GB2179442A - Formation density logging using two detectors and sources - Google Patents

Abstract

A gamma ray density sub (10) and method for measurement-while- drilling using a pair of gamma ray sources (22, 24) and detectors (30, 32), located symmetrically about the axis of the sub, and computation of the product of the counting rates obtained from the two detectors to indicate the average density of a formation sample (46) which surrounds a borehole traversing an earth formation (20). The sub is able to measure the density of the sample independent of the location of the sub within the borehole and of the chemical composition of the interfering materials lying between the formation sample and the detectors. A second set of detectors (40, 42) similarly receives scattered rays indicative of density of a second formation sample (47). The product of the count rates of the second detectors is divided into the first product to yield a density indication which is independent of borehole effects. <IMAGE>

Description

SPECIFICATION Formation density logging using two detectors and sources The present invention relates to logging of subterranean formations for determination of density using gamma rays. Particularly, this invention relates to determination of formation density without positioning the logging probe against the wall of the borehole traversing the earth formation. More particularly, this invention is useful for measurement of density while drilling.

Wireline gamma ray density probes are devices incorporating a gamma ray source and a gamma ray detector, shielded from each other to prevent counting of radiation emitted directly from the source. During operation of the probe, gamma rays (or photons) emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. In photoelectric- absorption and pair production phenomena, the particular photons involved in the interacting are removed from the gamma ray beam.

In the Compton scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss -being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered towards the detector. Many of these never reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photo-electric absorption process of the pair production process. The scattered photons that reach the detector and interact with it are counted by the electronic equipment associated with the detector.

The major difficulties encountered in conventional gamma ray density measurements include definition of the sample size, limited effective depth of sampling, disturbing effects of undesired, interfering materials located between the density probe and the sample and the requirement that the probe be positioned against the borehole wall. The chemical composition of the sample also affects the reading of conventional gamma ray density probes.

One prior art wireline density probe disclosed in US-A-3,202,822 incorporates two gamma ray detectors, one collimated gamma ray source and ratio-building electronic circuits, and is useful as long as the interfering materials, located between the detectors of the probe and the formation sample, are identical in thickness and chemical composition along the trajectories of emitted and received gamma rays. Non-uniformities in the wall of the borehole will interfere with the proper operation of the probe. Such non-uniformities can be caused by crooked holes, by cave-ins, and by varying thicknesses of the mudcake on the wall of the hole.

The prior art also includes US-A-3,846,631 which discloses a wireline density probe which functions regardless of the thickness and the chemical composition of materials that are located between the density probe and the sample. The method comprises passing of two gamma ray beams from two intermittently operated gamma ray sources into the sample, receiving the radiation backscattered from each of the two sources by two separate detectors, and building ratios of products of the four separate counting rates in such a manner that the numerical result is an indication of the density of the sample.

The critical dimension of the two-detector probe is the spacing between the detectors. If the interfering materials are non-uniform over distances comparable to the spacing of the two detectors, the measured density will be erroneous.

Neither of the wireline probes described above is disclosed as being useful for measurement while drilling and incorporation into a rotating drill string.

An object of this invention is to provide a method and apparatus capable of measuring the density of a subterranean formation while drilling a borehole traversing the formation.

In one aspect the invention provides a device for use in a borehole travering an earth formation including two gamma ray emitting means spread 180 apart about the device, said means emitting collimated gamma ray beams along two trajectories, the trajectories projecting in an azimuthally symmetric pattern about the axis of the device, intersecting at a first point on the axis of the device, and intersecting a first circle located in a sample of the formation to be measured, a first gamma ray detecting means oriented to receive emitted gamma rays scattered from two locations within the formation sample along a first two trajectories, the trajectories projecting in an azimuthally symmetric pattern about the axis of the device, intersecting a second point on the axis of the device and intersecting the first circle, and a means for determining the product of the counting rate of gamma rays received by the detecting means from each of the two trajectories as scattered from the two locations within the formation sample, wherein, the product is indicative of the average density of the formation sample.

