GB2099138A - Determining the bulk strength of strata - Google Patents
Determining the bulk strength of strata Download PDFInfo
- Publication number
- GB2099138A GB2099138A GB8114912A GB8114912A GB2099138A GB 2099138 A GB2099138 A GB 2099138A GB 8114912 A GB8114912 A GB 8114912A GB 8114912 A GB8114912 A GB 8114912A GB 2099138 A GB2099138 A GB 2099138A
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- GB
- United Kingdom
- Prior art keywords
- neutron
- log
- strata
- borehole
- lithology
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Links
- 239000011435 rock Substances 0.000 claims abstract description 32
- 238000000034 method Methods 0.000 claims description 29
- 238000001914 filtration Methods 0.000 claims description 10
- 230000004044 response Effects 0.000 claims description 10
- 238000009499 grossing Methods 0.000 claims description 7
- 230000008569 process Effects 0.000 claims description 6
- 238000004364 calculation method Methods 0.000 claims description 5
- 238000005553 drilling Methods 0.000 claims description 5
- 238000007620 mathematical function Methods 0.000 claims description 4
- 238000011179 visual inspection Methods 0.000 claims description 3
- 239000003245 coal Substances 0.000 description 11
- 239000000463 material Substances 0.000 description 7
- 238000000605 extraction Methods 0.000 description 4
- 239000001257 hydrogen Substances 0.000 description 4
- 229910052739 hydrogen Inorganic materials 0.000 description 4
- 238000005070 sampling Methods 0.000 description 4
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 3
- 239000004576 sand Substances 0.000 description 3
- 230000015572 biosynthetic process Effects 0.000 description 2
- 229910052500 inorganic mineral Inorganic materials 0.000 description 2
- 239000011707 mineral Substances 0.000 description 2
- 238000005065 mining Methods 0.000 description 2
- 238000003786 synthesis reaction Methods 0.000 description 2
- 239000004215 Carbon black (E152) Substances 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 210000005252 bulbus oculi Anatomy 0.000 description 1
- 230000009977 dual effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000012634 fragment Substances 0.000 description 1
- 229930195733 hydrocarbon Natural products 0.000 description 1
- 150000002430 hydrocarbons Chemical class 0.000 description 1
- 150000002431 hydrogen Chemical class 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000003208 petroleum Substances 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
- G01V5/04—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging
- G01V5/08—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays
- G01V5/10—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays using neutron sources
- G01V5/107—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays using neutron sources and detecting reflected or back-scattered neutrons
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- Physics & Mathematics (AREA)
- High Energy & Nuclear Physics (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Life Sciences & Earth Sciences (AREA)
- General Physics & Mathematics (AREA)
- Geophysics (AREA)
- Geophysics And Detection Of Objects (AREA)
Abstract
The bulk rock strength of strata is determined by logging a bore-hole with an instrument to obtain a neutron-neutron log of the hole and determining in conjunction with the lithology of the strata the strength by applying an inverse relation formula.
Description
SPECIFICATION
A method of determining the bulk strength of strata
This invention comprises a technique of determining the bulk strength of rocks encountered in boreholes and is a variation of the method disclosed in copending patent application
No. 8033883.
In all mineral extraction methods it is extremely useful to personnel involved in planning and working the extractions to be able to determine not only the nature and depth of various minerals and rocks but also their relative strength. Data pertaining to the relative strengths of rocks likely to be encountered in a scheme of operations is of importance on two accounts: firstly in ensuring that necessary support action is taken to enable the extraction to take place in a safe environment, and secondly that the extraction proceeds in the most economically efficient manner.
In order to determine the nature of strata it has been the practice for many years for boreholes to be drilled vertically from the surface to determine the exact positions and materials of the strata. In coal mining, boreholes may typically be up to 1400 metres deep and have a diameter of 0.25 metres.
Boreholes yield two types of rock samples: firstly "cutting samples" which are small fragments of rock produced by a tool which completely grinds the rock away in producing the borehole; secondly, continuous cylinders of core produced by a tool which cuts a narrow annulus of rock away. Since coring is roughly twice the cost of rock-bitting, a typical borehole may have the top section rock-bitted and the subsequent portion of interest cored.
