K08A109 well (K11 Zebra) PP evaluation
Femi Onita
EP201001302275
January 2010
Reviewed by
Oscar Kelder
Reema Mohanty
Walter Slijkerman
Summary and Conclusions
Summary
Form.
name
Interval
Gross
Net
N/G
Por
Sh
K
mAH(TV)
mAH(TV)
%
%
%
mD
2949.8
2953.8
3074.3
4.0
120.5
32.0
116.7
24.7
96.8
77.1
13.6
10.2
85.2
78.7
18.8
1.5
3106.3
3174.3
68.5
80
68.5
0.1
100.0
0.2
15.4
6.2
89.1
67.6
62.4
0.2
m
AHORT
m
TVMSL
ROCLT
ROSLU
ROCLA
3361.0
3365.0
3485.5
ROSLL
DC
3517.5
3586.0
Table 1 Petrophysical Evaluation Summary in K08A109 well
The K08-FA-109 exploration/ development well targeted the K11 Zebra prospect, which is part
of the K7K8 Gemini portfolio. The prospect is located at the South margin of the K08-FA
South field and is one of several Rotliegend pop-up structures within the K07-K08 area
The well was drilled with a constant azimuth of 220.34 and maximum inclination of 32. The
6”hole reservoir section was penetrated vertically in order to maximize column height and to
avoid drilling into the bounding fault. This vertical trajectory was then maintained to well TD
at 3663 mAHD in the Carboniferous.
The key subsurface risk in this well has been depletion; hence the logging programme was
planned in such a way as to minimize exposure of tools (differential sticking) and personnel.
Wireline logging has been proposed for this well rather than Logging while drilling (LWD)
due to the low maximum inclination of 32deg and its relatively lower cost.
K08-FA109 well
K08-FA Block 1
West
(High)
block
East block
not included
Zebra GIIP
Central
block
Figure 1: Map showing the outline of the K11 Zebra structure
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Well Information
The K08A109 well is of conventional well design with along-hole length of 3663m.The
maximum inclination of the well is 32 deg and reservoir section has been drilled with oil-based
mud (VCM). The reservoir tops during drilling came in about 25m shallower than prognosed.
See table below, which shows the actual relevant formation tops.
Formation Tops
North Sea
Chalk
Rijnland
Triassic
Zechstein
Ten Boer (ROCLT)
Upper Slochteren
(ROSLU)
Ameland (ROCLA)
Lower Slochteren
(ROSLL)
Carboniferous (DC)
TD
m AHORT
87.4
1062
2137
2339
2921
3357
3362
m TVMSL
32.6
962
1873
2042
2450
2946
2951
3482
3515
3071
3104
3527
3172
3663
3250
Table 2 Relevant Formation Tops in the K08A109 well
Base North Sea
Base Chalk
Base Vlieland
Top Bunter
Top Zechstein
Base Zechstein
Figure 2 Seismic cross-sections along well trajectory (depth
orientation). The section shows
the vertical penetration of the reservoir
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Data available
Log Data
Run
Log Description
Date
Interval (mAHRT)
GR-Impulse-Res
MWD
18-09-2009
7” casing to TD
1
GR-PEX-AIT
W/L OH
20-09-2009
TD to 7” casing shoe
2
GR-XPT
W/L OH
21-09-2009
10 data points
3
GR-CCL-SCMT
W/L CH
29-09-2009
TD to 7” casing shoe
Table 3: Logging runs in the K08A109 well. The contractor has been Schlumberger
For the log data acquisition, the impulse tool was run as part of the MWD tool string in the
reservoir section. Resistivity log data (Level A which gives 1 phase and 1 attenuation) from the
impulse tool helped to determine if gas was present in the reservoir before the eventual
Wireline data acquisition
The first run on Wireline was the GR-PEX-AIT, which was acquired without any operational
issues. Since depletion was the biggest risk in this well, a detailed monitoring of the drilling
fluid for kicks was carried out during drilling. Well Engineering and Petrophysics team later
agreed that the hole condition was fit enough to run Wireline with the nuclear source tools. The
GR-PEX-AIT run was successful and has been used as the primary depth reference for all
subsequent logging operations in the K08A109 well
The second wireline run was the GR-XPT. Based on the results of quick-look evaluation from
Wireline run1, the pressure points were picked. Twelve points in all were picked representing
all the reservoir zones.
