CORE SATURATION - OIL AND WATER
 
CORE Fluid saturations
Saturation of a particular fluid is the
proportion of that fluid compared to the porosity:
1: Swtr = Vwtr / PHI
2: Soil = Voil / PHI
3: Sgas = Vgas / PHI
4: Swtr + Soil + Sgas = 1.00
In the laboratory, it is easier to measure weight (mass)
than volume, although both are often recorded.
5: Fluid weight = Weight water + Weight oil
+ Weight gas
OR 6: Fluid weight = DENSwtr * Vwtr + DENSoil * Voil + DENSgas * Vgas
OR 7: Fluid weight = PHI * (DENSwtr * Swtr + DENSoil * Soil + DENSgas *
Sgas)
Saturations
determined from core analysis are full of problems, but the
measurements can be useful for certain situations. The problems are
related to the fact that usually cores are flushed, at least to some
degree, by mud filtrate
during the coring process. This means that much of the hydrocarbon
is removed and replaced by mud filtrate. Formation water in the core
is also altered by invasion. In water based muds, a tritium tracer
can be used while coring. This allows the lab to select samples with
the least invasion, based on the tritium remaining in the core.
Plastic sleeve coring or wrapping of the core upon retrieval
reduces gas loss and water evaporation. Properly handled and
measured in the lab, both residual oil and water saturations can be
useful qualitatively, and often even quantitatively. In an oil zone,
the residual water saturation from core may reflect the irreducible
water saturation in the zone, or at least the actual saturation in a
transition or depleted zone. The residual oil represents the
saturation to expect after an efficient water flood or aquifer
drive.
Cores
taken in oil based mud give a better view of irreducible water, as
these muds do not displace the water. However, gas expansion still
distorts the oil volume.ines
are log analysis.
Bakken “Tight Oil” example showing core porosity (black dots), core
oil saturation (red dots). core water saruration (blue dots), and
permeability (red dots). Note excellent agreement between log
analysis and core data. Separation between red dots and blue water
saturation curve indicates significant moveable oil, even though
water saturation is relatively high. Log analysis porosity is from
the complex lithology model and lithology is from a 3-mineral PE-D-N
model using quartz, dolomite and pyrite.
The main use for core analysis oil saturation is to estimate
minimum possible residual oil saturation, and to assist in locating
gas-oil and oil-water contacts. Gas and water zones have low
residual oil, unless they were once oil zones (recently or in
earlier geologic time). Oil saturation from core analysis is quite
useful in tar sand and sometimes in heavy oil evaluations, where
flushing is minimal.
Cores are stored in boxes on shelves in warehouses. In hot
climates, I have seen oil leaking from core to core, making the
presence of oil in a core somewhat equivocal.
CORE saturation measurements
 A
common method for direct measurement of saturation in a core sample
is the distillation retort method. Core samples are heated, fluids
are vaporized and condensed into a graduated glass receptacle. This
is a rapid method to determine oil and water volumes. Unfortunately,
high temperature (1100 F) may destroy the sample and drives off clay
bound water (CBW). Clay bound water may be estimated by observation
of water volume versus time - pore water is recovered first and clay
bound water later, as the temperature increases..
In a core drilled with water base mud, the oil volume is
divided by the porosity to obtain a residual oil saturation.
Similarly, a water saturation is determined but the sum of Soil +
Swtr will not equal 1.00 due to evaporation of water prior to the
measurement. In an oil based core, the sum of fluid volumes gives
total porosity (PHIt).
In both cases, coking and cracking of the oil reduces oil
volume, resulting in low estimated oil saturation. Core lab
companies scale the recovered oil by a factor to account for this.
The scale factor (KSF) varies from about 1.08 for light oil to 1.28
for heavy oil. Final results are calculated from:
8: Swtr = (Vwtr - CBW) / PHIe
9: Soil = (Voil * KSF) / PHIe
PHIe is usually determined by an independent lab method from
a very nearby core sample.
 The
solvent extraction method is somewhat similar. The core sample is
held in a thimble above a source of solvent, which is heated. The
solvent vapour mobilizes the water, dissolves the oil, and all are
condensed, recovered, and measured.
