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PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 1
Petroleum Engineering 311Reservoir Petrophysics
Thomas A. Blasingame, Ph.D., P.E.Department of Petroleum Engineering — Texas A&M University
College Station, TX 77843-3116 (USA)+1.979.845.2292
Subsurface Measurement of Porosity24 September 2007
PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 2
Porosity: Grain Density Data
Discussion: Grain Density DataAssumes "perfect" materials.Use for matrix volume calculations (lab).Use for density log calculations (well logs) — must know lithology.
PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 3
Porosity: Travel Time and Density Data
Discussion: Travel Time and Density DataTravel time is inversely related to velocity.Water slows the response the most.Correlate density and travel time?
PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 4
Porosity: Travel Time — Density Correlation
Discussion: Travel Time — Density CorrelationCorrelation is not definitive (unique).Probably due to "averaging" of matrix materials.Interesting to note the narrow range of properties.
Travel Time vs Density
0
20
40
60
80
100
120
140
160
180
200
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.25
Density, g/cc
Travel Time. μsec/ft
Water
Salt
Quartz (Sandstone)
LmSt
Dolm
Anhydrite
PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 5
Porosity: Density Log
Discussion: Density LogDenser the rock, the fewer gamma rays reach the detector.Must KNOW lithology to use porosity relations.Can be affected by gas.
)()(
or ) ()(
for Solving
) ( )( gRearrangin
) (
)1(
fluidmatrix
Logmatrixdensity
matrixfluid
matrixLogdensity
density
matrixfluiddensitymatrixLog
matrixmatrixfluiddensity
matrixdensitymatrixfluiddensity
matrixdensityfluiddensityLog
ρρρρ
φρρρρ
φ
φ
ρρφρρ
ρρρφ
ρφρρφ
ρφρφρ
−
−=
−
−=
−=−
+−=
−+=
−+=
Governing Eqs: Density Log
PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 6
Porosity: Sonic Log
Discussion: Sonic LogMeasures time required for compressional sound waves to travel through one foot of formation.Sound travels more slowly in fluids than in solids. Pore space is filled with fluids. Travel time increases as porosity increases.
)()(
or ) ()(
for Solving
) ( )( gRearrangin
) (
)1(
fluidmatrix
Logmatrixsonic
matrixfluid
matrixLogsonic
sonic
matrixfluidsonicmatrixLog
matrixmatrixfluidsonic
matrixsonicmatrixfluidsonic
matrixsonicfluidsonicLog
tttt
tttt
tttt
ttt
ttt
ttt
Δ−Δ
Δ−Δ=
Δ−Δ
Δ−Δ=
Δ−Δ=Δ−Δ
Δ+Δ−Δ=
Δ−Δ+Δ=
Δ−+Δ=Δ
φφ
φ
φ
φ
φφ
φφ
Governing Eqs: Sonic Log
PETE 311 (07C) — Subsurface Measurement of Porosity Slide — 7
Porosity: Lithostatic Gradient
Discussion: Lithostatic GradientThis is the weight of the overburden — imagine a column of fluid filed rock extending to the surface, this is the weight gradient (converted to a pressure per unit depth).The lithostatic gradient of (1 psi/ft) is THOUGHT to be the maximum pressure per unit depth. However, severely overpressured reservoir systems have shown reservoir pressure gradients as high as 1.05-1.10 psi/ft.
Governing Eqs: Lithostatic Gradient
(psi/ft) 1.01062 (psi/ft) 0.91796 (psi/ft) 0.09266
)g/cc 65.2( g/cc)](psi/ft)/( 433.0[ ))2.0(1( )g/cc 70.1( g/cc)](psi/ft)/( 433.0[ )2.0( g/cc 65.2 and g/cc 700.1 Using
g/cc)](psi/ft)/( 433.0[ ))2.0(1( g/cc)](psi/ft)/( 433.0[ )2.0( 2.0 Assume
g/cc)](psi/ft)/( 433.0[ )1( g/cc)](psi/ft)/( 433.0[
)( )()(
≈+≈
−+≈
==
−+≈=
−+≈
+≈
ssw
ssw
ssw
matrixfluidclithostati p/hp/hp/h
ρρρρ
φρφρφ
