Quick-Look Log Interpretation
S c h l u m b e r g e r P r i v a t e
S c h l u m b e r g e r P r i v a t e
E. Standen NExT Training Copyrght 2003, NExT
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Basal Quartz Quartz Example – Valley Valley Fill Sequence Sequence Rmf = 2.6 @ 60F, BHT BHT = 130F B a s a l Q u a r tz N o . 1 0 6 / 2 8 / 2 0 0 2 1 0 : 0 2 :0 6 A M DEPTH FT
G R ( G A P I) I) 0 .
IL D ( O H M M ) 1 5 0 .
0 .2
C A L I ( IN IN ) 6 . S P 1:500
-2 0 0 .
P H ID ID ( V / V ) 2 0 0 0 .
0 .4 5
IL M ( O H M M ) 1 6 .
0 .2
0 .
0 .2
(M V )
- 0 .1 5 P H IN IN S S
2 0 0 0 .
0 .4 5
(V /V) - 0 .1 5
S F L ( O H M M ) 2 0 0 0 .
S c h l u m b e r g e r P r i v a t e
5 4 0 0
5 5 0 0
Copyrght NExT 5 6 02003, 0
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Rock Matrix, Porosity & Fluids
S c h l u m b e r g e r P r i v a t e
Rt = Rw
Rt = Ro
Rt = F Rw / Sw
2
S c h l u m b e r g e r P r i v a t e
Ro = F Rw where
F=a/
m
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Archie’s Equation Empirical constant (usually near unity)
S w
n
Water n saturation, fraction Saturation exponent (also usually near 2) Copyrght 2003, NExT
a Rw
Resistivity of formation water, -m
m
Rt Porosity, fraction
Cementation exponent (usually near 2)
Resistivity of uninvaded formation, -m 4
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Resistivity & Lithology - Saturation • Low Resistivity is a water-wet formation. • Wet Sands/Carbonates
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• Shale
• High Resistivity is a formation with no water. • Low Porosity – no water • Hydrocarbon present – low volume of water (Swirr)
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• Or, VERY FRESH water
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Hydrocarbon Identification from Resistivity and SP.
High Resistivity => HC
or Tight? (check Φ)
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Clean Low Resistivity => Water-Wet
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Quick-look HC Identification & Flow Unit Analysis • Highlight the deep resistivity log. • Highlight Sonic or Density log as Porosity. • Both Sonic and Density read higher in Gas
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• In a porous, wet zone (ie. Low Resistivity and High Porosity) overlay the porosity on the deep resistivity log, keeping the logs parallel and on depth. • Hydrocarbon is indicated where separation occurs – high resistivity and high porosity.
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• If you change the relative position of the porosity and resistivity curves it implies a change in Rw. Copyrght 2003, NExT
Gamma Ray
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Neutron – Density Porosity Log
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Trace Density or overlay on a light table.
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Since we are dealing with log-compatible overlay scales, the density curve on the resistivity scale now defines Ro, the wet resistivity of the formation.
S c h l u m b e r g e r P r i v a t e
S c h l u m b e r g e r P r i v a t e
Overlay Logs Here Copyrght 2003, NExT
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HC
HC 5400
Water Wet
S c h l u m b e r g e r P r i v a t e
Water
1
hc?
hc?
5500
hc
HC
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Water Wet
5600
Sw Calculations • Get Rw from the SP or Rwa in a 100% wet zone. • Compute Sw from Deep Resistivity and Density or Sonic porosity. • Or • Compute Sw from Deep Resistivity and the average of Neutron and Density porosity total). • Do not mix porosities in your computations.
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• If shale resistivity is much lower than Rt in the hydrocarbon zone, be aware that no correction for the shale effect on Rt has been made and you should consider a shaly-sand interpretation model.
