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Publication History:
This article is based on
"Crain's Petrophysical
Pocket Pal" by E. R. (Ross) Crain, P.Eng., first
published in 1987, and updated annually until 2016.
This
webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
permission. |
Devonian Carbonate Reef WITH GAS / OIL /
WATER
This example is
Exercise 3 from
Crain's Practical Quantitative Log
Analysis for Conventional Reservoirs - a Video
Course for CEOs, geoscientists, engineers, and
petrophysicists looking to improve their oil-finding skills.
This example is more complex than first
appearances would indicate. Initially, the resistivity and
porosity logs show a 70 meter pay zone, but study of tests,
production history, and workover history on this and adjacent
wells paint a different story. Water break through has begun in
the lower portion, defined by lower resistivity values at 2158,
2166, and 2173 meters.
The extra high resistivity from 2108 to 2115
meters is a gas cap, which probably extends down to 2130 meters.
Since production rates are severely penalized by government
regulations when GOR is too high, this interval cannot be
completed, leaving only a short interval between 2138 and 2174
meters available for production – about half the “net pay”
interval. Completing too close to the water contact is also
unwise, further restricting the completion interval.
The PE curve indicates clean dolomite, while the
density neutron shows little separation for dolomite in the gas
zone at the top. The raw logs indicate limestone with 8 %
porosity when the zone is really dolomite with 10 – 12%
porosity. This is caused by gas effect canceling the dolomite
effect. Porosity must be computed from the special "Gas in Heavy
Minerals" algorithm over this interval or the results will be
far too pessimistic. Many computer aided log analysis programs
cannot do this without some help from you. Beware of
underestimating porosity in the “Gas in Dolomite” environment.
Although visual observation would provide good porosity values
in the oil and water zones, it is completely inadequate in the
gas and solvent zones.
rAW
LOGS and PETROPYSICAL RESULTS for EXERCISE 3
 
Induction and sonic logs on dolomite reef. Spikes on deep and shallow resistivity
and poor quality sonic
(shaded red) suggest fractures, confirmed
in the core and production rates. Sonic log from offset well
is shown in red to assist in editing the log. Deep induction
sonde error was poorly set and curve pegs at 2000 ohm-m, so
medium induction should be used instead.
Visual log analysis for
Exercise 3, based on Crain's Rules
 
Density neutron PE log
on the dolomite reef. Limestone scale log (left) shows reduced
separation in top 15 – 20 meters, suggesting limy dolomite, but
PE confirms pure dolomite, so it must be gas. Dolomite scale
presentation shows gas crossover at top of reef, minor crossover
for 42 API oil, and no crossover in water zone (so logs are
properly calibrated for dolomite). The gas-oil contact is not
perfectly clear based on crossover. Test shows G/O below 2125
and residual oil in core suggests 2127 to 2130.
 
Computed results over gas interval. Results on
left underestimate porosity in dolomite with gas due to a
conflict between gas crossover and dolomite separation. Results
on the right uses the gas correction given in Section 5.04.
Black dots show core porosity. Some more sophisticated programs
can handle this problem, but only if you allow gas to be
present.
Final petrophysical analysis of the dolomite
reef. Black dots are core data, showing heterogeneous nature of
the reservoir (and the difficulty in comparing the
beer-can-sized core sample to a barrel-sized log reading).
Discussion
A depth plot of the density neutron log on a dolomite scale
helps point this out by creating the gas cross over effect.
Beware of dolomite scales in a limestone rock – the crossover on
the density neutron DOES NOT indicate gas in this situation.
Some very expensive mistakes have been made by inappropriate use
of dolomite scale logs.
There is also a small amount of crossover on the dolomite scale
log in the oil zone caused by the light gravity crude. Notice
that there is no crossover in the water zone, demonstrating that
the dolomite scaling is correct.
Although not shown on any of the depth plots, the porosity from
the sonic log would be very optimistic in some levels, caused by
cycle skipping in fractured rock. In other levels, the porosity
would be several percent too low due to the sonic's inability to
see all vuggy porosity. These observations indicate how
difficult it is to analyze older carbonate wells where the sonic
may be the only porosity log.
