1. Petrophysical evaluation of the Rotliegend in K15-FB-107
exploration/development well of the K15-FB-NE prospect
Richard G. Hakvoort
December 2006
Doc. Nr. EP

2. Logging in the K15-FB-107 reservoir section:
Hole conditions: 6” hole, VCM oil based mud, 135 deg C.
LWD GR-ARC resistivity
Finished on Saturday December 2, 2006
Approximate GWC picked from resistivity
TD called on the basis of this approximate GWC
Wireline GR-PEX150-AIT
Executed on Sunday December 3, 2006
PEX150 consists of neutron and litho-density rated for 150 deg C (standard PEX rated for 125 deg C)
The down-run gave good GR and AIT resistivity, but bad neutron and density
The up-run gave good-quality GR-AIT-Den-Neut-Cal
AIT resistivity identical to ARC resistivity over the entire logging section
Wireline depth is about 8.5 m deeper than LWD (driller’s) depth. The wireline depth is taken as the depth reference (also for future cased hole logging operations). All logs are shifted downwards to be on depth with the GR-PEX wireline logging run dated Dec. 3, 2006.
Wireline GR-SRFT pressures
Executed on Monday December 4, 2006
First tool failed in the hole, which cost about 8 hours of rig-time; back-up SRFT tool functioned o.k.
Good-quality pressure measurements were obtained at a minor cost compared to what XPT would have cost!

3. Petrophysical Parameters (from K15-FB evaluation)
Reference: PP evaluation of the ROSL in K15-FB, Hakvoort, March 2006, Doc. Nr. EP
ma from core plugs: 2.69 g/cc in K15-FB-North
JDA Archie parameters (m = 1.9, n = 1.65) and with Rw = m
PORSHDEN user program used (joined evaluation of porosity and HC saturation) with p=0.5, f=0.5, fw = 1.15 g/cc, mf = 0.8 g/cc, HC = 0.2 g/cc
GR cutoff 75 API (optimized per well)
Next slides show the measured logs and PP evaluation results for K15-FB-107
Petrophysical sums and averages K15-FB-107
Note: The ROSLL has only been logged partially, as TD is in the ROSLL.

4. log permeability (mD) – using K15-FB-N k- relation
SRFT mobility (mD/cP)
K15-FB saturation-height function

5. FWL B A
FWL interpreted from logs at 3975 m TVMSL, identical to the K15-FB-North FWL C

6. K15-FB-107 SRFT pressure measurements

7. RFT data in K15-FB-107 (K15-FB-NE) and K15-FB-101 (K15-FB-North, 1979 undepleted, and 1987 partly depleted)
ROCLT
ROSLU
ROSLL (gas leg)
ROSLL (water leg)
Observations:
The K15-FB-107 SRFT pressures are indicative of depleted reservoir  the gas and water gradients are no longer visible in the plot & the FWL can not be determined from the pressure measurements.
The depletion is about 60 bar (compared to K15-FB-North virgin pressures) & the depletion is more the closer you get to the aquifer it is likely to be depletion due to aquifer depletion (due to production in either K15-FB or K12-FB)
One ROCLT (Ten Boer) pressure measurement in K15-FB-107 coincides with the virgin K15-FB-101 pressure. The mobility for this pressure point was reported zero. This may mean either of two things:
This pressure point is unreliable and should be ignored.
This pressure point is indicative of the fact that tight reservoir has remained undepleted and is therefore still at virgin pressure.

8. Saturation-height function
Format fit function:
with Sh and  in fraction, HAFWL in m.
Parameter Table B a b  FWL m TVMSL
K15-FB-N
K15-FB-NE
K15-FB-N(E) saturation-height function
Notes:
The FWL in K15-FB-North is based on K15-FB-101 RFT data
The FWL in K15-FB-North-East is estimated from logs and fit to cap curves (no virgin RFT data in K15-FB-107).
The saturation-height function in K15-FB-NE has been taken identical to the K15-FB-N saturation-height function; this fits nicely on the log data

9. depleting aquifer (due to K15-FB-N or K12-FB gas production)
Possible petrophysical explanation of gas below FWL in K15-FB-107 by depleting aquifer and expanding gas
K15-FB-107 well
K15-FB-NE gas reservoir
expanding gas A
FWL C B C
depleting aquifer (due to K15-FB-N or K12-FB gas production)
The points A, B and C are also depicted in the K15-FB-107 log plot:
In the transition zone  originally, low saturations (in line with low saturation from sat-height function). Due to gas influx from up-dip, the saturations have increased  relatively high log HC saturations.
High-perm streaks in the water leg  originally, zero saturation (below the FWL). Due to gas influx from up-dip, the HC saturation has increased  present-day log evaluation shows gas.
Low-perm streaks below the FWL. Gas influx from up-dip is restricted due to low mobility  gas saturation remains low.

10. Conclusions:
Data acquisition in K15-FB-107 was successful. Both high-quality LWD logs and high-quality wireline logs and good-quality SRFT pressure measurements have been acquired.
The SRFT has proven very cost effective compared to the XPT, even when including the cost of 8 hours rig-time due to failure of the first SRFT tool. Because of the hole diameter (6”), the MDT (5” OD) was not very feasible.
The logs have been evaluated consistently with the K15-FB log evaluation. The resulting average porosity (11% in the ROSLU) is slightly lower than expected on the basis of offset data.
The mobility values measured by the SRFT coincide very well with the permeability derived from the log porosity by applying the K15-FB-N k- relationship. The expected reservoir kh (ROSLU) is between 400 and 2500 mDm.
The SRFT pressures show depleted reservoir (60 bar or 10-15% of the GIIP). Probably this happened via aquifer depletion (via a shared aquifer with either K15-FB-North or K12-FB).
The petrophysical logs show a FWL at 3975 m TVMSL. This is consistent with the geological understanding of field.
The logs show gas below FWL. The explanation brought forward by petrophysics is that this gas below FWL is caused by the aquifer depletion in combination with expanding gas in K15-FB-North-East.