In another aspect the invention provides a method of determining the average density of a sample of earth formation surrounding a borehole including the steps of lowering a device into the borehole to a location adjacent to the sample; emitting gamma rays into the formation from the device along two trajectories projecting in an azimuthally symmetric pattern about the axis of the device, intersecting at a first point on the axis of the device and intersecting a first circle located in the formation sample; counting the emitted gamma rays scattered from the formation sample back to the device along a first set of two trajectories projecting in an azimuthally symmetric pattern about the axis of the device, intersecting at a second point on the axis of the device and intersecting the first circle; and determining the product of the two count measurements, wherein the product is indicative of the average density of the formation sample.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional representation of a device in accordance with the present invention for logging densities in a formation traversed by a rotating drill string, in which the device may be located; and Figure 2 is a schematic representation of electronic circuitry required to detect, count and process the scattered photons.

A gamma density sonde or so-called sub 10 is shown in Fig. 1 as interconnected between an upper drill string 12 and a lower drill string 14. Rotation of the drill string 12, 14 causes a drill bit 16 to form a borehole 18 traversing an earth formation 20.

The sub 10 includes a first gamma ray source 22, and a second gamma ray source 24. The two sources are situated about the sub in an azimuthally symmetric pattern, i.e. 1800 apart. The sources are collimated to form trajectories which are also azimuthally symmetrical. The trajectories are oriented to pass through a first point 28 located on the axis 29 of the sub 10. The term trajectory as used herein indicates not only the actual path of travel of the gamma ray but also a line of extension behind the source as well as beyond the detector.

The plurality of sources may be single primary source from which the emitted gamma rays are collimated to form the two symmetrical gamma ray beams.

The sub 10 further includes a first set of detectors including a first gamma ray detector 30 and a second gamma ray detector 32. The detectors are situated about the sub 10 in an azimuthally symmetrical pattern which is in axial and azimuthal alignment with the first and second sources 22 and 24. The detectors are collimated to receive gamma rays scattered from the formation along trajectories which are also azimuthally symmetrical. The trajectories are oriented to intersect the axis 29 of the sub 10 at a second point 36.

The trajectories from the sources will intersect the first set of detector trajectories at a first circle 38 about the sub 10. The first circle falls in a plane which is perpendicular to the axis of the sub 10, the plane intersecting the axis 29 at a third point 39. The second point 36 is positioned an axial distance away from the first point 28 and the first and second points 28, 36 are preferably on opposite sides of the third point 39.

The sub 10 includes a second set of detectors including a third gamma ray detector 40, and fourth gamma ray detector 42. This second set of detectors is situated about the sub 10 in an azimuthally symmetrical pattern which is also in axial and azimuthal alignment with the first and second sources, 22, 24 and the first set of detectors 30 and 32.

The second set of detectors will receive gamma rays along a third set of two trajectories which are azimuthally symmetric about the sub 10 and are oriented to intersect the axis 29 at a fourth point 45 and to intersect a second circle 72. Preferably, the fourth point 45 and the first point 28 are on oppoosite sides of the third point 39. Each trajectory of the third set should be parallel to a corresponding trajectory of the second set.

The first and second sets of detectors are shielded from the sources to pevent the emitted gamma rays from eaching the detectors directly from the sources.

The first circle 38 and the second circle 72 formed in the formation 20 will be the centre of the formation samples 46 and 47 respectively which are to be measured for density.

In a method of this invention the sub 10 rotates about its axis 29 as gamma rays 48 are emitted into the sample by the first source 22 and gamma rays 50 by the second source 24.

The emitted collimated beams of gamma rays from a first cone-shaped region of formation which is irradiated.

In the formation 20, some of the gamma rays 48 and 50 are scattered by the sample formation 46 toward the first set of detectors. Gamma rays 54 are scattered at location 56 in formation sample 46 toward and received by the first.detector 30. Gamma rays 58 are scattered at location 62 in formation sample 46 towards and received by the second detector 32.

Since the two collimated sources 22, 24 are symmetrically located, there is only one right conical region irradiated during the sub's rotation. The two collimated detectors 30, 32 receive emitted gamma rays scattered from the formation sample 46 back to the sub 10 along trajectories forming a second cone, inverted with respect to the first cone.

The thickness of the cones is determined by the collimator's diameter. The circle 38 formed by the intersection of the cones has as its centre point 39 on the axis 29 of sub 10. At a given instant of time, two small sectors 66, 68 of the formation sample, each 180 apart, will be sampled.

