data collected to these known physical values.

Some embodiments of the present invention rely not on taking a single reading of a directional characteristic of a core or corresponding borehole at either the beginning or end of a core run, but rather taking two or more separate readings of such directional characteristic, at least one at the commencement of the core run and at least one at the end of the core run. As a consequence, a core sample is able to be marked at opposite ends with a mark indicative of the lowest point of the core, or otherwise have its orientation characterised by data logged at the beginning and end of a core run.

Throughout this specification the expression "directional characteristic of a core or borehole" is used to denote either: (a) an indication of core orientation or corresponding core orientation data; or (b) an indication of the azimuth and/or inclination of a borehole from which the core is cut.

When the core is physically marked, respective lines can then be drawn on the core sample from those points parallel to a central axis of the core sample. If the lines overlie each other or are closely adjacent, a Geologist or other suitably trained and skilled personnel is able to assume that the orientation provided by the overlaid or closely adjacent lines is correct. However, in the event that the lines are spaced by more than the predetermined minimum distance or angle, a Geologist or other suitably trained and skilled personnel is able to conclude that one of the readings of core orientation taken is incorrect.

To determine whether the incorrect reading is that taken at the commencement of the core run or at the end of the core run, the Geologist or other suitably trained and skilled personnel can refer to adjacent core samples. In this regard, the orientation of adjacent core samples should be continuous; that is, the orientation indicated at the commencement of a core run should be continuous with the orientation indicated at the end of the previous core run. If no error is detected in the orientation of the previous core sample, and no error is detected in the orientation of a subsequent core sample, then it is expected that there will be either a misalignment between the orientation indication at the end of the core run of a previous core sample with the indication of orientation at the commencement at the present core sample or a misalignment with the orientation reading at the end of the core run of the present core sample and the indication of orientation at the commencement of a core run of the next core sample. This mismatch is likely to be clearly visible or discernable, allowing the Geologist or other suitably trained and skilled personnel to disregard that particular orientation indication.

In order to provide directional characteristics of a core or borehole at both the commencement and end of a core run

■ either at least two devices for obtaining such directional characteristics are required or a single device can be used, but operated both at the beginning and end of the core run. This may comprise one or more core orientation devices, one or more borehole survey tools, or a combination thereof .

Borehole survey tools provide information relative to the azimuth, inclination and/or other structural and environmental data of a borehole that may be required during the core production process and allow the correlation of such data to the physical core sample and the core orientation data at the completion of each core run. Various borehole survey tools could be deployed downhole either at the commencement of the drilling process or at its completion when the core sample contained in the inner core tube is retrieved via the overshot and the wireline, and then, if necessary, deviating the borehole path to ensure that the maximum value of such data is achieved at all times .

The table in Figure 2 provides a non-exhaustive matrix of combinations of orientation devices and borehole survey tools (referred to collectively as "tool" or "tools") that may be used to provide the directional characteristics of the core or borehole at the beginning and end of each core run. The table also indicates the possible location of the tools .

In the table, the location "in front of bit" indicates that a tool is initially disposed downhole of the drill bit 18. During use, this tool will, however, be pushed into the core barrel assembly 12.

The reference to "lifter" in the table indicates that a tool is located adjacent the core lifter case 22.

The reference to "tube back end/head assembly" means that the tool could be included at an uphole end of the inner core tube 20, or in an adaptor between the inner core tube 20 and the spear head assembly 26, or in a custom-made head assembly.

The location "overshot" in the table is reference to a tool being located in or adjacent to the overshot 14. It would be appreciated that the overshot is tripped at the beginning and end of every core run for the purposes of lowering an empty inner core barrel assembly 12 into the drill string 13 at the commencement of the core run, or alternatively withdrawing a core barrel assembly 12 with a cut core at the end of the core run . Thus , any tool disposed in or carried with the overshot may be operated at any one or both of the beginning or end of the core run.

The table also makes reference to particular types of tools which are explained below.

Profile record: this refers to a tool that marks a core face and/or records a core face . Examples of such devices include the above-mentioned VAN RUTH type system, clay or plasticine pads, scribe or pencil for physically marking the core face. In use the profile record would be aligned, keyed or synchronized with a further tool or device which records or measures orientation and/or azimuth indications or data.