Fractures are indicated by skips on the sonic log and spikes on
the density log, as well as low resistivity spikes on both the
deep and shallow resistivity curves. These are also the most
likely places for water break through. The lithology crossplots
show the effect of vugs, fractures, and gas on the sonic log, as
well as confirm the dolomite lithology. Some of the density
spikes caused by hole breakout at fractures still show up on the
final results. These could have been edited to reflect reservoir
conditions instead of borehole effects.
Below are the well history and core data for this well. A
detailed match to porosity from core is usually not possible due
to heterogeneity of the reservoir and the difference in rock
volume seen by logs compared to the core. A good match to
average porosity and total pore volume can be achieved by
adjusting the target matrix density in the computer program. A
permeability match may be possible if pore geometry is uniform
throughout the interval. Heterogeneity, fractures, and vuggy
porosity usually prevent a reasonable permeability from log
analysis.
"META/KWIK"
QUICKLOOK SPREADSHEET RESULTS for EXERCISE 3
The best tool for quicklook log analysis is a spreadsheet.
Pick parameters and log data from the raw logs shown above. When
data entry is complete, the answers are instantly available.
Metric Units Version
English
Units Version
META/KWIK data and results for Exercise
3
WELL HISTORY
Fossil
Joffre 15-22-39-26W4 Twin Well in
10-22-39-26W4
KB
Elev: 908.1 m Logs: DIL-SP, FDC-CNL-GR, BHCS-GR, GR
(COMPL)
Log
depths in METERS
Testing
Record 15-22
DST #1
2120.0 – 2130.0 m Inflate Straddle VO: 1.0/2.0 SI:
28.0/27.0 min
FP: 6550.0/6223.0 kPa SIP: 16410.0/16410.0 kPa HP:
23718.0/23063.0 kPa
High
permeability; No formation damage;
Blow
description: none given, closed chamber.
Recovery: 188m clean condensate 192m mud.
Perf #1
/ Acid Squeeze 2138.5 – 2145.0, 2150.0 – 2155.0, 2158.0 –
2161.6 m
Prod
Test 2138.5 – 2161.6 m VO: 24.0/24.0 No Shut In, No
Pressures
Recovery: 447.2 m3 clean uncontaminated oil
Perf
#2 2107.5 – 2111.0 m Swab Test
Recovery: 5.7 m3 Unknown recovery, Perfs Ineffective???
Perf
#3 2121.0 – 2125.0 m Production Test No Pressures
Recovery: 13.5 m3 Unknown recovery;
Gas 67
600 m3/d (2.387 mmcf/d)
Perf
#4 2171.5 – 2176.5, 2178.0 – 2179.0 Production Test No
Pressures
Gas
9600 m3/d (0.339 mmcf/d)
Perf
#5 2166.0 – 2170.0 m Production Test No Pressures
“not
set”
Perf
#6 2166.0 – 2174.6 m Production Test No Pressures
Recovery: 52.6 m3 clean uncontaminated oil
NOTE:
Bridge plugs between these tests were not reported.
Well history listing for Carbonate Reef Example,
CUMULATIVE PRODUCTION STATISTICS
Cum’l Gas
Oil
Water Inj CO2
10-22 8.4 Bcf 3.6 MM bbl
1.4 MM bbl
15-22 1.9 Bcf 1.4 MM bbl 0.07 MM bbl
5.5 Bcf
Log
analysis production predictions in carbonates are difficult,
and may be impossible, as in this case.
