Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Predicting reservoir system quality and performance

  | chapter = Predicting reservoir system quality and performance

  | frompg  = 9-1

  | frompg  = 9-46

  | topg    = 9-156

  | topg    = 9-47

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

==What is the Archie equation?==

==What is the Archie equation?==

Archie<ref name=ch09r2>Archie, G., E., 1942, Classification of carbonate reservoir rocks and petrophysical considerations: AAPG Bulletin, vol. 36, no. 2, p. 218–298. A classic paper written way before its time.</ref> developed his famous equation to calculate, from well log parameters, the water saturation (S_w) of the uninvaded zone in a formation next to a borehole. The Archie equation can be expressed as follows:

Archie<ref name=ch09r2>Archie, G. E., 1952, [http://archives.datapages.com/data/bulletns/1949-52/data/pg/0036/0002/0250/0278.htm Classification of carbonate reservoir rocks and petrophysical considerations]: AAPG Bulletin, vol. 36, no. 2, p. 218–298. ''A classic paper written way before its time.''</ref> developed his famous equation to calculate, from well log parameters, the [[water saturation]] (S_w) of the uninvaded zone in a formation next to a borehole. The Archie equation can be expressed as follows:

:<math>\mbox{S}_{\rm w}{}^{\rm n} = \frac{\mbox{R}_{\rm w}}{(\Phi^{\rm m} \cdot \mbox{R}_{\rm t})}</math>

:<math>\mbox{S}_{\rm w}{}^{\rm n} = \frac{\mbox{R}_{\rm w}}{(\Phi^{\rm m} \times \mbox{R}_{\rm t})}</math>

where:

where:

Even though numerous other relationships have been developed over the years, the Archie equation remains the most flexible and generally useful approach. Yet its proper application requires knowledge of its limitations. The equation was empirical in origin and therefore needs modification in rock–fluid combinations different from Archie's experiments. Modifications need to be made in rocks with the following characteristics:

Even though numerous other relationships have been developed over the years, the Archie equation remains the most flexible and generally useful approach. Yet its proper application requires knowledge of its limitations. The equation was empirical in origin and therefore needs modification in rock–fluid combinations different from Archie's experiments. Modifications need to be made in rocks with the following characteristics:

* Non-Archie pore geometries (i.e., not intergranular or intercrystalline)

* Non-Archie pore geometries (i.e., not intergranular or intercrystalline) (See [[Pore systems]].)

* Conductive minerals such as clays and pyrite

* Conductive minerals such as [[clay]]s and [[Wikipedia:Pyrite|pyrite]]

* Very fresh formation waters

* Very fresh (i.e., nonsaline) formation waters

==Caveat==

==Caveat==

==Deriving values for Archie variables==

==Deriving values for Archie variables==

Values for the five Archie variables are relatively easy to derive when a formation is thick, has a clay-free matrix, and/or is dominated by intergranular or intercrystalline porosity (Archie porosity). Formations that are thin bedded (i.e., below limits of logging tool resolution), have clay in their matrix, or have moldic, vuggy, or fracture porosity require adjustments. The table below lists the five variables and methods for deriving or estimating them.

Values for the five Archie variables are relatively easy to derive when a formation is thick, has a clay-free matrix, and/or is dominated by intergranular or intercrystalline porosity (Archie porosity). Formations that are thin bedded (i.e., below limits of logging tool resolution), have clay in their matrix, or have moldic, vuggy, or [[fracture]] porosity require adjustments. The table below lists the five variables and methods for deriving or estimating them.

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Step

! Step || Find || Use… || If… || Then…

! Find

! Use…

! If…

! Then…

|-

|-

| 1

| 1 || n

| n

|

| *  2.0 for Archie porosity *  1.8 (or less) for rocks with clayey matrix or fractures  *  4.0 for very strongly oil-wet rocks

*  2.0 for Archie porosity

*  1.8 (or less) for rocks with clayey matrix or [[fracture]]s

*  4.0 for very strongly [http://www.glossary.oilfield.slb.com/en/Terms/o/oil_wet.aspx oil-wet] rocks

| Not sure of rock type

| Not sure of rock type || Use 2.0

| Use 2.0

|-

|-

| 2

| 2 || R_w

| R_w

|

| *  Value calculated from SP log *  Estimated from R_w catalogs *  Estimated from wet zone R_o value *  Measured from water sample

*  Value calculated from [[Basic_open_hole_tools#Spontaneous_potential|spontaneous potential]] (SP) log  

 

*  Estimated from R_w catalogs  

| Thin beds, hydrocarbons in zone, or fresh formation waters make SP calculations uncertain

*  Estimated from wet zone R_o value  

| Use thin-bed correction or another method

*  Measured from water sample

| Thin beds, hydrocarbons in zone, or fresh formation waters make SP calculations uncertain || Use [[thin-bed correction]] or another method

|-

|-

| 3

| 3 || Φ || Value derived from cores, density, density–neutron, or sonic logs (See [[Basic open hole tools]].) || Density–neutron log matrix setting does not match formation matrix || Use density– neutron crossplot (See [[Determining porosity from density-neutron logs]].)

| Φ

| Value derived from cores, density, density–neutron, or sonic logs

| Density–neutron log matrix setting does not match formation matrix

| Use density– neutron crossplot

|-

|-

| 4

| 4 || m

| m

|

| *  2.0 for Archie porosity *  1.7–2.0 for shaly sandstones *  2.0–2.5 for porosity with connected vugs *  2.5–3.0 for nonconnected moldic porosity *  ~1.0 for fractured rocks

*  2.0 for Archie porosity  

 

*  1.7–2.0 for shaly sandstones  

| Not sure of rock type or pore geometry

*  2.0–2.5 for porosity with connected [http://www.merriam-webster.com/dictionary/vug vugs]

| Use 2.0

*  2.5–3.0 for nonconnected [[Porosity#Carbonate_pore_systems|moldic porosity]]

*  ~1.0 for fractured rocks

| Not sure of rock type or pore geometry || Use 2.0

|-

|-

| 5

| 5 || R_t || Value derived from deep resistivity log such as RILD or RLLD || Beds are thin, invasion occurred or borehole has washouts || Use chartbook corrections

| R_t

| Value derived from deep resistivity log such as RILD or RLLD

| Beds are thin, invasion occurred or borehole has washouts

| Use chartbook corrections

|}

|}

* [[Calculating Rw from SP logs]]

* [[Calculating Rw from SP logs]]

* [[Constructing a Pickett plot]]

* [[Constructing a Pickett plot]]

==References==

==References==

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

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

[[Category:Predicting reservoir system quality and performance]]

[[Category:Predicting reservoir system quality and performance]]