Borehole logging system and method

Abstract

A borehole logging method comprises processing density measurements taken by at least one probe at intervals along a borehole to determine the gravitational effects of geologic formations intersected by the borehole; and removing the determined gravitational effects from gravity measurements taken by at least one probe at intervals along the borehole to determine the gravitational efforts of geologic formations from the borehole.

Claims

What is claimed is: 1 . A borehole logging method comprising: processing density measurements taken by at least one probe at intervals along a borehole to determine the gravitational effects of geologic formations intersected by the borehole; and removing the determined gravitational effects from gravity measurements taken by at least one probe at intervals along the borehole to determine the gravitational efforts of geologic formations from the borehole. 2 . The method of claim 1 , wherein the density and gravity measurements are taken independently, at different times 3 . The method of claim 1 , wherein the density and gravity measurements are taken simultaneously. 4 . The method of claim 1 , comprising, prior to said processing, taking density measurements at first vertical intervals along the borehole and taking gravity measurements at second longer vertical intervals along the borehole. 5 . The method of claim 4 further comprising pre-processing the gravity measurements prior to said processing. 6 . The method of claim 6 wherein said pre-processing comprises correcting the gravity measurements to compensate for the effects of at least one of instrumental drift, effects of the Earth's tides and the normal variation with depth of the Earth's gravitational attraction. 7 . A borehole logging system comprising: a density probe taking density measurements within a borehole; a gravity probe taking gravity measurements within the borehole; and processing structure receiving the density and gravity measurements from the probes and processing the measurements to determine the gravitational effort of geologic formations remote from the borehole. 8 . The borehole logging system of claim 7 wherein said processing structure processes the density measurements to calculate gravitational effects of geologic formations in the vicinity of the borehole and removes the calculated gravitation effects from the gravitational measurements thereby to determine density variations remote from the borehole. 9 . The borehole logging system of claim 8 wherein the density and gravity probes take the density measurements and gravity measurements at vertical intervals along the borehole. 10 . The borehole logging system of claim 9 wherein the density probe takes the density measurements at first vertical intervals and wherein the gravity probe takes the gravity measurements at second vertical intervals, the first vertical intervals being smaller than the second vertical intervals. 11 . The borehole logging system of claim 10 wherein said processing structure pre-processes the gravity measurements prior to removing the calculated gravitational effects therefrom. 12 . The borehole logging system of claim 11 wherein during pre-processing, said processing structure corrects the gravity measurements to compensate for the effects of at least one of instrumental drift, effects of the Earth's tides and the normal variation with depth of the Earth's gravitational attraction. 13 . The borehole logging system of claim 11 wherein during pre-processing, said processing structure corrects the gravity measurements to compensate for the effects of instrumental drift, effects of the Earth's tides and the normal variation with depth of the Earth's gravitational attraction 14 . The borehole logging system of claim 7 , wherein the density probe is one of a gamma-gamma density probe and a neutron gamma density probe. 15 . The borehole logging system of claim 10 , wherein the density probe is one of a gamma-gamma density probe and a neutron gamma density probe. 16 . The borehole logging system of claim 13 , wherein the density probe is one of a gamma-gamma density probe and a neutron gamma density probe. 17 . The borehole logging system of claim 7 , wherein the density probe and the gravity probe are combined in a single sonde. 18 . The borehole logging system of claim 10 , wherein the density probe and the gravity probe are combined in a single sonde. 19 . The borehole logging system of claim 13 , wherein the density probe and the gravity probe are combined in a single sonde. 20 . The borehole logging system of claim 7 wherein said density probe and gravity probe are separate sondes. 21 . The borehole logging system of claim 10 wherein said density probe and gravity probe are separate sondes. 22 . The borehole logging system of claim 13 wherein said density probe and gravity probe are separate sondes.