The method gives an accurate water saturation, can be done
as part of the core cleaning process, and is non-destructive. The
method is slow and can take several days. Oil saturation is
determined by an indirect method, as follows:
10: Swtr = Vwtr / PHIe
11: Voil = ((WTinit - WTdry) - Vwtr *
DENSwtr)
/ DENSoil
12: Soil = Voil / PHIe
Only in rare cases will Soil + Swtr = 1.00 - the balance is
Sgas, usually air that entered the core during transport and
storage.
SAMPLE CORE ANALYSIS REPORT
|
02181815W4R |
#27771 |
780118 |
|
Revised Analysis - Soil and Swtr from
Original Analysis |
|
S# |
Top |
Base |
Len |
Kmax |
K90 |
Kvert |
Poros |
GrDen |
BkDen |
Soil |
Swtr |
Lithology |
|
|
feet |
feet |
feet |
mD |
mD |
mD |
frac |
Kg/m3 |
Kg/m3 |
frac |
frac |
|
|
1 |
3499.19 |
3500.17 |
0.98 |
370.0 |
316.0 |
264.0 |
0.255 |
2850 |
2378 |
0.129 |
0.448 |
SS VF |
|
2 |
3500.17 |
3501.16 |
0.98 |
445.0 |
425.0 |
326.0 |
0.248 |
2680 |
2263 |
0.123 |
0.450 |
SS VF |
|
3 |
3501.16 |
3502.17 |
1.02 |
764.0 |
751.0 |
231.0 |
0.248 |
2670 |
2256 |
0.111 |
0.520 |
SS VF |
|
4 |
3502.17 |
3503.16 |
0.98 |
445.0 |
417.0 |
127.0 |
0.234 |
2670 |
2279 |
0.129 |
0.479 |
SS VF |
|
5 |
3503.16 |
3503.88 |
0.72 |
479.0 |
411.0 |
84.0 |
0.241 |
2700 |
2290 |
0.110 |
0.504 |
SS VF PRY |
|
6 |
3503.88 |
3504.57 |
0.69 |
860.0 |
790.0 |
172.0 |
0.242 |
2680 |
2273 |
0.118 |
0.466 |
SS VF |
|
7 |
3504.57 |
3504.67 |
0.10 |
|
0.1 |
0.1 |
|
|
|
|
|
SHALE |
|
8 |
3504.67 |
3505.26 |
0.59 |
|
0.1 |
0.1 |
|
|
|
0.151 |
0.398 |
RUBBLE |
|
9 |
3505.26 |
3505.49 |
0.23 |
486.0 |
402.0 |
261.0 |
0.246 |
2670 |
2259 |
0.134 |
0.358 |
SS VF SH INC |
|
10 |
3505.49 |
3505.98 |
0.49 |
355.0 |
326.0 |
8.3 |
0.207 |
2640 |
2301 |
0.143 |
0.268 |
SS VF SHBKS |
|
11 |
3505.98 |
3506.96 |
0.98 |
376.0 |
192.0 |
32.2 |
0.240 |
2650 |
2254 |
0.131 |
0.471 |
SS VF |
|
12 |
3506.96 |
3507.88 |
0.92 |
250.0 |
245.0 |
17.6 |
0.218 |
2640 |
2282 |
0.156 |
0.399 |
SS VF CARB INC |
|
13 |
3507.88 |
3508.47 |
0.59 |
491.0 |
0.1 |
0.1 |
0.237 |
|
|
0.119 |
0.389 |
SS VF |
|
14 |
3508.47 |
3508.87 |
0.39 |
304.0 |
0.1 |
0.1 |
0.219 |
|
|
0.136 |
0.422 |
SS VF CARB BK |
|
15 |
3508.87 |
3509.88 |
1.02 |
309.0 |
288.0 |
127.0 |
0.230 |
2850 |
2425 |
0.132 |
0.440 |
SS VF |
|
16 |
3509.88 |
3510.87 |
0.98 |
845.0 |
340.0 |
135.0 |
0.237 |
2660 |
2267 |
0.131 |
0.323 |
SS VF SH INC |
|
17 |
3510.87 |
3511.88 |
1.02 |
298.0 |