CORE ANALYSIS DATA FOR 10-22-39-26W4
|
10223926W4 |
|
|
|
|
|
|
|
|
|
|
|
|
S# |
Top |
Base |
Len |
Kmax |
K90 |
Kvert |
Porosi |
GrDen |
BkDen |
Soil |
Swtr |
Lithology |
|
|
meters |
meters |
meter |
mD |
mD |
mD |
frac |
kg/m3 |
kg/m3 |
frac |
frac |
|
|
25 |
2122.00 |
2122.28 |
0.28 |
120.00 |
50.70 |
28.80 |
0.101 |
2810 |
2627 |
0.001 |
0.412 |
DOL I VUG CARB VFRAC |
|
26 |
2122.28 |
2122.64 |
0.36 |
11.30 |
5.64 |
8.23 |
0.064 |
2830 |
2713 |
0.001 |
0.182 |
DOL I PPV LV VFRAC |
|
27 |
2122.64 |
2122.86 |
0.22 |
547.00 |
82.00 |
92.20 |
0.105 |
2830 |
2638 |
0.106 |
0.212 |
DOL I VUG STY VFRAC |
|
28 |
2122.86 |
2123.05 |
0.19 |
2110.0 |
2110.0 |
2110.0 |
0.147 |
2810 |
2544 |
0.000 |
0.103 |
DOL I PPV SV CARB |
|
29 |
2123.05 |
2123.47 |
0.42 |
5350.0 |
2880.0 |
32.70 |
0.146 |
2810 |
2546 |
0.087 |
0.174 |
DOL I VUG CARB STY VFRAC |
|
30 |
2123.47 |
2123.67 |
0.20 |
560.00 |
166.00 |
443.30 |
0.148 |
2790 |
2525 |
0.080 |
0.353 |
DOL I MV LV CARB VFRAC |
|
31 |
2123.67 |
2124.10 |
0.43 |
16.00 |
11.30 |
12.00 |
0.074 |
2820 |
2685 |
0.001 |
0.247 |
DOL I VUG CARB VFRAC |
|
32 |
2124.10 |
2124.53 |
0.43 |
15.90 |
14.20 |
11.06 |
0.104 |
2840 |
2649 |
0.000 |
0.205 |
DOL I VUG |
|
33 |
2124.53 |
2124.80 |
0.27 |
5.27 |
3.36 |
1.02 |
0.048 |
2890 |
2799 |
0.000 |
0.133 |
DOL I PPV SV ANHY |
|
34 |
2124.80 |
2125.18 |
0.38 |
267.00 |
113.00 |
11.70 |
0.129 |
2830 |
2594 |
0.001 |
0.290 |
DOL I VUG |
|
35 |
2125.18 |
2125.44 |
0.26 |
192.00 |
130.00 |
11.80 |
0.079 |
2840 |
2695 |
0.113 |
0.271 |
DOL I VUG STY |
|
36 |
2125.44 |
2125.70 |
0.26 |
421.00 |
95.50 |
25.90 |
0.071 |
2830 |
2700 |
0.001 |
0.410 |
DOL I VUG STY VFRAC |
|
37 |
2125.70 |
2126.00 |
0.30 |
572.00 |
572.00 |
1282.0 |
0.129 |
2830 |
2594 |
0.001 |
0.560 |
DOL I VUG |
|
38 |
2126.00 |
2126.21 |
0.21 |
10000 |
10000 |
5.81 |
0.116 |
2830 |
2618 |
0.001 |
0.273 |
DOL I VUG |
|
39 |
2126.21 |
2126.42 |
0.21 |
2.49 |
2.12 |
0.81 |
0.070 |
2830 |
2702 |
0.000 |
0.250 |
DOL I VUG |
|
40 |
2126.42 |
2126.75 |
0.33 |
55.60 |
30.80 |
2.12 |
0.097 |
2840 |
2662 |
0.053 |
0.191 |
DOL I VUG |
|
41 |
2126.75 |
2126.95 |
0.20 |
82.20 |
17.00 |
1.88 |
0.144 |
2840 |
2575 |
0.043 |
0.072 |
DOL I VUG |
|
42 |
2126.95 |
2127.19 |
0.24 |
196.00 |
48.10 |
0.44 |
0.133 |
2840 |
2595 |
0.062 |
0.198 |
DOL I VUG |
|
43 |
2127.19 |
2127.38 |
0.19 |
8.35 |
7.63 |
0.06 |
0.118 |
2840 |
2623 |
0.077 |
0.196 |
DOL I VUG |
|
44 |
2127.38 |
2127.70 |
0.32 |
1840.0 |
1700.0 |
0.21 |
0.103 |
2830 |
2642 |
0.047 |
0.207 |
DOL I VUG |
|
45 |
2127.70 |
2127.94 |
0.24 |
23.60 |
20.50 |
0.23 |
0.117 |
2830 |
2616 |
0.001 |
0.182 |
DOL I VUG FOSS |
|
46 |
2127.94 |
2128.09 |
0.15 |
27.90 |
21.00 |
0.96 |
0.153 |
2830 |
2550 |
0.108 |
0.432 |
DOL I VUG |
|
47 |
2128.09 |
2128.38 |
0.29 |
107.00 |
8.50 |
0.07 |
0.130 |
2830 |