The materials produced by the coring method are used to give a variety of information, the most obvious of which is the precise nature of the strata through which the coring tool has passed.
Another important piece of information is the
Fracture Index Log (sometimes referred to as
Crack Density log) which is derived from the core and which is used to indicate the strength of the rock.
It is conventional to record from an examination of the core sample the exact depth of every fracture in the core when compiling a
Fracture Index log. This affords the opportunity at a later date to express the- Fracture Index as the number of fractures per unit length over any chosen unit length (e.g. 40 cm., 60 cm., 100 cm., 200 cm., etc.) The Fracture Index Log may be used as the basis to produce other logs e.g. Rock Quality Designation (R.Q.D.).
Once a borehole has been made further information can be obtained relating to the strata by logging the borehole. One log which is often obtained is a neutron-neutron log whilst another is a caliper log.
The neutron-neutron log is obtained by using a standard down-hole geophysical tool. The
equipment comprises a neutron source and a
neutron detector which are separated by a fixed
distance characteristic of that particular tool
(hence short-spaced neutron-neutron, long
spaced neutron-neutron etc.). In principle the
source continually emits fast neutrons. Neutrons
lose more energy per collision when the nuclei
with which they collide have comparable mass.
Hence the rate of energy loss (moderation) of
fast neutrons is proportional to the density of
protons (which have nearly the same mass as the
neutrons). After the neutrons have been slowed
(to so-called thermal energies), they may be
captured by nuclei which then emit high energy
capture gamma rays.
The prime response of fast neutrons in the
borehole environment is to be slowed by
interaction with hydrogen in combination with
other elements (most commonly with oxygen as
water). Therefore, making the detector respond to
thermal neutrons (neutron-neutron tool) or
alternatively capture gamma rays (neutron
gamma tooi) gives an indication of the amount of
hydrogen present in the borehole environment which is identified as a hydrogen index. The output
of the detector is used to produce a neutron or
gamma log. Neutron logs are often scaled in
counts per second (c.p.s.) or American Petroleum
Institute units (A.P.I.) or Standard Neutron Units
(S.N.U.) as is explained later. This hydrogen index
has been applied as indicated in our copending
application, No. 80/33883, to give the bulk rock
strengths of the strata.A synthesis of this may be
obtained and it is an object of the present
invention to provide a method of obtaining this
synthesis.
According to the present invention a method of
determining the bulk rock strength of strata
includes the steps of drilling a borehole in strata,
moving an instrument along the borehole and
determining a Neutron-Neutron log of the strata
through which the instrument passes,
determining for the strata surrounding the hole
the lithology of the strata, progressively modifying
the Neutron-Neutron log by a predetermined
mathematical function (the calibration constant)
for each lithology group encountered in the
borehole to produce a modified Neutron-Neutron
log which is an indication of the bulk rock
strengths of the strata through which the
borehole was drilled.
The bulk rock strength Z can be given in the
form:
a
Z+b
x
where a and b are derived calibration constants
for each lithological group or sub-division of a
group and x is the Neutron-Neutron log. It will be
appreciated that the Bulk Rock Strength log may
be considered as being effectively a synthesized
Fracture Index (or Crack Density) log.
The above form of the relationship is not fundamental to the technique-any polynomial of the form Z=f(x) (or other mathematical function) which can be fitted to the data can be used and the appropriate calibration constants determined-there is no reason why different polynomial should not be fitted to different lithology groups or sub-divisions. An inverse relationship is expressed only for simplicity's sake.
