Formation Evaluation: Recent Advances in Logging

Jean-Marc Donadille, Schlumberger

Outline

• Conventional formation evaluation• Porosity• Saturations• Permeability

• Petrophysical challenges: when does conventional interpretation fall short?

• New measurements• Advanced gamma ray induced spectroscopy• Dielectric dispersion• A few NMR applications

• Examples of new formation evaluation

Porosity

(1950’s… Neutron log: 1940’s…)

Saturations+ = Φ

Water saturation: Φ= 𝑆𝑤

Hydrocarbon saturation: Φ= 𝑆ℎ

𝑆𝑤 + 𝑆ℎ = 1

Water is conductiveHydrocarbons are resistive

Resistivity has good sensitivity to saturation

Resistivity log: 1920’s…

Saturations - quantification

Gus Archie (1952): 𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

𝑅𝑤 𝑆𝑎𝑙𝑤, 𝑇

m: pore tortuosity. Original Archie paper: m = 2 for sandstone

n: saturation exponent. Original Archie paper: n= 2 for sandstone

Many extensions to Archie formula many saturation equations, mainly for shale But the principle remains the same…

Impact on oil in place𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

𝑅𝑡 = 20 𝑜ℎ𝑚.𝑚, 𝜙 = 20%

𝑆ℎ = 1 − 𝑆𝑤

Choose nominal values of 𝑆𝑎𝑙𝑤 = 10 𝑝𝑝𝑘,𝑚 = 2, 𝑛 = 2 𝑆𝑤 = 61%, 𝑺𝒉 = 𝟑𝟗%

If instead, choose 𝑆𝑎𝑙𝑤 = 15 𝑝𝑝𝑘,𝑚 = 1.8, 𝑛 = 2… 𝑆𝑤 = 42%, 𝑺𝒉 = 𝟓𝟖%

If instead, choose 𝑆𝑎𝑙𝑤 = 8 𝑝𝑝𝑘,𝑚 = 2.2, 𝑛 = 2.5… 𝑆𝑤 = 86%, 𝑺𝒉 = 𝟏𝟒%

Oil in place increase by 𝟒𝟗%…

Oil in place decrease by 𝟔𝟒%…

Permeability How easily can fluids flow in the system?

Link expected to m and n

Not much to quantify it…

Petrophysical challenges𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

Complex mineralogy

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Bitumen / Kerogen

Permeability

𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

𝑆𝑤𝑛 =

1

𝜙𝑚𝑅𝑤𝑅𝑇

Advanced gamma ray induced spectroscopy

Co

un

ts

Energy

Inelastic

Capture

Neutron source

… …

Advanced spectroscopy answers (from inelastic):

Carbonate: split of calcite (CaCO3) and dolomite (CaMg(CO3)2) (Mg yield)

Shale oil and shale gas clay splitting (Al yield)

ClayQFM

CalciteDolomitePyrite

Unconventionals (kerogen) Total Organic Carbon (C yield)

Petrophysical challenges

Complex mineralogy

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Bitumen / Kerogen

Permeability

Petrophysical challenges

Complex mineralogy

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Bitumen / Kerogen

Permeability

Dielectric dispersionE

E=0

Matrix Oil Water Salt’s ions

Dielectric polarization is sensitive to:

Water-filled porosity: 𝜙𝑤 = 𝜙. 𝑆𝑤

Water salinity: 𝑆𝑎𝑙𝑤

Water phase tortuosity: 𝑀𝑁

Shallow measurement sensitive to the invaded zone.But MN can be used with deep resistivity to estimate oil in place:

𝜙𝑤 =𝑅𝑤𝑅𝑡

1𝑀𝑁

Petrophysical challenges

Complex mineralogy

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Bitumen / Kerogen

Permeability

Petrophysical challenges

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Permeability

Complex mineralogy

Bitumen / Kerogen

NMR applications

𝑇2. 𝑑𝑇2 = 𝜙

Direct correlation between free fluid / bound fluid and permeability:

𝑘 = 𝑎𝜙4𝑓𝑟𝑒𝑒 𝑓𝑙𝑢𝑖𝑑

𝑏𝑜𝑢𝑛𝑑 𝑓𝑙𝑢𝑖𝑑

light

oil

tar heavy

oil

intermediate

oil

Constituent viscosity

Petrophysical challenges

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Permeability

Complex mineralogy

Bitumen / Kerogen

Petrophysical challenges

Fresh water

Unknown / Varying water

Pore tortuosity

Wettability

Permeability

Complex mineralogy

Bitumen / Kerogen

Summary

• Conventional formation evaluation worked fine for easy reservoirs

• Reservoirs currently put on production do not tend to be easy ones…

• Fair progress in interpretation development during the last 10 years to adjust to new challenges

• It should be just a start…

• Keeps petrophysics fun!