GB2550400A - Method of correcting mineral ore density logs - Google Patents

Abstract

A method of producing a density log which corrects for the effects of non-fixed borehole tubing includes carrying out long 24 and short 23 spaced density logs, using gamma detectors and a gamma ray source, along a length of borehole. The logs 23,34 are first corrected for the nominal dimensions and properties of the tubing are then combined into a log 34 which compensates for variations in the log from one discrete length of tubing to another or variations within a single length of tubing. The tubing may be PVC or glass fibre pipe, and may be drill pipe, drill rods or reverse circulation rods.

Description

METHOD OF CORRECTING MINERAL ORE DENSITY LOGS

The invention reiates to a method of correcting one or more density iogs of mineral ore bodies; and to apparatuses for carrying out such a method. in the technical field of mineral production there are numerous important, technical reasons tsr idontifyfng the nature of mineral ore bodies in or adjacent to a formation.

It is generally considered desirable to: acquire a good quality density log of a borehole in the vicinity of mineral ore bodies,

Before completion of a borehole it is possible to obtain accurate density logs in open-hole, This is so even when there Is mudcake in the borehole or the logging tool is “stood off from the wail of the borehole. Udder these circumstances It is possible to compensate the density log for example using bne of more of the techniques disclosed in ' The Dual-Spaced Density Log Characteristics, Calibration and Compensation” * Samworih, The Log Analyst, February 1992.

When prospecting for minerals by: drilling boreholes It is known to use liners made e.g. of polymeric or fibreglass materials to line the resulting bores. One purpose of such liners, which are sometimes tarred to as "tubing", Is to provide a constant diameter along the length of the borehole; and another is to avoid problems such as caving-in of sections of the drilled cavity. Alternatively it is possible to leave the drill pipe used during forming of the borehoie temporarily ip place in the borehole for this purpose, The nature and characteristics of tubing and driii pipe will be familiar to the person of skiii in the art. in boreholes drilled for the purpose of extracting fluids such as oil, gas or water from under the ground or a sea; bed, completion of the borehoie involves the insertion of casing:, which is a series of hollow imetal tubes that are Joined end to end in the borehole and fixed in place using cement interposed between the exterior of the tubes and the interior of the borehole.

Such casing of a borehole presents; particular problems when if is desired to log the formation using an energy-emitting sonde and one or more receivers of returned energy that has travelled through the rock of the ferrnation. Some techniques however, such as that disclosed in US 7,328,108, have proved highly successful in compensating for the effects of casing.

Ars :;istisi'pesst -of of US 7,328,108 relies on ihe fast that the casing is ted in position: by the cement.

When fining boreholes in mineral ore bodies, however, engineers generally do not fix the polymeric: / fibreglass liners in place, with the result that they tend to “fioaf (i,e, move In up^ndHiown and/or side-to-side directions) in the borehole, if the drill-pipe Is being used for this purpose, it can move in a similarway.

Similar problems arise during creation of the boreholes. Thus togging difficulties arise when considering; drill pipe and/or drill rods, that are also examples of non-fixed tubing that may be present in a borehole during e,g, borehole drilling operafiohS, It may be required to produce density fogs through drill pipe or drill rods. The method of the invention is useable regard lass of the non-fixed tubing type

Publication no GB2516855 & describes; a method of compensating a density log for the effects of non-fixed tubing In the region between the tubing; and the geological formation.

Unexpectedly the inventors have established that contrary to the expectation in the art if is desirable to compensate for effects of non-fixed tubing (a) from one adjacent tubing length to another along the length of the borehole and/or (b) from one part of a given length to another along the length in question. Such effects are non-trivia! and can be significant.

To this end according to a first aspect of the invention there is provided a method of producing a corrected density log, in a borehole in a geological formation extending; through or adjacent one or more mineral ore bodies, for ihe effects of non-fixed tubing in the borehole, the method comprising correcting and combining a plurality of density logs obtained using a gamma ray source inside the tubing and relating to a length of weii including the non-fixed tubing therein,: the method including the steps of: (a) correcting each of the said plurality of density logs for the nomine! dimensions and properties of the tubing, the said density fogs resulting from; use of a plurality of density detectors corresponding in number to the number of density togs and Ihe Correcting utilising gamma logs: (bj combining the thus-corrected density togs to compensate for variations in the log from one adjacent length of the said non-fixed tubing to another along the borehole; and (c) yielding a resultant output.