The cased hole logging (GR-CCL-SCMT) was done to evaluate cement quality and integrity.
This was in particular important in this well since the K08A109 well was the first offshore well
to use cemented completions. A good cement isolation was therefore a pre-requisite for
producing the well
Log Quality
The caliper log shows good and stable hole condition especially in the reservoir section.
Environmental corrections have been applied to the Neutron and density logs. The Density
correction log also falls within the allowable +/-0.1g/cc except in the DC shale (where the
HDRA log reading became high likely due to the unsteady hole condition in the
Carboniferous). In all, the logs are adjudged to be of good quality. The figure 3 below shows
the raw logs acquired in the K08A109 well before petrophysical processing.
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Figure 3: Shows the raw logs before processing. In track 2 the MWD resistivity (in red) and Wireline AIT
resistivity (in black) overlie each other after depth shifting MWD-Res. The caliper log is in track 3 and it shows a
relatively smooth hole while the density correction log (HDRA) is in track 3. The HDRA is high in DC probably
due to hole quality in the Carboniferous
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Log Evaluation and Interpretation
Porosity and HC saturation
The PORSHDEN user programme in Techlog has been used to process the porosity and HC
saturation. The PORSHDEN user programme is a modified Waxman-Smits equation, which
gives an iterative solution strategy for computing hydrocarbon saturations and density porosity
that have been corrected for invasion effect (See Ref 3). The parameters used in the
programme are shown below. These parameters are in line with regional parameters for the
South K08 block cluster where the K08A109 well has been drilled into See Ref 1. The average
porosity and average HC saturation are shown in Table 1.
Parameters
a
mf
n0
B
qvf
qvp
qvmax
Rw
p
f
Rmf
rhomaleft
porleft
rhomamid
pormid
rhomaright
porright
rhogas
Rhofw
Cluster 3
1
1.92
1.55
19
0.0018
2
1
0.016 m
0.7 (as big gas effect is seen on N/D separation
0.3
100000 m
2.72 (g/cc)
0.05 (v/v)
2.7 (g/cc)
0.1 (v/v)
2.69 (g/cc)
0.15 (v/v)
0.2 (g/cc)
1.15 (g/cc)
Table 4: PORSHDEN parameters used for processing Porosity and HC saturation
Permeability
The regional K08 block Porosity-permeability relationship has been used to determine the
permeability. The ambient permeability generally fits better to well test derived KH in the K08
block hence it has been used to predict permeability. The ambient permeability has been
converted into insitu- corrected permeability based on the equation 1 below. This equation has
been derived from stress core permeability data in K08-3 well (See ref 1)
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The equation for converting the ambient permeability to insitu (based on K08-3 core plugs) is:
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Log K stress  1.05 * 10 Log(K amb ) - 0.13 ………………………Equation 1
K08 block Por-perm
100000
10000
Poro perm plot
1000
K08-1B
Permeability(mD)
K08-3
K08-8
100
K08A 102
K08A 201
10
Fit_Arith Aver
Fit_Geom Aver
K08- 7
1
K08- 12A
0.1
0.01
0.001
0
0.05
0.1
0.15
0.2
0.25
0.3
Insitu Por(v/v)
Figure 4 Ambient
permeability vs. Core porosity. The red line shows the Arithmetic fit line with
kambient = 1027  2.3 and the blue line shows the Geometric fit line with the relation
the relation
kambient = 1027.6  3.05
Saturation Height Functions and FWL
FWL
The K11 Zebra prospect shares the same FWL of 3340mTVDSS with the K08FA field. This is
based on spill point analysis and average GWC in the K08FA field. However, the K08A109
well did not penetrate GWC.
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SHF
Log derived (lambda approach) SHF has been used for the saturation height modelling in
K08A109. This is the approach used in all K08 block wells as it predicts better HC saturation
and compares better to resistivity derived HC saturation.
Equation 2 shows the Lambda equation and equation 3 shows the input data into the Lambda
model.
Sh  max( 0, 1  B 
10a b
( HAFWL )
)
…………………..…..Equation 2
Sh and Ø are in fraction
HAFWL is 3340m
a= 0.5, b=3.5, B=0 and = 0.35
…………………………..Equation 3
Figure 5 below show the composite log based on the final evaluation of the K08A109 well.