The Neutron-Neutron log (or neutron-gamma) tool response is contributed to by the drilling mud within the borehole in addition to material within the borehole wall. The drilling mud contribution is partly controlled by borehole diameter. Variation in borehole diameter (rugosity) therefore may partly contribute to the Neutron-Neutron tool response variation. The rugosity is measured by logging the borehole with a caliper log which follows the contours of the borehole and gives an output which is directly related to the width of the borehole. Rugosity may, in part, be related to lithological type and in this situation the effect of varying rugosity in the calculation of a bulk rock strength log from the Neutron-Neutron log is already accommodated by deriving the calibration constants of the above equation for each lithology group.However rugosity may vary independently of lithology and therefore the bulk rock strength log could be derived from a mathematical relationship in which bulk rock strength is both a function of Neutron-Neutron response and caliper value. This may simply be achieved by obtaining the calibration constants of the above equation as a function of caliper value. Thus, if required, the method can include the further step of using a caliper log to provide information to correct for borehole rugosity when deriving the bulk strength log.
The lithology of the strata which the borehole encounters may be obtained from a Fracture
Index log of the core, this is preferably obtained by visual examination of core produced in forming the bore-hole although it may be obtained by a geologist's interpretation of (other) conventional down-hole geophysical logs or by computer processing of (other) conventional down-hole geophysical logs.
The bulk rock strength log may be also derived from a neutron-gamma log. The Neutron-Neutron or Neutron-Gamma log is preferably filtered and a filtering factor may be applied to the Fracture
Index log for the purpose of establishing calibration constants.
The method may inciude the further step of applying a filtering factor to the lithology log with the provision that the filtering factor may only smooth over continuously similar sections of a particular sub-group, and with the further provision that any lithology units contaminated as a result of the smoothing process are eliminated from calculations of the calibration constants.
In order that the invention may be readily understood one application of the method thereof to the logging of the borehole will now be described, by way of example only, with reference to the four charts in the accompanying drawings.
In the drawings: Figure 1 shows a plot of the Neutron-Neutron log against depth;
Figure 2 shows a plot of the Fracture Density log against depth;
Figure 3 is a plot of the Neutron-Neutron log of
Figure 1 against the Crack Density log of Figure 2, for one lithology group; and
Figure 4 is a plot of the Bulk Rock Strength and fracture density. Bulk Rock Strength is obtained by modifying the Neutron-Neutron log according to this invention, superimposed on the synthetic
Fracture Density log for the purposes of comparison.
A borehole was drilled vertically from the surface through strata known to contain coal measures to the depth of 1100 metres. The first 400 metres were drilled by rock-bitting where the drill carried a drill bit which ground away at the material which was continuously flushed away to the surface. Below 400 metres drilling was continued by a coring drill which ground out an annulus of rock which was flushed to the surface but retained a core of material in the barrel of the drill. This barrel was periodically withdrawn to the surface for examination and removal of the core.
A distance log was kept of the position of the drill in the borehole so that the nature of the material produced by the coring technique could be directly identified with the location of the strata.
The lithology of the strata was then classified by visual inspection. A series of numbers was then accorded to the materials of the strata according, in this example only, to the following sub-divisions:-
Lithology Lithology number
Coal 1
M/S S.M 2
M/S SL silty 2
S/E. M/S S.M. 3
S/E. M/S. SL. silty 3
M/S silty 4
M/S Silty with ironstone bands 5
Silts fine gr. 5
Silts med. gr. 5
Silts coarse gr. 6
Sand fine gr. 6
Sand med. gr. 6
Sand coarse gr. 7
Grit 7
Conglom 7
(M/S=Mudstone) (S/E=Seatearth)
(S.M.=smooth)
(SL.=slightly)
(GR.=grain size)
The lithology log was then entered into a computer at the chosen basic sampling interval of 20 cm. in this example, using the relevant lithology numbers.
It will be appreciated that, once obtained, the lithological constants or numbers shown can be applied, with modifications if necessary, to other boreholes in the same or other localities.
Once the borehole has been drilled it can be logged by slowly winching a device from the bottom of the borehole surface whilst recording the variations in electrical signals which the device transmits to surface recording instruments via the (dual purpose) winching cable.
Examples of various types of geophysical logs are given in a paper entitled "Exploration 2000" by R. H. Hoare in "The Mining Engineer" August 1979, Pages 131-140.
One log taken is the Neutron-Neutron log which is obtained by winding a Neutron-Neutron log device down the borehole and logging the
Neutron-Neutron tool response from 400 metres downwards to give a log as shown in Figure 1. In this figure the ordinate gives the Neutron-Neutron response, the ordinate is on a scale of 0 to 400 while the abscissa gives the depth in metres.