This aspect of the invention compensates for fog variations that are discernible when comparing the sections of a log corresponding to distinct, adjacent sections of drill pipe or drill rod.

According to a second aspect of fie invention there is provided a method of producing a corrected density fog, in a borehole in a geological formation extending through or adjacent one or more mineral ore bodies, for the effects of non-fixed tubing in the borehole, the method comprising correcting and combining a plurality of density logs obtained using a gamma ray source inside the tubing and relating to a length of well including the non-fixed tubing therein, the method including the steps of; (a1) correcting each of the said plurality of density iogs for the nominal dimensions and properties of the tubing, the said density iogs resuSing from use of a plurality of density detectors corresponding in number to the number of density logs and the correcting utilising gamma iogs; (fo1} combining the thus-corrected density iogs to compensate for variations in the log along a length of the said non-fixed tubing; and (d) yielding a rasuitanf output*

This aspect of the invention compensates for variations that are discernible within a section of a log corresponding to a single length of drill pipe or drill rod.

Thus the method offers an improved technique for the non-fixed tubing measurement of the density of formations, specifically in mineral ore bodies.

As used herein the terms :;dni! pipe" and ‘‘drill rod1' are essentially interchangeable. The inventfon Is applleebie to and useable in relation to a ii types of non-fixed tubing that may befoneoonterab in downhole and borehole situations. This inciudes but is not limited to drill pipe:,; drill rod and: so-called “rm&mB circulation rads”, which Iasi-mentioned are special: types of dni! rod the Individual lengths of which include two concentric tubes defining an annulus that surrounds an elongate cylindrical chamber. Such tubing types: are used for fluid circulation In boreholes, with fluid typically being pumped In a downhole direction in the annulus and: In an uphoie direction via the central cylindrical chamber, in one preferred embodiment: ol the method of the Invention the mineral ore body is iron ore. The Invention however is appiscahis In other types of mineral ore body as welt:

Frefsrabiy the gamma ray source is Caesium-137. in other embodiments of the invention it may be Cobait-80. in a particularly preferred embodiment of the invention the tubing is or includes a polymeric pipe, espeeiaiiy PVG- pipe. The tubing may also be made p other materials such as fibreglass. Alternatively the tubing may be or include driii pipe and/or one or more driii rods and/or one or more reverse circulation rods as mentioned. The driii pipe, driii rod or reverse circulation rod if present is preferably made of a metal such as steel or aluminium.

Preferably step (a) includes one or more of the steps of: (d) modeiiing the effect of the tubing using a modelling database; or (a) calibrating the logs using a tubing calibration database.

These techniques are advantageously reliable.

Alternatively, the step (a) may optionally include correcting the logs for effects of the: tubing using an iterative downhole calibration technique that is database-independent.

The Iterative calibration technique may offer advantages in terms of computer processing power and response times.

Conveniently step (b) includes the step of: (f) approximating the integrated geometric factor ((3} of the borehole / density detector combination to an exponential function of the density log penetration depth.

There is a detailed description of this technique in the paper by Samworth mentioned hereinabove. The entirety of this paperis Incorporated herein by reference.

Preferably step (b) further Includes (g) further approximating the exponential function to linear form.

There is a description of this technique in the aforementioned paper by Samworth. instead of the steps (f) and (g) specified herein, step tb} of the method of the invention may alternatively include the step (h) of. approximating the integrated geometric factor (G) of the density measurement to a series ef straight iines.

The respective method steps (f) and (g) or (h) lend themselves to computation by different computational methods, ft is possible for the logging engineer within the scope of the invention to use the method that is most appropriate to the prevailing circumstances. in a preferred embodiment, tine method is carried out: Using a single tool. Such a tool may contain ail of the logging devices necessary to carry out the essential and preferred steps defined herein.

By hjorapacf is meant a tool whose outside diameter is iess than about iymm |j e. 214 inches). Such a tool is capable of more easily accessing: narrow and otherwise difficult boreholes, than a tool of conventional diameter (i.e. about 89mm or 314 inches or greater).

