gas hydrate evaluation / cyclostratigraphy · 2008-03-13 · water saturation of sample f formation...
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Gas hydrate evaluation / Cyclostratigraphy
Alberto Malinverno
Well Logging Principles and ApplicationsG9947 - Seminar in Marine Geophysics
Spring 2008
Outline
• Introduction to gas hydrates
• Estimating gas hydrate abundance with downhole logs
- Electrical resistivity
- Acoustic
- Nuclear magnetic resonance (NMR)
• Cyclostratigraphy with downhole logs
Fire and ice
Clathrate (Chemistry) a compound in which molecules of one component are physically trapped within the crystal structure of another.ORIGIN 1940s: from L. clathratus, from clathri ‘lattice-bars’.
GH stabilityNo gas hydrate abovethis depth
Gashydratestabilityzone (GHSZ)
Water + free gas
Water + gas hydrate
Kvenvolden & Grantz, 1990
{
Archie’s equation (1)
R0 Resistivity of fully water- saturated sampleRw Resistivity of water
F Formation factor > 1a Tortuosity coefficient ≈ 1m Cementation exponent ≈ 2
Archie’s equation (2)
R0 Resistivity of fully water- saturated sampleRw Resistivity of waterRt Resistivity of sample
Sw Water saturation of sampleF Formation factor > 1a Tortuosity coefficient ≈ 1m Cementation exponent ≈ 2n Saturation exponent ≈ 2
Archie’s equation
R0 Resistivity of fully water- saturated sampleRw Resistivity of waterRt Resistivity of sample
Sw Water saturation of sampleF Formation factor > 1a Tortuosity coefficient ≈ 1m Cementation exponent ≈ 2n Saturation exponent ≈ 2
Needed for evaluation
• Porosity and “true” formation resistivity (from downhole logs)
• Formation water resistivity (from local pore water salinity and temperature)
• Coefficients a, m, and n
0.2 0.4 0.6 0.8
50
100
150
200
250
Depth
(m
bsf)
Porosity1
Resistivity (ohm m)0 0.2 0.4 0.6
50
100
150
200
250
GH saturation
R0 R
t
Fig. 6 - Malinverno et al.
10
a b c
Rw from measured T and salinity baseline
a = 2.14, m = 1.07 from fitting R0 to Rt (20-190 mbsf)
n = 2.42 ± 0.28 from matching chlorinity results in 9 cm-thick sand
Sh = 1 !!
a Rw!mRt
"1n
GH
Bulk modulus K relates compressional stress and strain
Shear modulus μ relates shear stress and strain
Ks, µs Elastic moduli of the rock grainsKf Bulk modulus of fluidρs, ρf Rock grain and fluid densitiesA Pore shape aspect ratio ≤ 1β, β' ≈ 1
R1 = 1 Ω · mR2 = 100 Ω · m
Unit CubeHorizontal
current flowVertical
current flow
Harmonic meanArithmetic mean
R2
R1