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Porosity—Wireline, LWD General

Sonic Tool
Porosity Evaluation—Open Hole

Purpose This chart is used to convert sonic log slowness time (∆t) values into those for porosity (φ). Description There are two sets of curves on the chart. The blue set for matrix velocity (vma) employs a weighted-average transform. The red set is based on the empirical observation of lithology (see Reference 20).For both, the saturating fluid is assumed to be water with a velocity (vf) of 5,300 ft/s (1,615 m/s). Enter the chart with the slowness time from the sonic log on the x-axis. Move vertically to intersect the appropriate matrix velocity or lithology curve and read the porosity value on the y-axis. For rock mixtures such as limy sandstones or cherty dolomites, intermediate matrix lines may beinterpolated. To use the weighted-average transform for an unconsolidated sand, a lack-of-compaction correction (Bcp) must be made. Enter the chart with the slowness time and intersect the appropriate compaction correction line to read the porosity on the y-axis. If the compaction correction is not known, it can be determined by working backward from a nearby clean water sand for which the porosity isknown.

Example: Consolidated Formation Given: ∆t = 76 µs/ft in a consolidated formation with vma = 18,000 ft/s. Find: Porosity and the formation lithology (sandstone, dolomite, or limestone). Answer: 15% porosity and consolidated sandstone. Example: Unconsolidated Formation Given: Unconsolidated formation with ∆t = 100 µs/ft in a nearby water sand with a porosity of 28%. Find: Porosity of theformation for ∆t = 110 µs/ft. Answer: Enter the chart with 100 µs/ft on the x-axis and move vertically upward to intersect 28-p.u. porosity. This intersection point indicates the correction factor curve of 1.2. Use the 1.2 correction value to find the porosity for the other slowness time. The porosity of an unconsolidated formation with ∆t = 110 µs/ft is 34 p.u.

Lithology Sandstone Limestone Dolomitevma (ft/s) 18,000–19,500 21,000–23,000 23,000–26,000

∆tma (µs/ft) 55.5–51.3 47.6–43.5 43.5–38.5

vma (m/s) 5,486–5,944 6,400–7,010 7,010–7,925

∆tma (µs/m) 182–168 156–143 143–126

Por

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201

Porosity—Wireline, LWD

Sonic Tool
Porosity Evaluation—Open Hole

Por-1
(customary, former Por-3)

vf = 5,300 ft/s 50 Time average Field observation 50

401.1 1.2 1.3
) ne sto e (lim

40

1.4 1.5 1.6 Bcp Porosity, φ (p.u.) 20 30

30
te
Do lom i
Ca lci t

e

Porosity, φ (p.u.) 20 vma (ft/s)
26 , 23 000 21 ,000 19 ,000 18 ,500 ,00 0

Q

e ton ds n sa rtz ua

10

10

Por

0 30 40 50 60 70 80 90 100 110 120 Interval transit time, ∆t (µs/ft)

0 130

© Schlumberger

202

Porosity—Wireline, LWD

Sonic Tool
PorosityEvaluation—Open Hole

Por-2
(metric, former Por-3m)

vf = 1,615 m/s 50 Time average Field observation 50

40

1.1 1.2 1.3
Do l
ite om

40

30

ite alc C

ne sto nd sa tz ar Qu

1.4 1.5 1.6 Bcp Porosity, φ (p.u.) 30

Porosity, φ (p.u.)
8,0 00 7 6,4 ,000 5,9 50 00 5,5 D 00 Ce C ol Qu men alci omit te e a rt t z s ed q an ua ds ton rtz s an e ds ton e

20 vma (m/s)

2010

10

0 100

0 150 200 250 300 350 400 Interval transit time, ∆t (µs/m)

Por

© Schlumberger

Purpose This chart is used similarly to Chart Por-1 with metric units.

203

Porosity—Wireline, LWD

Density Tool
Porosity Determination—Open Hole

Por-3
(former Por-5)

ρf (g/cm3) 1.1

1.0 0.9 0.8

30

ρ

Porosity, φ (p.u.)

20

φ=

ρma – ρb ρma – ρf

10

02.8 2.6 2.4 Bulk density, ρb (g/cm )
3 *Mark of Schlumberger © Schlumberger

ρ

ma

=2 mi te = 2 . 71 ) (c .68 alc =2 ite .6 5 ) (q ua rtz sa nd sto ne )
ma

1.2 40

ρ

ma

ρ

ma

=2 = 2 .87 (d .83 olo

ρ

ma

2.31

2.2

2.0

Por

Purpose This chart is used to convert grain density (g/cm3) to density porosity. Description Values of log-derived bulk density (ρb)...
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