The invention is also considered to reside in data: acgaired by the method steps defined herein.

According to a further aspect of the invention: there is provided a borehole fogging tool and data processing apparatus combination comprising a density sonde secured in the tool, the density sonde including a caliper for urging the density sonde into contact with Tie inferior surface of a casing string, the density sonde being operatively conneciabie to one or more programmable devices that are programmed to carry out at least steps (a) --(b) of Claim 1, or at least steps (al) - (bl) of Claim 2 hereof.

Such a logging foci is of course advantageously suited to carrying out the method of the invention as defined herein.

Advantageous, optional features of the invention are defined in Claims 15 to 19 hereof,

There now follows a description of preferred embodiments Of the invention, by way of nop-limmng example, with reference being made to the accompanying drawings in which:

Figure 1 is a schematic view of a wireline tool, according to an aspect, of the invention, that Is Mpable ef carrying cut the method of the invention;

Figure 2 is a plot of the integrated geometric factor G, characteristic of fractional; contribution to the density measurement, against penetration depth in: a formation,: that illustrates some principles underlying the invention;

Figure 3 is a spine and ribs piot derivable through use of the method of the inmnion; and

Figure 4 Is an exemplary* non-limiting density log showing improvements brought about through practising of the invention. A method according to the invented involves the use of a logging tool 10 as shown In Figure 1, that may fee deployed in a borehole and subsequently used to log the borehole, A typical logging; operation involves lowering, pumping or otherwise conveying the too! to the total depth: of the borehole using one or more of the conveyance techniques described herein and/or as would be familiar to the person of skill in the art; and logging the borehole during withdrawal of the tool to the surface.

An electronics section of ifie tool may include one or more driver circuits capable of effecting telemetry of the logged data via a conventional: armoured wireline hy means of which the tool Is connected to a surface location.

As is well known in density logging, tools such as that shown in Figure: 1 produce so-called “shoht spacetf and "long spaced" logs using respective receivers (la, energy detectors) that are spaced relatively close to, and relatively far from, a gamma energy source.

Regardless of the precise method of conveying data to the surface location, the method of the invention involves ire Mowing actions; 1. Correcting each density log (i.e. the short and long spaced density logs) lor the presence of the known nominal dimensions and properties of the tubing.

As noted herein this may be achieved through per se known modelling and/or calibration database techniques, or by iterative methods. 2. Ofemfeinihg the tubing-corrected logs in such a way as to compensate for the effects of variations in the tagged values: apparent between adjacent lengths of the tubing and/or apparent from place to place along a given length of tie tubing, such as but not limited' to variation In? concentricity of the toner member of reverse circulation rods.

This-is achieved by firstly approximating the integrated geometric factor (G) of the borsheMdnsity detector combination to an exponential function of density measurement penetration depth, as illustrated by figure 2 which shows such an epproxirnation in a plot of G against penetration distance measured radially from the gamma source.

(1) where

Ihiow using geometric-factor theory and assuming that the too! stands off the borehole wall, the apparent measured density is given by;;

(2) where

Since the analysis considers only a: two-part situation, by definition of geometric factors:

13)

Combining Equations 2 and 3 gives:

(4)

Using the relationship in Equation 1 for G gives:

(5)

It;; Is possible to estimate -g , put since ris unknown and variable, it is prefetaoiP to rearrange Equation 5 to eliminate it;

(6}

This is true for both detectors, and if there exist pamliet standoff phditidns ih® f !s are the same, thus:

(7) where the suffices L arid ^ refer to the long-· and short- spaced detectors..

When rearranged, this yields

Note that the k’s only appear as the ratio k$ ikL, This means that only the ratio of the penetration depths is involved in Equation 8 (this can be derived from Figure 3). To a first approximation then, the compensation remains vaiid even if the penetrations change, as long as:thei ratio stays constant;

Plotting Equation 8 gives a borehoie-known “spine and ribs” plot as shown In Figure 3 Although there is a need to estimate ρ.(ίί. it: is apparent that for corrections up to 0,2-0.25 g/ce, the locus of the correctionvery similar, even if pS!, varies madsediy. The ribs rejoin the spine when p {p^.,

As a further refinement It is possible further to approximate the equation of “exponential-G" to linear forth, as a further simplification.