Profile with Ori : this refers to a device that records a core face profile correlated to an orientation reading. Such a tool is exemplified by Applicant's EZY-MARK (TM orientation system described in International Publication

No. WO 2005/078232) .

Profile with Ori+Az : this refers to a device that records core face profile correlated to an orientation reading together with the taking of a borehole azimuth reading. Such a device, for example, could constitute the above- mentioned EZY-MARK orientation system coupled with an azimuth measurement and reading device.

Ori reading: this refers to an orientation tool that takes a reading of core orientation such as for example the BALLMARK system mentioned above.

Ori reading+Az : this refers to an orientation tool which includes an azimuth module for taking a reading of the azimuth of the hole. Az : this refers to an azimuth tool per se to provide readings of the azimuth of a borehole. The azimuth tool may be magnetic, gravitational, optical or inertial (gyro) .

The specific location of a tool is not necessarily indicative of the timing of providing corresponding orientation data. Thus, having a tool "in front of the bit" does not necessarily mean that the tool is operated only at the commencement of a core run. For example, placement of the tool "profile with Ori+Az" at the location "in front of the bit" can provide both the first and second indications of core orientation. In particular, the "profile with Ori" component can provide profile and orientation data at the commencement of the core run, constituting the first indication of orientation of the core. The azimuth component of the tool may also be operated at the commencement of the core run so as to tie the azimuth reading at the commencement of the core run with the recorded profile and orientation data, but significantly the tool can also be operated at the end of the core run to provide an azimuth reading of the borehole, thus providing the second indication of core orientation at the end of the core run.

In addition, it is to be appreciated that the tools are coupled with the core barrel assembly 12 or the overshot 14 and thus tripped through the drill string at the beginning and end of every core run. There is no requirement to trip the drill string per se, which of course is a much more difficult and time-consuming operation. It will be also noted in the enclosed table that in some instances only one tool is required to provide both the first indication of core or borehole orientation at the commencement of the core run and the second indication of core or borehole orientation at the end of the core run. For example, combination C in the table shows the existence of an azimuth tool only at the location "in front of bit". The azimuth tool, however, may be operated to provide azimuth information of the borehole at both the commencement and end of the core run. Similarly, combination J indicates a tool providing profile with orientation data located at the lifter. This device may provide a profile of the core at the commencement of the core run together with orientation of the core at the commencement of the core run and orientation at the end of the core run.

In the above-described embodiment, separate directional characteristics are taken at the commencement and end of a core run. However, in a further variation multiple directional characteristics relating to the core and/or borehole may be taken at substantially the same time at the commencement or end of a core run, particularly for the purposes of auditing the tools used for obtaining core orientation data. As mentioned above, the tools used to obtain core and/or hole orientation data may be mechanical, electronic or optical devices. Electronic orientation devices are becoming popular in some sectors of the market. Many of these devices rely on a drill rig operator operating a timing device so that the reading from the electronic device can be taken at the time corresponding to either the commencement or end of the core run. However, when the electronic device is used by itself, there is an assumption that the device is providing accurate orientation information. While errors in orientation data obtained from such electronic devices can be detected, this can only be done by comparison with other data collected by Geologist or other suitably trained and skilled personnel which is analysed off-site, days or weeks after the drilling has been completed. The present embodiment proposes a system to enable auditing, particularly of such electronic devices, on site so that errors in readings can be detected within one to two core runs after the core run that may have provided a faulty- reading .

To this end, the present embodiment proposes the use of both a mechanical gravity-based orientation tools such as the BALLMARK system or the EZY-MARK system, together with an electronic orientation device such as the ACE system manufactured by Australian Mud Company Pty Ltd. For example, with reference to Figures 1 and 2, both the mechanical and electronic orientation tools may be provided adjacent the lifter 22 or the head assembly 26. The mechanical tool will be operated by a physical action of the core drill, e.g. lowering of the core drill on to the toe of the hole, or during a core break. The electronic tool typically will provide readings for the entire time it is in a bore hole and a timer is operated to flag or mark the reading at a point in time corresponding to time of operation of the mechanical device.