287.0 |
75.3 |
0.218 |
2650 |
2290 |
0.146 |
0.422 |
SS VF SH INC |
|
18 |
3511.88 |
3512.87 |
0.98 |
139.0 |
0.1 |
0.1 |
0.208 |
2650 |
2307 |
0.103 |
0.354 |
SS VF |
|
19 |
3512.87 |
3513.79 |
0.92 |
139.0 |
0.1 |
0.1 |
0.174 |
|
|
0.073 |
0.418 |
SS VF |
|
20 |
3513.79 |
3514.38 |
0.59 |
|
0.1 |
0.1 |
|
|
|
0.096 |
0.441 |
RUBBLE |
|
21 |
3514.38 |
3515.07 |
0.69 |
65.1 |
0.1 |
0.1 |
0.257 |
|
|
0.119 |
0.387 |
SS VF |
|
22 |
3515.07 |
3515.16 |
0.10 |
|
0.1 |
0.1 |
|
|
|
|
|
SHALE |
|
23 |
3515.16 |
3516.18 |
1.02 |
1050.0 |
385.0 |
385.0 |
0.254 |
2670 |
2246 |
0.044 |
0.492 |
SS VF |
|
24 |
3516.18 |
3516.77 |
0.59 |
385.0 |
471.0 |
471.0 |
0.220 |
2660 |
2295 |
0.042 |
0.501 |
SS VF |
|
25 |
3516.77 |
3517.46 |
0.69 |
835.0 |
183.0 |
183.0 |
0.237 |
2670 |
2274 |
0.050 |
0.531 |
SS VF CARB INC |
|
26 |
3517.46 |
3518.28 |
0.82 |
901.0 |
644.0 |
644.0 |
0.238 |
2650 |
2257 |
0.046 |
0.487 |
SS VF |
|
27 |
3518.28 |
3519.07 |
0.79 |
438.0 |
103.0 |
103.0 |
0.240 |
2690 |
2284 |
0.079 |
0.494 |
SS VF CARB INC |
|
28 |
3519.07 |
3519.99 |
0.92 |
1430.0 |
278.0 |
278.0 |
0.251 |
2660 |
2243 |
0.063 |
0.501 |
SS VF |
|
29 |
3519.99 |
3520.58 |
0.59 |
|
0.1 |
0.1 |
|
|
|
0.052 |
0.563 |
RUBBLE |
|
30 |
3520.58 |
3521.46 |
0.89 |
1050.0 |
951.0 |
951.0 |
0.258 |
2570 |
2165 |
0.055 |
0.516 |
SS VF |
|
31 |
3521.46 |
3522.48 |
1.02 |
382.0 |
61.5 |
61.5 |
0.210 |
2690 |
2335 |
0.064 |
0.450 |
SS M P/SCARB INC |
|
32 |
3522.48 |
3523.47 |
0.98 |
570.0 |
48.9 |
48.9 |
0.186 |
2680 |
2368 |
0.058 |
0.408 |
SS M P/SCARB INC |
|
33 |
3523.47 |
3524.48 |
1.02 |
|
0.1 |
0.1 |
|
|
|
0.082 |
0.411 |
RUBBLE |
|
34 |
3524.48 |
3525.47 |
0.98 |
3149.0 |
321.0 |
321.0 |
0.209 |
2590 |
2258 |
0.051 |
0.391 |
SS VF |
|
35 |
3525.47 |
3526.48 |
1.02 |
|
0.1 |
0.1 |
|
|
|
0.073 |
0.360 |
RUBBLE |
|
36 |
3526.48 |
3527.47 |
0.98 |
285.0 |
48.8 |
18.8 |
0.170 |
2690 |
2403 |
0.046 |
0.481 |
SS M P/S |
|
37 |
3527.47 |
3528.16 |
0.69 |
193.0 |
0.1 |
0.1 |
0.169 |
2770 |
2471 |
0.042 |
0.548 |
SS M P/S CARB |
|
38 |
3528.16 |
3528.88 |
0.72 |
|
0.1 |
0.1 |
|
|
|
0.066 |
0.462 |
RUBBLE |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Arithmetic Averages |
0.78 |
602.9 |
228.6 |
140.2 |
0.227 |
2679 |
2297 |
0.095 |
0.443 |
|
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Use the
oil saturation (Soil) data in this core analysis example to find
the oil - water contact.
|