The considerations of the standoff used only a two-part geometric·factor equation. Therefore:, the form of G matters little for penetrations deeper than the Standoff, since this appears solely as (1 - G), Therefore, it is possible to consider a simpler form of G that should be reasonable for modest corrections (i.e„ a linear form as in Figure 3). For smalt penetrations and, therefore, small standoffs:

# where k:~ constant, in this caSe);; as in Equations 4 and 5

(10)

Rearranging as before,

(11)

Eliminating r by using both detectors,

(12)

Rearranging and; simplifying gives:

(13)

Note here that: has canceiied out.

The spine^od-cibs piot for this linear G model a iso appears in Figure 3. Again, the compensation locus varies little tram the previous ones for modest corrections. Thus, the compensation is not a strong function of the form of G.

Referring now to Figure 1 there is shown a wireline tool 10 that is, in conjunction with data processing apparatus to which it is oQnnecfohie, capable of carrying oof the method steps herein.

Toot 10 can be configured in two ways for use in air-filled or liquid-filled boreholes, in the aiMilled borehole configuration a natural gamma detector is at the top of the tool, as exemplified by numeral 11A, so as to be remote from and not be influenced by the radioactive source at the bottom of the looi. In the fluid-filled borehole configuration the natural gamma detector Is further down the tool e.g. at point 11B so as to minimise the length of unlogged hole at the bottom of the hole.

The gamma detector in each case therefore in effect is secured in series to a density sonde 13 including a per se known cahper mechanism (riot shown) urging the sonde 13 into contact with the casing of the borehole; and a radiation source 12 that as is known to the person of skill in the art provides energy for the creation of iog data.

Tool 10 includes per se known short and Song: spaced detectors.

Tool 10 may include a per se known cartridge (not shown in Figure 1) containing an electronics section whose functions might include signal conditioning and amplification. However the primary means of obtaining useabio data from the tool of Figure 1 is by way of a per se known armoured: wireline (not shown in Figure 1), on an end of which the tool is driveable into a cased borehoie. The wireline transmits electrical power to the tool 10 and permits dele telemetry.

Thus the tooi 10 includes electronics whose function concerns the telemetry of logging data via the wireline to e.g. a surface location. At the surface location the wireline may connect fo brie of more programmed devices (such as a digital computer) that are capable of carrying out the method steps of the invention other than those carried out by the sondes.

The tooi 10 preferably has a maximum diameter In the so-called "compact” or “slim-hole” range, ie, less than about 57mm (2¾ inches). However other, greater topi: Component diameters are possible within: the scope of the iovenioh., in use the: tool110 ;s passed as aforesaid into the; borehole,· This may be by the action of gravity or ag. by pumping the tool using fluid in the borehole. The too! is then withdrawn along the borehole towards a surface location and it logs the borehole as it travels. Withdrawal of the tool 10 may,, depending on the too! type, occur by pumping, by winding In wireline on which the tooi is supported, or by using other techniques,.

It should pe understood1 that, although many boreholes extend downwardly into rock formations this used not be the case. Inclined and horizontal boreholes are commonplace, as are boreholes the directions of which are non-constant. The methods and: apparatuses of the: ihVehtidh are beneficially useable in a wide variety of classes of borehole ineluding but not limited to those described herein,,

The advantageous effects of the invention: are apparent in Figure 4, which is an exemplary density log produced from the output of a density logging tooi that also includes or at least is operatively associated with a detector of natural Gamma radiation. The logging tod! typidaiiy would he operated in accordance with one of the depldphent and logging methods described herein, or in accordance with other methods as would be known to the person of skill in the art.

As is weii known, a density fogging tool operates primarily by emitting gamma raysinto the formation surrounding a borehole and detecting the radiation returned to it that has passed through the formation. This process induces electrical voltages in detectors forming part of the logging tooi (which may be a single, integrated device, or may be a series of devices that are joined one to another or are supported at intervals on supporting structure). The voltages may be telemetered as signals to a surface location using one or more signal transmission methods known to persons of skill Id: the logging art.

