Changes

==Basic concepts==

==Basic concepts==

The borehole gravity meter (BHGM) can be described simply as a deep-investigating density logging tool. Applications range beyond this simple description to include detection of oil- and gas-filled [[porosity]] and detection and definition of remote structures, such as salt domes, faults, and reefs.

[[file:borehole-gravity_fig1.png|thumb|300px|{{figure number|1}}An example of a BHGM log. The sharp difference In density between 6330 and 6370 ft is caused by porosity not detected by the gamma-gamma [[density log]]. The broader difference anomaly observed over the length of the logged interval is explained by the structural influence of the reef complex.]]

One of the great advantages of the BHGM as a density logging tool is that it is practically unaffected by near-borehole influences, which are the scourge of nuclear tools: casing, poor cement bonding, rugosity, washouts, and fluid invasion. Another advantage is the fundamental simplicity of the relationships among gravity, mass, rock volume, and density. Complex geology can be easily modeled so that the response of a range of hypothetical models can be studied and understood before undertaking a survey.

The borehole [[gravity]] meter (BHGM) can be described simply as a deep-investigating density logging tool. Applications range beyond this simple description to include detection of oil- and gas-filled [[porosity]] and detection and definition of remote structures, such as salt domes, faults, and [[reef]]s.

What is actually measured is referred to as ''BHGM apparent density'', which is a simple function of the measured vertical gradient of gravity. To obtain an apparent density measurement, gravity is measured at two depths. The accuracy of the computed density depends on the accuracy of both measured differences: gravity and depth. Operationally, BHGM surveys resemble vertical seismic profiling (VSP) surveys. The BHGM is stopped at each planned survey level, and a 5- to 10-min reading is taken. The blocky appearance of the log reflects the station interval (Figure 1).

One of the great advantages of the BHGM is that it is the only logging method that can directly measure density at a significant distance away from a well. It is also the only logging method that can reliably obtain density through well casing.<ref name=Chapin, David A. and Mark A. Ander>Chapin, D. A. and M. A. Ander, 1999, [http://archives.datapages.com/data/specpubs/beaumont/ch15/ch15.htm Applying borehole gravity methods], in Exploring Oil and Gas Traps: [http://store.aapg.org/detail.aspx?id=545 AAPG Treatise of Petroleum Geology 3], p. 15-15,</ref> It is practically unaffected by near-borehole influences, which are the scourge of nuclear tools: casing, poor cement bonding, rugosity, washouts, and fluid invasion. Another advantage is the fundamental simplicity of the relationships among gravity, mass, rock volume, and density. Complex geology can be easily modeled so that the response of a range of hypothetical models can be studied and understood before undertaking a survey.

[[file:borehole-gravity_fig1.png|thumb|{{figure number|1}}An example of a BHGM log. The sharp difference In density between 6330 and [[depth::6370 ft]] is caused by porosity not detected by the gamma-gamma density log. The broader difference anomaly observed over the length of the logged interval is explained by the structural influence of the reef complex.]]

What is actually measured is referred to as ''BHGM apparent density'', which is a simple function of the measured vertical gradient of gravity. To obtain an apparent density measurement, gravity is measured at two depths. The accuracy of the computed density depends on the accuracy of both measured differences: gravity and depth. Operationally, BHGM surveys resemble [[Checkshots_and_vertical_seismic_profiles#Vertical_seismic_profiles|vertical seismic profiling (VSP)]] surveys. The BHGM is stopped at each planned survey level, and a 5- to 10-min reading is taken. The blocky appearance of the log reflects the station interval ([[:file:borehole-gravity_fig1.png|Figure 1]]).

The log is not continuous. BHGM measurements are taken at discrete depths usually at intervals of 10 to [[length::50 ft]], depending on the resolution required. While the BHGM has remarkable resolution in the measurement of density over intervals of [[length::10 ft]] or more (less than 0.01 g/cm³), surveys requiring closer vertical resolution must sacrifice density resolution.

The log is not continuous. BHGM measurements are taken at discrete depths usually at intervals of 10 to [[length::50 ft]], depending on the resolution required. While the BHGM has remarkable resolution in the measurement of density over intervals of [[length::10 ft]] or more (less than 0.01 g/cm³), surveys requiring closer vertical resolution must sacrifice density resolution.

