Devonian Carbonate Reef WITH GAS
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.

 


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.



 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.

 

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.

 

 

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.

      

Most computer software does not handle gas in dolomite very well and underestimate porosity and over estimate water saturation. The illustration on the next page show the difference between no gas correction and a reasonable gas correction. The math is given earlier in this Handbook in Section 5.05.

 



 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).

 

 

       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

 

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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