Such signals; may be plotted as exemplified in Figure 4 to create a graphieai log the main pans of which are described below. For the avoidance of doubt however as referred to herein a fog may also be a table containing agt numerical values corresponding; to1 the detector signals; computer files or databases· or any of a range of other data format types. The invention is applicable to ail types of density log, regardless of the method of storage of the date Wore# and regardless of whether, and If so in what form, the log is graphically presented.

The log visible in Figure 4 represents a length of borehole, as is signified by the depth Information printed between the left hand 20 and middle 21 tr acks of the log.

The left hand track 20 shows In graphical form plotted against borehole depth the output of the Gamma detector that detects natural Gamma radiation in the formation surrounding the borehole. The nature of the: Gamma trace is known to the person of skill: in the art and therefore is not discussed In detail herein.

The middie track 21 shows a density log recorded in a borehole haying non-flxed tubing therein, in a form not modified or improved by the method of the Invention.

The middle track 21 consists of two traces, being the near detector plot 23 (dotted iine) and the far detector plot 24 (dashed iine| representing density as determined by the near detector and the far detector respectively of the logging tool

The near and far detector plot lines show several artefacts that the inventors have discovered are the result of variability of the tubing, which as noted may be drill pipe, drill rod, reverse circulation rod or a range of other non-fixed tubing types.

Near the top of the middle track 21, which represents the more uphoie end of the logged length of borehole, two signal spikes 26a, 26b are apparent in the near detector plot. These spikes represent cdllato ih: the non-flxed tubing that significantly perturb the log in the vicinity of the ooiars.

Similar spikes 27a, 27h are visible in the far detector log plot 24.

As is well known, the collars represent the; points at which adjacent lengths of the non-fixed tubing are joined together. Hence they are spaced at equal intervals along the middie log track 24, as signified generally by numerais 28, 29 and 31. At each of the points 28, 29 and 31 each of the near and far detector plots exhibits a spike that is similar to the spikes 26a, 26b, 27a, 27b; but for ease of viewing the spikes have not been individually labelled.

As is apparent from comparison of the values of the near and far density logs represented: by the plots 23 and 24 at various points along the middle track 21, the recorded density value changes each time the plots pass through the location of a collar. This implies that the discrete adjacent lengths of fubihf: differ from one another in terms of their attributes, in a way that is severely adverse to the quality of the overall log.

The most stark difference in this regard is between the region of the log labelled 32, corresponding to one particular length of non-flxed tubing; and that of the adjacent non-fixed tubing length labelled 33. Λ dramatic difference between the average near and far density values is apparent that could notbeaceounted for by geological factors.

The right hand track 22 of Figure 4 shows the beneficial effects of the method and apparatus of the invention.

Track 22 plots at 34 (solid line) a compensated density value obtained using the method of the invention, The anomalies apparent in middle track 21 from comparing adjacent negated tubing lengths are entirely eliminated from plot 34 ofthe right hand track 22.

The fight hand track 22 also includes a dotted line 36, which is a plot of the borehole logged In openhole and then subjected to a compensation technique of the general kind described in the paper by Sam worth above. Plot 36 may be used as a comparison reference since ft relates to the situation of no tubing being present in the borehole. As a result no tubing effects arise, and plot 36 is a good representation of the true density of the formation surrounding the botehole.

Riot 34, being compensated: for tubing effects in accordance with the method of the Invention closely resembles plot 36, demonstrating that the inventive method and apparatus provide significant benefits compared with prior art techniques. in addition tubing-derived log variations within the length of a single length of tubing may also be compensated using the method and apparatus of the invention. To this end thefefbre the method of the invention additionally is applicable to compensate for variations such as the variability in concentricity of the reverse-circulation rods, In the iog along a length of non-Uxed febing, as mentioned herein»

The listing or discussion of an apparently prior-published document In this specification should not necessarily be taken as an acknowledgement that the document is part: of the state of the art or is common general knowiedge.

Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, he regarded as having been disclosed in combination with any and all preferences and options for ail other aspects, features and parameters of the invention.