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• An alternative to individual computations is to plot porosity and resistivity on a Picket Plot. Copyrght 2003, NExT
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Rwa Method • Rwa is the apparent water resistivity assuming all zones are 100% wet. • If Sw = 100% then: Rwa =
**2 x Rt
• If the zone is 100% wet then Rwa will go to a minimum value. • If hydrocarbon is present then Rwa > Rw. • (Rwa will be less than Rw in low porosity zones!)
• In hydrocarbon zones Sw = Rw/Rwa Copyrght 2003, NExT
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S c h l u m b e r g e r P r i v a t e
Rwa Computation for BQ Example Using Rild and PhiD (density porosity) Basal Quartz No.1 06/26/2002 5:09:08 PM DEPTH FT 0.
GR(GAPI)
ILD(OHMM)
PHID( V/V)
150. 0.2
2000. 0.45
CALI (IN)
-0.15
ILM(OHMM)
6.
PHINSS(V/V)
16. 0.2
2000. 0.45
SP(MV)
-0.15
SFL (OHMM)
-200.
0. 0.2
2000.
S c h l u m b e r g e r P r i v a t e
Rwa (ohmm) 0.002
1:500
20.
5400
Rwa = .025 ohmm
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S c h l u m b e r g e r P r i v a t e
Note that where PhiD goes to zero 13 Rwa goes lower than Rw.
Pickett Plot – ILD vs PhiD Basal Quartz No.1 Rw = 0.025 ohmm ILD / PHID Interval : 5340. : 5608. 1. 0.9 0.8 0.7 0.6 0.5 0.4 0.3
S w = 1 0 0 %
Hydrocarbon Zones plot above Sw=100% line.
M=2
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120.
Water zones
0.2 D I H P
GR 150.
0.1 0.09 0.08 0.07 0.06 0.05 0.04
90.
60.
0.03
30.
0.02
0.01 0.01
0.1
1.
10.
100.
0. 1000.
ILD 446 points plotted out of 537 Well NExT Copyrght 2003, Ba sa l Qua rtz No. 1
Depths 5 34 0. F - 5 60 8. F
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Simple Shaley-Sand Model Φtotal = Φeffective
HC
Clean Sand Matrix (Quartz)
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In a clean sand the irreducible water volume is a function of the surface area of the sand grains and therefore, the Irreducible grain size. water Bound water
Clean Sand Matrix (Quartz)
Φeffective
Clay + Silt
In a shaley-sand the addition of silt + clay usually decreases effective porosity due to poorer sorting and increases the irreducible water volume with the finer grain size. In addition, there is clay bound water that is non-effective porosity that adds conductivity to the formation.
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Φtotal
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Quick-Look Shaley-Sand Analysis Sw = 1/ T**2 x Rw/Rt total = (PhiN + PhiD)/2 effective = total x (1 – Vsh) In a clean formation PhiN = PhiD and Phi-Total is Phie. In a shaley formation PhiN + PhiD / 2 usually increases slightly as shale volume increases (Shale total porosity is usually higher than the total porosity of a clean sand until significant compaction occurs). As shale increases Rt will decrease so the net effect on the saturation computation is minimal as shale volume increases. Copyrght 2003, NExT
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S c h l u m b e r g e r P r i v a t e
Archie’s Equation
S w
a Rw n
S c h l u m b e r g e r P r i v a t e
m Total
Rt S c h l u m b e r g e r P r i v a t e
As Shale (clay) volume increases – What is the effect on Sw? Up to about 20% Vshale not much effect will be seen on Sw as long the porosity input is Total Porosity, not Effective porosity. Copyrghtas 2003, NExT 17
Bulk Volume Water • What is the volume of water in the formation? • Answer: Sw x Φ = BVW • Assume basic Archie:
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• Sw**2 = (1/Φ**2) * Rw/Rt
• Sw**2 x Φ**2 = Rw/Rt •
or Sw*Φ=
Rw/Rt
• Rt is on a logarithmic scale - it is inversely proportional to BVW. – low Rt = high BVW and high Rt = low BVW.