==Applications==

==Applications==

The spectrum of BHGM applications is defined on one extreme by density logging and on the other by remote sensing of structure. The first extreme sometimes focuses strictly on formation and reservoir evaluation questions, while the other extends to basic exploration. Figure 1 is an example of both applications. In fact, the purpose of the survey was to detect carbonate porosity in a reef environment that was missed by the other logs. For this objective, the useful radius of investigation of the measurement is on the order of [[length::50 ft]]. The sharp negative density anomaly observed between 6330 and [[depth::6370 ft]] suggests porosity obscured by near-borehole effects or poor volume sampling. However, the broad departure of the BHGM and gamma-gamma logs over the depth range of the logged section is typical of a structural effect, in this case the edge of the reef complex which is within a few hundred feet.

The spectrum of BHGM applications is defined on one extreme by density logging and on the other by remote sensing of structure. The first extreme sometimes focuses strictly on formation and reservoir evaluation questions, while the other extends to basic exploration. Figure 1 is an example of both applications. In fact, the purpose of the survey was to detect carbonate porosity in a [[reef]] environment that was missed by the other logs. For this objective, the useful radius of investigation of the measurement is on the order of [[length::50 ft]]. The sharp negative density anomaly observed between 6330 and [[depth::6370 ft]] suggests porosity obscured by near-borehole effects or poor volume sampling. However, the broad departure of the BHGM and gamma-gamma logs over the depth range of the logged section is typical of a structural effect, in this case the edge of the reef complex which is within a few hundred feet.For more information about this log and for more examples of applications, see [[Borehole gravity applications: examples]].

===Density logging===

===Density logging===

Borehole gravity density measurements are unhindered by casing, poor hole conditions, and all but the deepest fluid invasion. The BHGM measurement samples a large volume of rock, which provides a density-porosity value that is more representative of the formation. This is especially beneficial in carbonate and fractured reservoirs<ref name=pt07r48>Rasmussen, N. F., 1975, The successful use of the borehole gravity meter in Northern Michigan: The Log Analyst, v. 16, n. 5, p. 3–10.</ref>. BHGM surveys have been used to find hydrocarbon-filled porosity missed by other logs in both open and cased holes. Gas-saturated sands are a particularly easy target<ref name=pt07r16>Gournay, L. S., Maute, R. E., 1982, Detection of bypassed gas using borehole gravimeter and pulsed neutron capture logs: The Log Analyst, v. 23, n. 3, p. 27–32.</ref>.

Borehole gravity density measurements are unhindered by casing, poor hole conditions, and all but the deepest fluid invasion. The BHGM measurement samples a large volume of rock, which provides a density-porosity value that is more representative of the formation. This is especially beneficial in carbonate and fractured reservoirs.<ref name=pt07r48>Rasmussen, N. F., 1975, The successful use of the borehole gravity meter in Northern Michigan: The Log Analyst, v. 16, n. 5, p. 3–10.</ref> BHGM surveys have been used to find hydrocarbon-filled porosity missed by other logs in both open and cased holes. Gas-saturated sands are a particularly easy target.<ref name=pt07r16>Gournay, L. S., and R. E. Maute, 1982, Detection of bypassed gas using borehole gravimeter and pulsed neutron capture logs: The Log Analyst, v. 23, n. 3, p. 27–32.</ref>

The wide radius of investigation has also been successfully used to determine gas-oil and oil-water contacts in reservoirs where other measurements have been ineffective<ref name=pt07r60>van Popta, J., 1990, Use of Borehole gravimetry for reservoir characterization and fluid saturation monitoring: Society of Petroleum Engineers, SPE 20896.</ref><ref name=pt07r51>Schultz, A. K., 1989, Monitoring fluid movement with the Borehole gravity meter: Geophysics, v. 54, n. 10., p. 1267–1273., 10., 1190/1., 1442586</ref>. BHGM density measurements have been used to calculate hydrocarbon saturations: the larger the fluid density contrast, the larger the measured effect. Gas saturations are therefore the easiest to measure. The difference in densities measured by the gamma-gamma log and the BHGM can be used to calculate the difference in oil saturation between the invaded and undisturbed zones, which can in turn give an estimate of movable hydrocarbons.