2592 |
0.000 |
0.285 |
DOL I VUG |
|
48 |
2128.38 |
2128.79 |
0.41 |
533.00 |
338.00 |
102.00 |
0.075 |
2840 |
2702 |
0.000 |
0.504 |
DOL I PPV MV |
|
49 |
2128.79 |
2129.26 |
0.47 |
40.20 |
11.30 |
5.19 |
0.068 |
2830 |
2706 |
0.046 |
0.130 |
DOL I VUG STY VFRAC |
|
50 |
2129.26 |
2129.76 |
0.50 |
2340.0 |
1800.0 |
99.70 |
0.097 |
2820 |
2643 |
0.068 |
0.370 |
DOL I VUG |
|
51 |
2129.76 |
2130.32 |
0.56 |
1670.0 |
708.00 |
532.00 |
0.122 |
2820 |
2598 |
0.055 |
0.398 |
DOL I VUG CARB VFRAC |
|
52 |
2130.32 |
2130.83 |
0.51 |
62.30 |
19.80 |
6.16 |
0.086 |
2810 |
2654 |
0.000 |
0.427 |
DOL I VUG CARB VFRAC |
|
53 |
2130.83 |
2131.14 |
0.31 |
2110.0 |
1770.0 |
698.00 |
0.142 |
2820 |
2562 |
0.000 |
0.534 |
DOL I VUG CARB |
|
54 |
2131.14 |
2131.60 |
0.46 |
226.00 |
20.90 |
6.76 |
0.075 |
2830 |
2693 |
0.121 |
0.338 |
DOL I VUG |
|
55 |
2131.60 |
2131.94 |
0.34 |
37.50 |
16.30 |
5.62 |
0.075 |
2840 |
2702 |
0.118 |
0.037 |
DOL I VUG |
|
56 |
2131.94 |
2132.15 |
0.21 |
90.40 |
36.40 |
7.00 |
0.062 |
2830 |
2717 |
0.204 |
0.082 |
DOL I VUG |
|
57 |
2132.15 |
2132.54 |
0.39 |
30.80 |
16.60 |
1.92 |
0.073 |
2830 |
2696 |
0.261 |
0.104 |
DOL I PPV LV VFRAC |
|
58 |
2132.54 |
2132.67 |
0.13 |
81.90 |
48.10 |
88.90 |
0.129 |
2840 |
2603 |
0.180 |
0.072 |
DOL I PPV SV VFRAC |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Arithmetic Averages |
0.36 |
875.1 |
672.8 |
165.8 |
0.104 |
2830 |
2640 |
0.054 |
0.260 |
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Core data listing for Carbonate Reef Example – partial
listing over gas-oil contact
Notice the high permeability streaks on the core analysis caused by
fractures. Lower values show matrix permeability. Vuggy porosity
is mentioned often, suggesting that it would be difficult to
obtain a good porosity analysis from the sonic log. Use the Oil
Saturation column (Soil) to pick the gas-oil contact. Compare to
the crossover on the dolomite scale density neutron log.
Core porosity vs permeability crossplot
shows wide range in permeability caused by fractures. Production history plot show data for original well (large symbols) and
twin well put on production during injection phase (smaller
symbols).
On the core porosity versus permeability plot,
the data scatter is large due to natural fractures. The high
porosity – low perm data points represent matrix permeability.
The lower porosity – high perm points indicate the fracture
permeability.
A solvent flood
was begun after Year 3 to maintain pressure in the twin well,
then production was resumed during Year 6. IPR was 2100 bbl/day
but declined very steeply and continued to decline on the same
trend after the solvent flood was terminated. We also need the
production graph for the twin well to see the overall
performance.
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