The received gamma rays 54, 58 will- react with the first set of detectors 30, 32 and cause electrical pulses. The pulse amplitudes are proportional to the energy of the received gamma rays. If it is desired to provide counting rates- indicative of only those rays which have been scattered only once in the sample 46, these pulses would be amplified by preamplifiers and amplifiers, and fed to discriminators (not shown), which are set to pass only those pulses having energy levels of gamma rays that were scattered at the location 56, towards the detector 30, and at location 62 towards the detector 32. Gamma rays that underwent multiple scattering prior to entering the detectors 30, 32 will be rejected by the discriminators.The -output of the detectors and, if used, the discriminators, leads to the gates, which provide individual counting rates of received gamma rays from the two detectors 30, 32. This arrangement is shown generally in Fig. 2.

The product of the counts in the near detectors 30 and 32 and in the far detectors 40 and 42 and the quotient of the products is produced using the electronics schematically shown in Fig. 2. The counters 80-83 convert the current pulses produced in the detectors into digital voltage pulses by means of amplifiers and voltage discriminators (not shown) and then store the counts. The counts from the near detectors 30, 32 are stored in counters 80 and 81; the counts from the far detectors 40 and 42 are stored in counters 82, 83. Inputs into the counters are voltage counts from the detectors and voltage levels from the clock 75.

The clock 75 is preset to produce a pulse at regular intervals, for example every 30 seconds.

When it sends a pulse to the counters 80-83 and to the multipliers 84, 85 the counts in 80-81 and the counts in 82-83 are multiplied together by the multiplers 84 and 85 respectively. Multipling device 84 computes the product of counts in counters 80, 81; multiplying device 85 computes the product of counts in counters 82, 83. The dividing device 86 computes the quotient of the products produced by devices 84 and 85 once every time a pulse is signalled from the clock 75. The output of divider 86, i.e. the ratio of the outputs of multipliers 84, 85 may then be plotted versus time by a suitable plotting device 87.

The individual counts from the detectors 30, 32, 40 and- 42 may vary with time due to the sub's location within the borehole as caused by rotation of the drill string off of the axis of the borehole.

In a method of this invention, the two instantaneous counts from the first set of detectors 30, 32 are multiplied by multiplier 84 resulting in a constant value thus indicating elimination of variables with time, such as the thickness of mud through which the emitted gamma rays must pass to be received at the detectors, and the movement of the sub in relation to the borehole wall.

The sub, in an off-axis position, will receive gamma rays 48, which have scattered from the formation sample 46, at detector 30. These rays 48 will have travelled through a different amount of mud and formation than gamma rays 50 from source 24. However, the sum of the path lengths through mud, and the sum of the path lengths through the formation are constant provided that the diameter of sub 10 is substantially similar to the diameter of borehole 18.

A density log for measurement while drilling applications should be accurate to within about 0.1 g/cm3. Since formation density is typically 2.5 g/cm3, the accuracy required is about 4%. If vertical resolution required for the log is 1 5 cms, a required counting rate may be estimated as follows:

where .f is the statistical variation of the product N,N2 N, is the total count at detector 30, and N2 is the total count as detector 32.

Assuming N, > N2=N then, from the field of statistics and

Solving for N No 1250 counts Each density log measurement should detect an average of 1250 counts per measurement and there should be a measurement every 15 cms. At 18 metres per hour drilling rate, each measurement will therefore be completed in 30 seconds.

Therefore, each detector 30, 32, 40, 42 should have sufficient sensitivity such that about 43 counts per second are registered. Alternatively, each source may be adjusted to emit at a rate such that the detectors receive at the required rate of 43 counts per second.

To compensate for borehole effects on the measurement of the average density for the formation samples 46 and 47, a method of this invention would include use of the counts from the second set of detectors 40, 42. The product of these two counts (output of multiplier 85) would be used to form a ratio (output of divider 86) between the product of the first set of detectors and the product of the second set of detectors. Alternatively, the product of the two ratios of a detector of the first set to a corresponding detector of the second set may be used to determine the average density. This is shown generally in Fig. 2.

A similar arrangement for the second set of detectors 40, 42 may be included in the sub 10 for receiving, discriminating, counting, storing and using the gamma rays received by the second set, as shown in Fig. 2.