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application No. 61/393,339 to Seigel filed on Oct. 14, 2010 and U.S. Provisional Application No. 61/393,717 to Seigel et al. filed on Oct. 15, 2010, both entitled “Application of Gamma-Gamma or Neutron-Based Densities to Borehole Gravity Measurements”, the entire contents of which are incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention relates to borehole logging and in particular to a borehole logging system and method. BACKGROUND OF THE INVENTION [0003] Changes in the gravitational field of the Earth from place to place reflect the distribution of its diverse geologic materials, as a result of their differences in density. These gravitational changes can be observed through precise measurements by means of gravimeters, on the Earth's surface and, more recently, from aircraft and in boreholes. In mining applications, gravitational measurements have played an important role in the exploration, evaluation and development of mineral resources, including both metallics and non-metallics. Gravitational measurements are also of use in the exploration and production phases in the hydrocarbon field. [0004] Surface gravity measurements naturally provide more information about the distribution of densities of geologic material closer to the Earth's surface than about geologic material at greater depths. It is now common practice to explore and mine mineral deposits at depths of up to 1-2 km below the Earth's surface, and hydrocarbon bearing horizons are being exploited to even much greater depths. Gravity measurements made at the Earth's surface are of limited help in such deep programs. For this reason, borehole gravity sondes have been developed which are capable of providing high quality gravity measurements at such great depths, even under the extreme conditions of pressure and temperature prevailing at those depths. [0005] Borehole gravity measurements are affected by the distribution of rock densities, both in the vicinity of the borehole and remote from the borehole. For some applications, borehole gravity measurements are employed to provide quantitative, bulk-density data about the geologic formations being intersected by the borehole, and about fluids in reservoirs. Accurate bulk-density measurements can be of material value in several stages of mining activity. In the hydrocarbon field, they can yield information of great value in the evaluation of the hydrocarbon potential of specific horizons, or to monitor the progress of secondary recovery methods. [0006] For other applications, borehole gravity measurements can be interpreted to indicate the presence of density changes that are either remote from the borehole or that lie below the bottom of the borehole. Remote detection of non-intersected ore bodies is a common objective of borehole gravity measurements in mining applications. In the hydrocarbon field, information may be sought about the presence of salt domes and structural features, such as faults and folds, which have not been intersected by the borehole. Whereas the effects of remote geologic features are inherent in gravity measurements taken at intervals along the borehole, for lack of any unique interpretation of gravity data, it is theoretically impossible to resolve the contribution of remote density variations from the much larger contributions due to proximal sources, that is, the geologic formations actually intersected by the borehole. Only in the rare case that the geologic formations intersected by the borehole all have a uniform density, will the gravity effects of more remote geologic formations become resolvable. [0007] As will be appreciated, being able to determine the effect remote geologic formations have on gravity measurements taken along a borehole is desired as it provides insight as to the nature of the remote geologic formations. It is therefore an object of the present invention to provide a novel borehole logging system and method. SUMMARY OF THE INVENTION [0008] According to the following, the gravitational effects of geologic formations intersected by a borehole may be determined and removed from the gravity measurements taken along the borehole allowing the gravitational effects of geologic formations remote from the borehole to be resolved and revealed. [0009] Accordingly, in one aspect there is provided a borehole logging method comprising processing density measurements taken by at least one probe at intervals along a borehole to determine the gravitational effects of geologic formations intersected by the borehole; and removing the determined gravitational effects from gravity measurements taken by at least one probe at intervals along the borehole to determine the gravitational effects of geologic formations remote from the borehole. [0010] In one embodiment, the density and gravity measurements are taken independently, at either different times or simultaneously. The method may further comprise, prior to the processing, taking density measurements at first vertical intervals along the borehole and taking gravity measurements at second longer vertical intervals along the borehole. The method may further comprise pre-processing the gravity measurements prior to the processing to compensate for the effects of at least one of instrumental drift, effects of the Earth's tides and the normal variation with depth of the Earth's gravitational attraction. [0011] According to another aspect, there is provided a borehole logging system comprising a density probe taking density measurements within a borehole; a gravity probe taking gravity measurements within the borehole; and processing structure receiving the density and gravity measurements from the probes and processing the measurements to determine the gravitational effects of geologic formations remote from the borehole. DETAILED DESCRIPTION OF THE DRAWINGS [0012] Embodiments will now be described more fully with reference to the accompanying drawings in which: [0013] FIG. 1 is a side elevational view of a gamma-gamma density probe that has been lowered into a borehole; [0014] FIG. 2 is a side elevational view of a gravity sensor that has