The pressure points taken are also displayed in the plot. The sums and averaged based on this
evaluation are shown in Table 1
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K08A109 (K11 Zebra)
Figure 5: Shows the composite log of the K08A109 well. Track 4 and 5 show the density porosity and HC
saturation respectively. Pressure points are shown in track 6. Track 7 shows the log permeability (derived by
applying the -k relation to the log porosity) and the mobility (from pressure data). The Integrated Permeability
(Pseudo PLT) is displayed in track 8
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Fluid Contacts
Pressure data has been acquired in this well. The objectives for acquiring pressures were to
 Properly understand the degree of depletion in the field,
 To see if there is differential depletion between the ROSLU and ROSLL which will
help to fully understand fault juxtaposition theory in pop-up
 To establish optimum perforation conditions.
The XPT tool has been used for the pressure acquisition. This pressure tool was chosen
because of the high depletion anticipated in the field. The XPT is a slim tool with limited
surface contact with the wellbore and therefore will reduce the chance of tool sticking. It also
acquires pressure data fast and thereby save cost on rig time.
The pressure data acquired is shown in the table below
Well
Date
mAHD
mTVDSS
Form. Press
(bar)
Mobility
(md/cp)
Comment
K08A109
21-Sep-09
3371.00
2959.83
249.98
23.10
Good test
K08A109
21-Sep-09
3380.00
2968.83
250.00
39.60
Good test
K08A109
21-Sep-09
3408.00
2996.83
250.36
10.10
Good test
K08A109
21-Sep-09
3434.00
3022.83
250.73
1.30
Good test
K08A109
21-Sep-09
3462.00
3050.83
251.27
1175.00
Good test
K08A109
21-Sep-09
3479.00
3067.83
251.27
719.80
Good test
K08A109
21-Sep-09
3493.00
3081.83
-
0.90
Tight
K08A109
21-Sep-09
3508.50
3097.32
250.86
458.40
Good test
K08A109
21-Sep-09
3525.00
3113.82
250.57
455.00
Good test
K08A109
21-Sep-09
3547.00
3135.81
250.86
362.80
Good test
K08A109
21-Sep-09
3562.50
3151.30
251.11
447.30
Good test
K08A109
21-Sep-09
3576.00
3164.79
-
-
Tight
Table 5: Pressure data in K08A109. Pressure data has been acquired with XPT tool
Twelve points have been chosen based on the quick look evaluation result. The data points
have been spread across the reservoir zones: ROCLT, ROSLU, ROCLA and ROSLL. Two
data points were found tight: one from the ROCLA (Ameland) and the other from ROSLU
Interpretation of Pressure data
The reservoir pressure came in depleted at average pressure of approx. 250bar. This is about
115-bar depletion (Virgin pressure is 365 bar) but it is within the predicted pressure range of
150-365bar. Figure 6 below, shows a pressure depth plot. A 0.8bar baffle is observed in the
Ameland shale separating the ROSLU from ROSLL. However it is assumed that this 0.8 bar
baffle is not enough to suggest differential depletion between the two sand bearing zones and
hence the reservoir is treated as a single tank. PLT data will be acquired after a few months of
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production to help to further establish the depletion mechanism. Also, the behaviour of the well
will be closely monitored during its production life history.
Depth vs Formation pressure
2900.00
2950.00
Depth (mTVDSS)
3000.00
3050.00
K08A109 XPT plot
3100.00
3150.00
3200.00
3250.00
249.00
249.50
250.00
250.50
251.00
251.50
252.00
Pressure (bar)
Figure 6: Pressure plot of K08A109 well. Difference of 1bar can be seen in the ROCLA
K08A109
K08 Block Regional Pressure
2900.00
3100.00
K08- 7
3300.00
K08- 8
TVDSS(m)
K08- 9
K08- 12
3500.00
K08- 13A
K08- 14
K08A303
3700.00
K08A304
K08-15
fit line
3900.00
K08A109
4100.00
4300.00
240
290
340
390
440
Pressure(bar)
Figure 7: Shows the regional pressures plot in the K08 block. The K08A109 pressure data has been plotted alongside all
other virgin pressures in the K08 block. The result shows how much depletion has taken place before drilling this well
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References:
1. K08 Block Petrophysical Evaluation, Femi Onita, November 2009, EP200909329957
2. Well Functional Specifications K11 Zebra, 2009
3. PORSHDEN user manual, Richard Hakvoort, 2005
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