Changes

==How the tool measures gravity==

==How the tool measures gravity==

[[file:applying-gravity-in-petroleum-exploration_fig15-10.png|300px|thumb|{{figure number|1}}Fundamentals of measuring density using a borehole gravity sensor. From Schowalter;<ref name=Schowalter1979>Schowalter, T.T., 1979, Mechanics of secondary hydrocarbon migration and entrapment: AAPG Bulletin, vol. 63, no. 5, p. 723–760.</ref> courtesy AAPG.]]

[[file:applying-gravity-in-petroleum-exploration_fig15-10.png|300px|thumb|{{figure number|1}}Fundamentals of measuring density using a borehole gravity sensor. From Schowalter;<ref name=Schowalter1979>Schowalter, T. T., 1979, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0063/0005/0700/0723.htm Mechanics of secondary hydrocarbon migration and entrapment]: AAPG Bulletin, vol. 63, no. 5, p. 723–760.</ref> courtesy AAPG.]]

[[:file:applying-gravity-in-petroleum-exploration_fig15-10.png|Figure 1]] illustrates the fundamentals of measuring density using a [[borehole gravity]] sensor. Two gravity measurements, ''g''₁ and ''g''₂, are made downhole, separated in depth by Δ''z''. The value ''G'' is the universal gravity constant. Thus, the gravity gradient, Δ''g''/Δ''z'', is related directly to the density of the intervening layer. The result is a direct computation of the bulk density of that layer.

[[:file:applying-gravity-in-petroleum-exploration_fig15-10.png|Figure 1]] illustrates the fundamentals of measuring density using a [[borehole gravity]] sensor. Two gravity measurements, ''g''₁ and ''g''₂, are made downhole, separated in depth by Δ''z''. The value ''G'' is the universal gravity constant. Thus, the gravity gradient, Δ''g''/Δ''z'', is related directly to the density of the intervening layer. The result is a direct computation of the bulk density of that layer.

==Depth of investigation==

==Depth of investigation==

[[file:applying-gravity-in-petroleum-exploration_fig15-11.png|300px|thumb|{{figure number|2}}Depth of investigation is tunable by means of varying the separation between two gravity measurements, Δ''z''. After McCulloh et al.<ref name=ch15r9>McCulloh, T., H., Kandle, J., R., Schoellhamer, J., E., 1968, Application of gravity measurements in wells to problems of reservoir evaluation: Transactions of the 9th Annual SPWLA Logging Symposium. Fundamental work describing the distance of sources seen by the borehole gravity meter.</ref> Copyright: SPWLA.]]

[[file:applying-gravity-in-petroleum-exploration_fig15-11.png|300px|thumb|{{figure number|2}}Depth of investigation is tunable by means of varying the separation between two gravity measurements, Δ''z''. After McCulloh et al.<ref name=ch15r9>McCulloh, T. H., J. R. Kandle, J. E. Schoellhamer, 1968, Application of gravity measurements in wells to problems of reservoir evaluation: Transactions of the 9th Annual SPWLA Logging Symposium. Fundamental work describing the distance of sources seen by the borehole gravity meter.</ref> Copyright: SPWLA.]]

[[:file:applying-gravity-in-petroleum-exploration_fig15-11.png|Figure 2]] shows how the depth of investigation is tunable by means of varying the separation between two gravity measurements, Δ''z''. The rule of thumb is that 90% of the gravity effect can be imaged at a distance away from the borehole within five times Δ''z''.

[[:file:applying-gravity-in-petroleum-exploration_fig15-11.png|Figure 2]] shows how the depth of investigation is tunable by means of varying the separation between two [[gravity]] measurements, Δ''z''. The rule of thumb is that 90% of the gravity effect can be imaged at a distance away from the borehole within five times Δ''z''.

==Depth of investigation of various logging methods==

==Depth of investigation of various logging methods==

The following table, based on data from Beyer<ref name=ch15r1>Beyer, L., A., 1991, [[Borehole gravity]] Surveys: SEG Short Course notes, June, 350 p. Excellent source for general principles of borehole gravity. Very good figures and references.</ref> shows conservative estimates of the depth of investigation using various density logging tools. [[Borehole gravity]] can sample the farthest distance and investigate the most formation.

The following table, based on data from Beyer<ref name=ch15r1>Beyer, L. A., 1991, [[Borehole gravity]] Surveys: SEG Short Course notes, June, 350 p.</ref> shows conservative estimates of the depth of investigation using various density logging tools. [[Borehole gravity]] can sample the farthest distance and investigate the most formation.

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Logging Method

! Logging Method || Radial Distance for 90% Effect, in || Radial Distance for 90% Effect, cm || Formation Volume Investigated, ft³ || Formation Volume Investigated, m³

! Radial Distance for 90% Effect, in

! Radial Distance for 90% Effect, cm

! Formation Volume Investigated, ft³

! Formation Volume Investigated, m³

|-

|-

| Conventional 5.25-in. (13-cm) core

| Conventional 5.25-in. (13-cm) core || 2.6 || 6.6 || 1.5 || 0.04

| 2.6

| 6.6

| 1.5

| 0.04

|-

|-

| Gamma-gamma log

| Gamma-gamma log || 8 || 20 || 17 || 0.5

| 8

| 20

| 17

| 0.5

|-

|-

| Neutron log

| Neutron log || 14 || 36 || 40 || 1.1

| 14

| 36

| 40

| 1.1

|-

|-

| Sonic log

| Sonic log || 18 || 46 || 59 || 1.7

| 18

| 46

| 59

| 1.7

|-

|-

| Borehole gravity log

| Borehole gravity log || 600 || 1,500 || 78,532 || 2,224

| 600

| 1,500

| 78,532

| 2,224

|}

|}

==Logging procedure==

==Logging procedure==

In a typical logging operation, several downhole station locations are planned ahead of time. The tool is fitted to a conventional wireline and lowered to each station. Once the tool is stopped (in some wells it must be clamped to the side of the well), the measurement begins. The gravity values are telemetered to an operator at the surface. Because of vibrations, seismic activity, or residual tool movement, the measurements may take some time to settle to an acceptable noise level.

In a typical logging operation, several downhole station locations are planned ahead of time. The tool is fitted to a conventional wireline and lowered to each station. Once the tool is stopped (in some wells it must be clamped to the side of the well), the measurement begins. The [[gravity]] values are telemetered to an operator at the surface. Because of vibrations, seismic activity, or residual tool movement, the measurements may take some time to settle to an acceptable noise level.

==Tool limitations==

==Tool limitations==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Applying gravity in petroleum exploration]]

[[Category:Applying gravity in petroleum exploration]]