Once obtained the Neutron-Neutron log was then entered into a computer at the chosen basic sampling interval of 20 cm.
In order to determine the calibration constants the Fracture Index log was then entered into a computer at the chosen basic sampling interval of 20 cm. This log is shown in Figure 2 plotted against the depth in metres.
Prior to establishing the calibration constants a filtering factor is applied to both the Neutron
Neutron log and the Fracture Index log before cross-plotting. Filtering has been effected by smoothing over a number (L) of basic sample intervals such that:
L=1 no smoothing, raw data at basic sampling interval of 20 cm
L=3 smoothed over 40 cm. i.e. 20x(L-1) cm.
L=7 smoothed over 120 cm.
L=2 1 smoothed over 400 cm.
This amounts to simple running average smoothing other forms of filtering could equally well be applied.
For the particular lithology group or subdivision under examination in this example a range of 'L' values is used in the Neutron
Neutron/Fracture Index cross plots and the best relationship is selected (by a least square fit or by "eyeball"). The range of lithology groups is arbitrary, the number and range of groups can be varied to suit different geological environments and to obtain the most accurate calibration constants.
It is important to note that although large 'L' values smooth both the Neutron-Neutron and
Fracture Index logs they must not be permitted to smooth together different lithology sub-groups for calibrations. Consequently large 'L' values operate only on the Neutron-Neutron response and
Fracture Index for continuously similar sections of that lithology sub-group-any lithology units contaminated' as a result of the smoothing process are eliminated from calculation of the calibration constants.
Figure 3 shows a cross-plot of Neutron
Neutron response and Fracture Index for a specific lithological group including only the points uncontaminated by the smoothing process.
For the compilation of Bulk Rock Strength (z)the Neutron Neutron log is amended at each lithology sub-group according to the calibration coefficients a and b:
a
x
During the compilation of Bulk Rock Strength (z) all lithology group 1 (coal) are removed from consideration and replaced in the final printed log by "bars". This is preferred because (a) coal is hydrocarbon and therefore responds significantly differently on the Neutron-Neutron log to the other Coal Measure rocks; (b) the mechanical strength of coal is well established and presents no engineering difficulties and (c) the convention of displaying coal horizons in the finished Bulk
Rock Strength log as "bars" has some value in the case of stratigraphic identification in the finished
Bulk Rock Strength log.
Figure 4 shows a display of derived Bulk Rock
Strength A superimposed on the synthetic
Fracture Index B. The intermediate "bars" C are coal. The 'L' value used in Figure 4 is 21. The identity between the logs A and B indicates the accuracy of the invention. The Bulk Rock Strength log, which in this example is a synthesized
Fracture Index (Crack Density) log can readily be used to produce other rock quality and/or strength logs (e.g. R.Q.D.)
It will be appreciated from the foregoing that an accurate Bulk Rock Strength log can be readily computed from the Neutron-Neutron log and a lithology log and that a set of calibration constants has been derived which reiates to the
Coal Measure rocks. For use in sedimentary environments other than Coal Measure rocks, a relevant set of calibration constants can be derived from a suitable borehole by compiling a conventional Fracture Index log.
In this submission Fracture Index has been used as the reference or basis for the process of deriving a Bulk Rock Strength log from a Neutron
Neutron log. The Fracture Index log is not crucial to the process of deriving a Bulk Rock Strength log from a Neutron-Neutron log. The principle of this invention is that for purposes of calibration a
Bulk Rock Strength log can be derived from a
Neutron-Neutron log using any rock strength, rock quality or rock property log. A tolerable (although inferior ) Bulk Rock Strength log can even be derived from a Neutron-Neutron log using no calibration constants or additional rock quality log, that is to say that the Neutron-Neutron log alone represents a crude Bulk Rock Strength log.
The use of the invention thus enables an accurate indication of strength to be obtained more easily and speedily than hitherto.