been lowered into the borehole; and [0015] FIG. 3 is a side elevational view of a sonde, comprising a gamma-gamma density probe and a gravity sensor that has been lowered into the borehole. DETAILED DESCRIPTION OF THE EMBODIMENTS [0016] According to the following, a borehole logging system and method are provided that allow the gravitational effects of geologic formations remote from a borehole to be resolved and revealed. During the method, density measurements are taken along the borehole at intervals to yield a density log and are processed to determine the gravitational effects of the geologic formations intersected by the borehole. Gravity measurements are also taken at intervals along the borehole to yield a gravity log. The determined gravitational effects of the geologic formations intersected by the borehole are removed from the gravity measurements allowing the gravitational effects of geologic formations remote from the borehole to be determined. These determined gravitational effects signify density variations remote from the borehole and provide insight as to the nature of remote geologic formations. [0017] A number of techniques may be used to acquire the density measurements at intervals along the borehole in order to generate the density log. For example, gamma-gamma (GG) density measurements, neutron gamma (ND) density measurements or other non-acceleration based density measurements that provide information about the density of geologic formations within one meter or less of the borehole maybe used. GG density measurements however are the one most commonly preferred in practice for wallrock density determination and will be referred to hereinafter by example only. [0018] The model 2GDA-1000DX device, manufactured by Mt. Sopris Instruments, Denver, Colo., USA, is a suitable device to take GG density measurements at intervals along a borehole in order to generate the density log. This device comprises a source of gamma radiation (e.g. 100 milliCuries of Cesium 137), and one or two radiation detectors (typically thalium-activated NaI crystals), separated from the radioactive source by a distance of between about 20-35 cm. The detectors are shielded from direct gamma radiation emitted by the radioactive source. The emitted gamma radiation creates both back-scattered Compton radiation and photoelectric absorption in the geologic material surrounding the borehole. This secondary radiation impinges on the detectors. For a range of common densities of geologic materials (1-4 gm/cc), the count rate sensed by the detectors provides a reasonable linear estimate of the density of the geologic material surrounding the borehole, out to about 7-12 cm into the surrounding geological material. The typical accuracy of such gamma-gamma density measurements is of the order of 0.1 gm/cc, with a resolution of about 0.05 gm/cc. [0019] The Gravilog sonde offered by Scintrex Limited, Concord, Ontario, Canada is a suitable device to take gravity measurements at intervals along the borehole in order to generate the gravity log. The gravity measurements are typically pre-processed to undergo certain corrections in order to more accurately provide useful geologic information. These corrections include instrumental drift with time, the effects of the Earth's tides and the normal variation, with depth, of the Earth's gravitational attraction (the so-called Free-Air correction). After such corrections have been made to the gravity measurements, changes (Ag) in the resultant gravity data with depth are predominantly dependent on the density of the geologic formations being traversed by the borehole, and are given by the formula shown in Equation (1) below: [0000] Δ=−0.08382 ρΔz   (1) [0000] where: [0020] Δg is in units of milliGals; [0021] ρ is the local geologic formational density, in units of gm/cc; and [0022] Δz is the change in depth, in meters [0023] The change of resultant gravity with depth reflects the upward attraction (i.e. the reduction of gravity) of the geologic formations which overlie the gravity station (i.e. the position along the borehole at which the gravity measurement is made), and may be inverted to derive the mean bulk density of the geologic formations, for those applications where this is the factor of greatest interest. Inverting Equation (1) provides Equation (2) as follows: [0000] ρ=11.93 Δg/Δz   (2) [0000] where: [0024] ρ is the mean geologic formational density, in units of gm/cc. [0025] According to Equation (2), ρ is the mean value of the geologic formational density, the so-called “bulk density” between two gravity stations Δz meters apart in depth. The effective radius from the borehole of these bulk-density values is several times the separation between the two gravity stations, i.e. normally 10-100 meters. [0026] However, for applications requiring the detection of remote density variations, a basic impediment is the dominant effect of the near-borehole geologic formational densities on the gravity measurements. Attempts may be made to estimate the mean value of ρ to be used for the near-borehole geologic formation densities, in Equation (1) and therefore to estimate their contribution to the observed gravity measurements, but this is an exercise which is subject to considerable subjective error. [0027] It has been found that the gravity contribution of the near-borehole geologic formations can be removed through the use of the GG density log. Due to the shallow depth penetration of GG density measurements into the geologic material surrounding the borehole, the densities in the density log pertain only to the geologic formations actually intersected by the borehole. The densities derived from the GG density log can be designated as ρ γ and can be used in Equation (1) to determine the gravity effects that arise from these geologic formations, on the assumption of their being of a uniform GG density, tabular, horizontally lying and of very large horizontal extent. [0028] These calculated gravity effects can then be subtracted from the corrected gravity log measurements, on the basis of the measured GG densities. The residual gravity after this subtraction is attributed