It will be appreciated that once a core run has been completed, both the mechanical and electronic devices are returned to the surface with the core barrel assembly 12. A Geologist or other suitably trained and skilled personnel is then able to compare the orientation data from the mechanical tool with that of the electronic tool. Provided the readings are within a desired tolerance range, for example 5°, it is assumed that both tools are providing reliable orientation information. However, if the readings from the two tools differ by more than the desired tolerance range, then it is assumed that one or both is not providing an accurate orientation reading. Further tests may then be conducted to ascertain which device is providing faulty readings. However, if the mechanical device is one that itself provides self error checking such as the EZI-MARK system (which includes a plurality of balls in separate races) , identification of the tool in error may be simplified.

If it is determined that both devices are operating reliably, future core runs during the day may be conducted with only the mechanical tool or only the electronic tool. It will be appreciated that because the comparison between readings of the two devices is conducted on site, errors in equipment can be detected within one to two core runs . This will enable errors in the direction of the hole being drilled to be rectified on site during the drilling process ensuring that the cores extracted relate to the strata that is desired to be examined. In a minor variation, auditing of the mechanical and electronic devices can be performed at the surface (rather than downhole) by use of an appropriate jig or adaptor that enables firing or operation of the mechanical device. This also ensures the accuracy in the correlation of the orientation of a core sample with the azimuth position and inclination of the bore hole.

The above auditing feature may of course be extended to apply to any two separate tools or devices that may be use to measure, record or mark directional characteristics of a core or bore hole. For example in relation to core orientation two mechanical devices can be audited against each other, or two electronic devices. Indeed the devices may also be of the same type, e.g. two EZY MARK core orientation devices. Similarly in relation to bore hole directional characteristics, one may use two mechanical devices, two electronic devices or one of each to perform the auditing feature.

It would be appreciated and understood by those in the art that embodiments of this method can provide the orientation of a core sample, as well as the azimuth position, and inclination of the same core and/or borehole sample in the same process without significantly- interfering with the core production process. It further facilitates the in-situ auditing or correlation of any or all of these independent pieces of information during the core production process.

Claims

CLAIMS :

1. A method for orientating a geological core sample extracted from a borehole comprising: providing a first indication of a directional characteristic of the core or the borehole at the commencement of a core run; providing a second indication of a directional characteristic of the core or the borehole at the end of the core run; and, determining orientation of a core sample extracted from the borehole on the basis of both the first and second directional characteristics.

2. The method according to claim 1 wherein the first directional characteristic comprises (a) an indication of core orientation, or (b) an indication of borehole azimuth and/or inclination.

3. The method according to claim 1 or 2 wherein the second directional characteristic comprises (a) an indication of core orientation, or (b) an indication of borehole azimuth and/or inclination

4. The method according to any one of claims 1 - 3 further comprising providing a first directional characteristic measuring, recording or marking device; providing a second directional characteristic measuring, recording or marking device; and operating each device to provide respective indications of directional characteristics of either a core sample or bore hole at substantially the same time.

5. The method according to any one of claims 1 - 4 further comprising making a comparison between the first and second indications to provide an audit on the accuracy of the tools or devices .

6. The method according to claim 5 further comprising assigning a high confidence level in the first and second indications when a difference between the first and second indications is less than or equal to predetermined difference and assigning a low confidence level in the first and second indications when the difference is more than the predetermined difference.

7. The method according to any one of claims 4 - 6 wherein the first device is a mechanical device or an electronic device and the second device is a separate mechanical or electronic device.

8. A method of orientating a geological core sample comprising: providing a mechanical gravity based orientation device and an electronic based orientation device; disposing both devices down a borehole; and, operating both devices to take separate orientation readings at substantially the same time.

9. A method of orientating a core sample comprising: obtaining first and second separate indications of a directional characteristic of a core sample or corresponding borehole from which the core sample is cut, the first and second separate indications either (a) captured at substantially the same time at the commencement of a core run or at the end of a core run by respective separate tools or devices or (b) captured at different times, one at the commencement of a core run and a second at the end of the core run, using either separate or the same tool or device .