The type of gamma ray sources is also not an object of the invention, since different types are preferred for different applications. Capsule type sources containing the radioactive isotopes such as cobalt 60 and cesium 137, are the types of gamma ray sources most frequently used in gamma ray density probes.

The diameters of the borehole 18 and the sub 10 should be substantially equivalent. This can be accomplished by the use of stabilizers on the exterior of the sub which are then part of the relative diameter determination.

Various other alterations in the details of construction and the sequence of computations can be made without departing from the scope of the invention.

Claims (15)

1. A device for measuring the density of an earth formation traversed by a borehole, said device comprising gamma ray emitting means for emitting collimated gamma ray beams along a first set of two trajectories, said trajectories projecting in an azimuthally symmetric pattern about the longitudinal axis of said device, intersecting at a first point on said axis of said device, and intersecting a first circle located in a sample of said formation to be measured; first gamma ray detecting means oriented to receive emitted gamma rays scattered from two locations within said formation sample along a second set of two trajectories, said trajectories projecting in an azimuthally symmetric pattern about said axis of said device, and intersecting said first circle; and means for determining a first product of the counting rates of gamma rays received by said first detecting means from each of said two trajectories as scattered from each of said two locations within said formation sample, wherein said first product is indicative of the average density of said formation sample.

2. A device according to Claim 1 wherein said first circle lies in a first plane which is perpendicular to the axis of said device and intersects said axis at a third point.

3. A device according to Claim 1 or Claim 2 wherein said device is adaptable for use in a drill string.

4. A device according to any one of Claims 1 to 3 wherein said gamma ray emitting means comprises gamma sources, each source collimated to emit gamma rays along one of each of said first set of two trajectories.

5. A device according to Claim 4 wherein said sources are positioned in an azimuthally symmetric pattern about said device and lie in a second plane which is perpendicular to the axis of said device.

6. A device according to any one of Claims 1 to 5 wherein said first detecting means comprise two detectors, each said detector collimated to receive gamma rays along one of said second set of two trajectories.

7. A device according to any one of Claims 1 to 6 comprising additionally: second detecting means oriented to receive emitted gamma rays scatterd from said two locations within said formation sample along a third set of two trajectories, said trajectories intersecting at a fourth point on the axis of said device and intersecting a second circle about said axis, said second circle being intersected by said first set of two trajectories; means for determining a second product of the counting rates of gamma rays received by said second detecting means from each of said two trajectories as scattered from each of said two locations within said formation sample; and divider means for dividing said first product by said second product to provide a ratio which is indicative of an average compensated density of said formation sample.

8. A device according to any one of Claims 1 to 7 wherein said first point is spaced apart from said second point.

9. A device according to Claim 7 wherein said first point and said second point are spaced apart from said fourth point.

10. A device according to Claim 7 or Claim 9 wherein said first point is on one side of said third point and said second point is on the opposite side of said third point.

11. A device according to Claim 7 or Claim 9 wherein said first point is on one side of said third point and said fourth point is on the opposite side of said third point.

12. A method of determining the average density of a formation sample surrounding a borehole comprising: lowering a device into said borehole to a location adjacent to said sample; emitting gamma rays into said formation from the device along a first set of two trajectories projecting in an azimuthally symmetric pattern about the axis of said device, said first set of two trajectories intersecting at a first point on the axis of said device and also intersecting a first circle located in said formation sample; counting said emitted gamma rays scattered from said formation sample back to said device along a second set of two trajectories projecting in an azimuthally symmetric pattern about the axis of said device, said second set of two trajectories intersecting at a second point on the axis of said device and also intersecting said first circle; and determining a first product of said two counts, wherein said first product is indicative of the average density of said formation sample.

13. A method according to Claim 12 comprising the additional steps of: counting said emitted gamma rays scattered from said formation sample back to said device along a third set of two trajectories projecting in azimuthally symmetric patterns about the axis of said device, said third set of two trajectories intersecting at a third point spaced apart from said second point on said axis of said device, and also intersecting a second circle about said axis, said second circle also being intersected by said first set of two trajectories; determining a second product of said at least two counts; and determining the ratio between said first and second second products; wherein said ratio is indicative of an average compensated density of said formation sample.

14. A method according to Claim 12 and substantially as described herein with reference to the accompanying drawings.

15. A device for measuring the density of an earth formation traversed by a borehole, substantially as described herein with reference to the accompanying drawings.