• As long as BVW is changing with porosity you are not in the zone of irreducible water saturation. Copyrght 2003, NExT
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Assume ILD = Rt, then BVW is proportional to 1/Rt
Lowest BVW S c h l u m b e r g e r P r i v a t e
High Resistivity
Clean zone S c h l u m b e r g e r P r i v a t e
Low BVW Low Resistivity Copyrght 2003, NExT
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High BVW
Φ = 12%
Φ= 6 to 15%
Φ = 19% Φ = 19%
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Sw=100%
Φ = 18%
Sw=100%
Φ = 19%
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S c h l u m b e r g e r P r i v a t e
Ellerslie Example BVW Computation depth
Phi
Rt
Sw
BVW
BVW as Cap. Pressure 5350
0.12
15 0.372678
447
2500 S c h l u m b e BVW r g e r P r i v a t e
2000
5374 5378 5382
0.09 0.13 0.06
25 0.3849 27 0.25641 22 0.615457
346 333 369
5392 5396
0.12 0.18
28 0.272772 14 0.257172
327 463
W1500 V B1000 500 0
5 0 5 3
Water free production 5408
5420 5428
0.19
0.16 0.15 0.19
9 6 5 3
2 0 5 4
depth
7 0.344555
1.1 1.032154 1.5 0.942809
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5436
7 8 5 3
0.8 1.019206
S c h l u m b e r g e r P r i v a t e
655
We could plot Sw vs. depth as well, but saturation varies more with changes in porosity. BVW goes to a minimum when all 1651 rock types reach Swirr and is therefore, an easier number to 1414 use for determining water-free production. 21
1936
BVW related to Cap. Pressure Swirr x Porosity = BVW at irreducible saturation conditions. This means that when BVW approaches a low constant value for a formation it will produce water free above that point. Above the Swirr point, changes in BVW will reflect changes in pore size (grain size) or a change in HC fluid content. Remember that Swirr is 100 unique for each rock unit.
S c h l u m b e r g e r P r i v a t e
Pressure Or Depth
0
Swirr Swirr
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Low BVW
Hi BVW
Sw
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B a s a l Q u a r tz N o . 1 02/25/2003 3:22:07 PM DEPTH FT
S W ( D ec ) 0.
P H iT ( v / v ) 1.
0 .2 5
V W C L ( De c ) 0.
0.
0.
1.
0.
0.
P H IE ( D e c ) 0 .2 5 BVW 1:500
Capillary Pressure from Log Data
0 .2 5
1. P H IE ( D e c )
( D ec )
0. V S IL T ( D e c ) 1.
5400
S c h l u m b e r g e r P r i v a t e
1
5500
S c h l u m b e r g e r P r i v a t e
5600
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BVW Plot with Permeability – K4 “Buckles Plot” K= {70* Φe**2[(1-Swi)/Swi]}**2 Rock unit 1
Rock unit 2
Water zone Transition zone
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S c h l u m b e r g e r P r i v a t e
BVW Rules of Thumb • eg. For: Sw=20% & Φ=30%, BVW=600 • For water free production in clean zones… • Carbonates:
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• Oil : BVW= 150 to 400 • Gas: BVW= 50 to 300
• Course-grained Sands: • Oil : BVW = 300 to 600 • Gas : BVW = 150 to 300
• Very fine-grained Sands S c h l u m b e r g e r P r i v a t e
• Oil : BVW = 800 to 1200 • Gas : BVW = 600 to 900
• Note: This will depend on the position in the HC column. – Higher up gives a lower BVW. Copyrght 2003, NExT
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For our Sand Example • BVWirr ranges from 460 to 330. • Since we expect light oil & gas production from the zone we can estimate that the rock should be a coarse-grained sand. • Zones of higher BVW above the oil-water contact would indicate finer grain-size rock units. • Log saturations should match core capillary pressure data for any given rock type. Copyrght 2003, NExT
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S c h l u m b e r g e r P r i v a t e