Claims (20)

1. A method of producing a corrected density log, in a borehole in a geological formation extending through or adjacent one or more mineral ore bodies, for the effects of non-fixed tubing in the borehole, the method comprising correcting and combining a plurality of density tags obtained using a gamma ray source inside the tubing and relating to a length oi well iltciudirig the nan-fixed tubing therein, the method including the steps of: (a) correcting each of the said plurality of density togs for the norninai dimensions and properties of the tubing, the said density logs resulting from use of a plurality of density detectors corresponding in number to the number of density logs and the correcting utilising: gamma: logs; (b) combinihg the thus-corrected density lags to compensate for variations in the log from one adjacent length: of the said non-fixed tubing to another along the borehole:;: end (c) yielding a resultant output;

2. A method of producing a corrected density tag, in a borehole in a geological tarmattah extending through or adjacent one or more mineral ore bodies, for the effects of ποη-fixed tubing in the borehole,: the method comprising correcting and combinihg a plurality of density logs obtained using a pea ray source inside the tubing and relating to e length of weii including the nonffixed tubing: taerelrythe method Inducing the steps of: (a1) correcting each of the said plurality of density logs for the nominal dimensions and properties of the tubing, the said density logs resulting from use of a plurality of density detectors corresponding in number to the number of density logs and the correcting: utilising gamma idgs; (b1) containing the fbus-eofreeled density logs to compensate for variations in the tag along a length cf the said namlixed tubing; and f c1} yielding a resultant output.

3. A method according to Claim 1 or Claim 2 wherein the mineral ore body is iron ore.

4. A method according to any preceding claim wherein the gamma ray source is :0aesi:um~13?y

5. A method according to any of Claims i to 3 wherein the gamma ray source is Cobait-60,

6. A method according to any preceding claim wherein the tubing is or Includes PVC or glass fibre pipe.

7. A method according to any of Claims 1 to 5 wherein the tubing is or includes driii pipe and/er one or more drill rods and/or one or more reverse circulation rods.

8. A method according to any preceding eialm wherein the step (a) inciudes one or mom of the steps on (d) modeling the effect of the casing using a modeling database; or (e) calibrating the logs using a casing calibration database.

9. A method according to any of Ciaims 1 to 7 wherein the step (a) includes correcting the logs for effects of tubing using an iterative downhole calibration technique that is database-independent.

10. A method according to any preceding claim wherein the step (b) inciudes the steps of: (f) approximating: the integrated geometric factor (G) of the borehole / density detector combination: to an exponential function of density log penetration depth.

11. A method: according to Claim 10 including the further step of; (g) further approximating the exponential function to linear form.

12. A method according to any of Claims 1 to 8 wherein the step (b) includes the step of: (h) approximating the Integrated geometric factor (13) of the density measurement to a series of straight lines.

13. A method according to any preceding: claim when carried out using a single tool.

14. Data acquired by the method of any of Ciaims 1 to 13.

15. A borehole logging toe! and data processing apparatus combination comprising density sonde, the density sonde including a caliper for urging the density sonde into contact with the interior surface of a casing siring, the density sonde being operatively connectable to one or more programmable devices that are programmed to carry out at least steps (a) - (b) of Claim 1 or at least steps (a1) - (b1) of Claim 2. 1®> A borehole logging tool and data processing apparatus combination according to Claim 15 wherein one or more of the pmgmmrnable devices is programmed to carry out one or more of trie Mowing: steps of Claims 1, 2. 3, 4, 5, 6, 7. 8, 9. 10,11, 12 or 13; (c), (d), (e), (f), |g|: |l)>

17, A borehole logging tool and data processing apparatus combination according to Claim 15 or Claim 16 including secured In the too! a Gamma detector for detecting natural Gamma radiation,

18., A borehoie logging tool and data processing apparatus combination according to any of Claims 15 to 17 having secured to the tool an armoured wireline on which: the logging tool is supportable within a borehole tubing.

19. A borehole logging tool and data processing apparatus Combination according to Claim 18 wherein one or more of the programmable devices is remote from the logging too! arid is operatively connected thereto by means of the wireline.

20. A method generally as herein described, with reference to and/or as illustrated in the accompanying drawings,

21. A logging too! and data processing apparatus combination generally as herein described, with reference to and/or as Illustrated in the accompanying drawings.