The wide radius of investigation has also been successfully used to determine gas-oil and oil-water contacts in reservoirs where other measurements have been ineffective.<ref name=pt07r60>van Popta, J., 1990, Use of Borehole gravimetry for reservoir characterization and fluid saturation monitoring: Society of Petroleum Engineers, SPE 20896.</ref><ref name=pt07r51>Schultz, A. K., 1989, Monitoring fluid movement with the Borehole gravity meter: Geophysics, v. 54, n. 10, p. 1267–1273, 10, 1190/1, 1442586</ref> BHGM density measurements have been used to calculate hydrocarbon saturations: the larger the fluid density contrast, the larger the measured effect. Gas saturations are therefore the easiest to measure. The difference in densities measured by the gamma-gamma log and the BHGM can be used to calculate the difference in oil saturation between the invaded and undisturbed zones, which can in turn give an estimate of movable hydrocarbons.

===Remote sensing===

===Remote sensing===

A useful and practical rule of thumb for BHGM remote sensing applications is that a remote body with sufficient density contrast can be detected by the BHGM no farther from the wellbore than one or two times the height of the body. A salt dome with [[depth::15,000 ft]] of vertical relief would have a definitive signature a few miles away. A channel sand [[length::20 ft]] thick would be detectable no more than [[length::40 ft]] away. Local geology, and in particular the thickness of local density units, defines the effective radius of investigation of the BHGM.

A useful and practical rule of thumb for BHGM remote sensing applications is that a remote body with sufficient density contrast can be detected by the BHGM no farther from the wellbore than one or two times the height of the body. A salt dome with [[depth::15,000 ft]] of vertical relief would have a definitive signature a few miles away. A channel sand [[length::20 ft]] thick would be detectable no more than [[length::40 ft]] away. Local geology, and in particular the thickness of local density units, defines the effective radius of investigation of the BHGM.

Computer modeling of BHGM measurements can be used to develop relatively detailed salt dome flank or reef flank model interpretations. Modeling is particularly effective where seismic data can be integrated into the modeling process; a model is sought that is consistent with both data sets<ref name=pt07r34>Lines, L. R., Schultz, A. K., Treitel, S., 1988, Cooperative inversion of geophysical data: Geophysics, v. 53, n. 1, p. 820., 10., 1190/1., 1442403</ref>.

Computer modeling of BHGM measurements can be used to develop relatively detailed salt dome flank or reef flank model interpretations. Modeling is particularly effective where seismic data can be integrated into the modeling process; a model is sought that is consistent with both data sets.<ref name=pt07r34>Lines, L. R., A. K. Schultz, S. Treitel, 1988, Cooperative inversion of geophysical data: Geophysics, v. 53, n. 1, p. 820, 10, 1190/1, 1442403</ref>

==Theory==

==Theory==

==Operating limitations==

==Operating limitations==

The gravity meter itself is a delicate spring balance that measures changes in weight of a small proof mass. The meter must be leveled at each station, and it is accurately thermostatted at a temperature of about 126ΰC. The present LaCoste and Romberg meter cannot operate in a wellbore deviated from vertical by more than 14°. Table 1 shows a summary of maximum operating conditions for the meter.

The gravity meter itself is a delicate spring balance that measures changes in weight of a small proof mass. The meter must be leveled at each station, and it is accurately thermostatted at a temperature of about 126°C. The present LaCoste and Romberg meter cannot operate in a wellbore deviated from vertical by more than 14°. Table 1 shows a summary of maximum operating conditions for the meter.

{| class = "wikitable"

{| class = "wikitable"

==See also==

==See also==

* [[Seismic data acquisition on land]]

* [[Borehole gravity: uses, advantages, and disadvantages]]

* [[Borehole gravity tool]]

* [[Borehole gravity applications: examples]]

* [[Vertical and lateral seismic resolution and attenuation]]

* [[Forward modeling of seismic data]]

* [[Amplitude versus offset (AVO) analysis]]

* [[The gravity method]]

==References==

==References==

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]