to density changes that are remote from the borehole. This GG-derived residual gravity is referred to as GRG and embodies the effect of any departures, at a distance from the borehole, of the actual density distribution from the idealized, one-dimensional density model based on ρ γ . [0029] Typically, in a gravity log of a borehole, the gravity station highest in the borehole is designated as a base station for drift determination, and also is the arbitrary base level for all subsequent relative gravity measurements. If the corrected gravity values are defined as g, and the GG calculated gravity values are defined as g γ , then based on Equation (1), Equation (3) can be derived as follows: [0000] g γ =−0.08382Σρ γ δ  (3) [0000] where: [0030] the Σ summation is over the vertical depth from the base station to the moving station; and [0031] δ is the vertical spacing between successive values of ρ γ . [0032] Based on the above, GG-derived residual gravity GRG may be resolved as shown in Equation (4) below: [0000] GRG=g−g γ =g+ 0.08382Σρ γ δ  (4) [0033] The graph of GRG may be interpreted to derive basic information about density changes occurring remote from the borehole, not just laterally but also at depths below the bottom of the borehole. [0034] As well, the bulk-density values of ρ determined by Equation (2) using the gravity measurements, may be compared with the mean GG density values ρ γ over the same vertical intervals. Any differences between these two density values indicates an increase or decrease of density away from the borehole (usually laterally) from the geologic formations intersected by the borehole. These differences may be designated as GG-derived residual densities, or GRD, equal to ρ−ρ γ . However, GRD data are of lesser value than the GRG values, because they are not as readily subject to quantitative modeling. [0035] The practice of generating a GG density log of a borehole is a long established, standard procedure in the case of boreholes drilled for hydrocarbon purposes, so that in such boreholes at-hole density measurements are usually available for use. In the case of boreholes drilled for mining and other applications, GG density logs are not necessarily commonplace, and a density log may not be available. For such case, a GG density log will have to be carried out. For example, a GG sensor may be incorporated in the same sonde as a gravity sensor, or alternatively a GG density log may be carried out independently, either prior to or subsequent to the gravity log. Where it is intended that a borehole will be metal cased prior to conducting a gravity log, the GG density log will have to be done prior to the insertion of the casing. This is because GG density measurements are not possible in metal-cased boreholes, due to the effect of the casing. When carried out, GG density measurements are commonly recorded at short intervals down the borehole, typically of the order of 15 cm. Gravity measurements are typically made at much larger intervals, commonly 10 meters or more. The value of the GG ρ γ to be used will be the mean of the GG ρ γ values over the gravity station intervals. [0036] As will be appreciated, the subject method employs density logs to calculate the theoretical gravitational effect of geologic formations in the immediate vicinity of the borehole and then removes these gravitational effects from gravity measurements to reveal the presence of density changes which are remote from the borehole. The mean GG density values, over the vertical interval between successive gravity measurements, are compared with the gravity-derived bulk-density values, thereby to directly indicate an increase or decrease of density away from the borehole. [0037] FIGS. 1 to 3 show examples of probes that can be used to acquire density and/or gravity measurements at intervals along a borehole in order to enable density and gravity logs to be generated. In particular, FIG. 1 is a schematic drawing of a gamma-gamma density probe 2 , having a gamma source 3 and a radiation detector 4 that is suspended in borehole 5 by a cable 1 . Cable 1 incorporates a strength member, as well as electrical members which both control the operation of the probe 2 and transmit density measurements from the probe 2 to the top of the borehole. The region of geologic material 6 around the borehole 5 which influences the gamma-gamma density measurements is shown. [0038] In FIG. 2 , a gravity sensor 7 that is suspended in the borehole 5 by the cable 1 is shown. As is suggested by the configurations shown in FIGS. 1 and 2 , the density probe 2 and gravity sensor 7 allow independent density and gravity measurements to be taken along the same borehole. However, as shown in FIG. 3 , a sonde including both a gamma-gamma density probe 2 and a gravity probe 7 may be used, whereby both gamma-gamma density and gravity measurements can be made in one logging survey of the borehole. [0039] The gamma-gamma density measurements and the gravity measurements are transmitted via one or more of the electrical members in the cable 1 to processing structure (not shown). The processing structure processes the density and gravity measurements as described above to resolve the gravity contribution due to formations intersected by the borehole from the gravity contribution due to density variations that are remote from the borehole. [0040] The processing structure in this embodiment is a general purpose computing device comprising a processing unit, system memory (volatile and/or non-volatile memory), other non-removable or removable memory (e.g. a hard disk drive, RAM, ROM, EEPROM, CD-ROM, DVD, flash memory, etc.), a display and a system bus coupling the various computer components to the processing unit. The computing device may also comprise networking capabilities using Ethernet, WiFi, and/or other suitable network format, to enable connection to shared or remote drives, one or more networked computers, or other networked devices. [0041] Although embodiments have been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the scope thereof as defined by the appended claims.

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