Claims (13)
1. A method of determining the bulk strength of strata including the steps of drilling a borehole in strata, moving an instrument along the borehole and determining a Neutron-Neutron log of the strata through which the instrument passes, determining for the strata surrounding the hole the lithology of the strata progressively modifying the Neutron-Neutron log by a predetermined mathematical function (the calibration constants) for each lithology group encountered in the borehole to produce a modified Neutron-Neutron log to produce a Bulk
Rock Strength log which is an indication of the bulk strengths of the strata through which the borehole was drilled.
2. A method as claimed in claim 1 in which the bulk rock strength z is given the form:
a
z+b
x where a and b are derived calibration constants for each lithological group or sub-division of a group and x is the Neutron-Neutron response.
3. A method as claimed in claim 2 in which the bulk rock strength z is related to the Neutron
Neutron response x by any polynomial expression or mathematical function, i.e. z=f(x) with derived calibration constants for each polynomial expression used for each lithological group or sub-division of a group.
4. A method as claimed in claim 2 or 3 in which a caliper log is used to correct or allow for borehole rugosity in the calculation of the bulk rock strength.
5. A method according to any preceding claim in which the Neutron-Neutron log is replaced by a neutron-gamma log.
6. A method according to any preceding claim in which the lithologies encountered by the boreholes are obtained from visual inspection of core samples.
7. A method as claimed in claim 6 in which the
Fracture Index log is obtained by visual inspection of core samples.
8. A method according to any preceding claim in which the lithologies encountered by the boreholes are obtained by manual interpretation of conventional down-hole geophysical logs.
9. A method according to any one of claim 1 to 7 in which the lithologies encountered by the boreholes are detailed by computer processing of conventional down-hole geophysical logs.
10. A method according to any preceding claim incorporating the further step of applying a filtering factor to the Neutron-Neutron log.
11. A method according to any preceding claim incorporating the further step of applying a filtering factor to the Fracture Index log for purposes of establishing calibration constants.
12. A method according to any preceding claim incorporating the further step of applying a filtering factor to the lithology log with the provision that the filtering factor may only smooth over continuously similar sections of a particular lithology sub group and with the further provision that any lithology units contaminated as a result of the smoothing process are eliminated from calculations of the calibration constants.
13. A method of determining the bulk rock strength of rock in a strata substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8114912A GB2099138B (en) | 1981-05-15 | 1981-05-15 | Determining the bulk strength of strata |
| CA000388366A CA1161573A (en) | 1980-10-21 | 1981-10-20 | Method of determining the bulk strength of strata |
| US06/313,427 US4445032A (en) | 1980-10-21 | 1981-10-20 | Method of determining the bulk strength of strata |
| AU76678/81A AU542385B2 (en) | 1980-10-21 | 1981-10-21 | Method determining the bulk strength of strata |
| BR8106784A BR8106784A (en) | 1980-10-21 | 1981-10-21 | METHOD FOR DETERMINING THE GLOBAL STRENGTH OF STRATEGIES |
| DE19813141679 DE3141679A1 (en) | 1980-10-21 | 1981-10-21 | METHOD FOR DETERMINING THE STRENGTH OF MOUNTAIN LAYERS |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8114912A GB2099138B (en) | 1981-05-15 | 1981-05-15 | Determining the bulk strength of strata |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2099138A true GB2099138A (en) | 1982-12-01 |
| GB2099138B GB2099138B (en) | 1984-06-27 |
Family
ID=10521826
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8114912A Expired GB2099138B (en) | 1980-10-21 | 1981-05-15 | Determining the bulk strength of strata |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2099138B (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL1012787C2 (en) * | 1998-08-25 | 2001-08-30 | Schlumberger Holdings | Parametric modeling of well survey data to remove periodic errors. |
-
1981
- 1981-05-15 GB GB8114912A patent/GB2099138B/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL1012787C2 (en) * | 1998-08-25 | 2001-08-30 | Schlumberger Holdings | Parametric modeling of well survey data to remove periodic errors. |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2099138B (en) | 1984-06-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| 746 | Register noted 'licences of right' (sect. 46/1977) | ||
| PCNP | Patent ceased through non-payment of renewal fee |