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Vol. I, No. 3
MUMBAI
JANUARY, 2007
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From
Editors’ Desk
Seasons’ greetings
For nearly 80
years, the oil industry has relied on logging tools to reveal the
properties of the sub-surface. The arsenal of wireline measurements
has grown to allow unprecedented understanding of the reservoir.
However, the earlier developments in the wireline logging industry
were limited to innovations in resistivity and porosity measurements
only. But the speed of advances in technology accelerated
dramatically in the last two decades with the introduction of
imagers and scanners which can expose the formation to the extent
never thought before. A number of petrophysical properties like pore
size, pore size distribution, pore connectedness, permeability,
porosity distribution, porosity portioning, bound fluid volume,
capillary pressures, relative permeability and so on, can now be
fairly estimated. These properties have a direct bearing on the
hydraulic properties of the formation and hence they play an
important role during production and injection planning of the
field. Besides, these properties help us design a better reservoir
description.
A few years
back, hell used to break loose if a well was cased without logging.
There was a complete loss of information. The scene has drastically
changed now. We can now have a complete logging suite even in a
cased hole, which is no way inferior to the open-hole logs. Cased
hole resistivity has gone a long way in identifying the bypassed
hydrocarbon. Likewise reservoir saturation tool has also helped the
oil industry in assessing the saturation and fluid contacts behind
the casing.
Keeping pace
with the technological advancements, we bring forth in this first
issue some of the findings of new generation tools in the D-1 field
of ONGC. We will keep this trend in our forthcoming issues also,
wherein we intend to bring for you the new technology adaptation in
other oil companies as well. Some of the success stories related to
logging operations, like logging while fishing and MPBT in flowing
wells will also be covered in our forthcoming issues.
Finally, we
must confess that we cannot continue publishing such news letters
for long without your contribution and feedback. Please, therefore,
contribute to this letter in whatever area you feel relevant,
covering topics related to technological innovations, formation
evaluation and reservoir characterization. Let us come together and
share our knowledge in the interest of our fraternity and in our
individual interest as well. We look forward to a positive response
from all of you
We dedicate
this issue to Shri HK Gaur, one of the esteemed members of our
society who left for his heavenly abode on 23rd November,
2006.
Application of New Technology in D-1
field
The first
phase of drilling of D-1 field in Mumbai offshore is just over. The
field was discovered in 1976 but was placed under marginal category
from reserve point of view. The remoteness of this field from the
existing ONGC facilities coupled with the sourness of hydrocarbon
and low GOR further added to the delay in its development plan.
This field,
although in a development stage now, still exhibits surprises with
the drilling of each new well. It is basically a heterogeneous
carbonate formation of Early Miocene sequence with multi-layered oil
pools. The carbonate sequence is believed to have deposited in a
cyclic pattern with a deposition of low to moderate energy
wackestone and packstone to high-energy grainstone. Subsequent
diagenetic changes such as cementation, dissolution, dolomitization
and compaction have changed the depositional fabric of the
formation. It is, therefore, necessary to recognize the reservoir
characteristics of this field in order to build a precise reservoir
model.
In the early
exploratory stage of this field the new generation logging tools
were not available. Hence reservoir description was attempted with
the help of basic triple combo logging tools. But these tools,
besides their low resolution, do not reveal anything about pore
size, pore size distribution, pore connectedness, permeability,
fractures, channels, dolomitization and so on, without which the
reservoir description is just an approximation. The carbonates may
have a good vuggy (secondary) porosity containing hydrocarbon but it
cannot be produced if not connected. Sometimes carbonates also show
low resistivity hydrocarbon bearing zones because of high fraction
of bound water in micrites. These zones will be overlooked if we do
not have any alternative (non-resistivity based) saturation tool or
any other means of fluid identification like MDT sampler with
pump-out and LFA modules or MRF of CMR tool.
New generation
tools like CMR, FMI, ECS, DSI, MSCT and MDT (with dual packer, pump
out and LFA modules) have been used in the development wells of this
field for building up a better understanding of the reservoir. In
one of the wells, where CMR has been recorded, a good amount of
vuggy porosity represented by bin 7 and 8 in the figure below can be
seen in the zone of interest. These vugs are also connected, as can
be seen on the FMI log recorded in the same well.
 
The good
connectedness of these vugs results into a high permeable zone with
a good potential to deliver. The integration of these two
measurements can be used for partitioning the porosity into micro,
meso and macro classes. Making such categorization is important
because, when well connected, macro pores provide high initial
production rates but can then short circuit water floods, making it
difficult to produce the oil residing in the mesopores. Further, we
can also to define a high-resolution connectivity index to estimate
the permeability of the flow units. The permeability thus estimated
matches well with the production results. Simple permeability
estimation from industry established transforms like SDR and Timur-Coats
are also in good agreement with MDT mobility.
CMR has also
been used to identify the fluid type in its MRF mode, where
molecular diffusivity of the fluids is enhanced by increasing the
echo spacing of the measurements. This technique also gives the
saturation of the fluids in the flushed zone. At present this is a
station measurement but in the latest version of the tool it will be
a continuous log with multiple depths of investigation. It will,
therefore, provide a continuous independent measurement of Sxo,
which in turn, can be used to define variable m and n.
The DSI log,
although recorded to supplement VSP, has been used to compute
mobility and fracture in its Stoneley mode. In both the cases the
Stoneley waveform is forward modeled based on the tool and formation
properties and the modeled waveform is compared with the actual one.
The results are shown below. Mobility computed from Stoneley matches
well with the MDT mobility.
 
MDT dual
packer with pump out and LFA modules were used to recover formation
fluids at low porosity stations. The conventional single probe
method may not yield any result because of high drawdown against
less permeable zones. But dual packer exposes a much larger area for
sampling by inflating two packers (1m apart) in the well bore. A
drawdown is then created by pumping out the mud from this area, thus
allowing formation fluids to enter. The LFA provides an on-line
information about the type of fluid and helps in sampling the
representative formation fluid. The technique has been used in this
field to access low permeability zones and identify their fluid
contents.
Mineralogical
understanding of the formation is of utmost importance for carrying
out any petrophysical analysis, basically because the mineralogy of
the formation controls the matrix density and thus the porosity.
Amongst the post-depositional alterations which occur in carbonates,
the process of dolomitization is a long going process and in its
initial stages it may not get reflected on the conventional logs or
on cores. Dolomitization is a process wherein calcium ions of the
calcite are partially replaced by magnesium ions. But the magnesium
ions, being smaller in size than the calcium ions, create a void and
increase the porosity.
To
substantiate the occurrence of dolomitization, Elemental Capture
Spectroscopy (ECS) tool has been run in this field to find any
magnesium in the elemental concentration. The results do show the
presence of magnesium, thus confirming the presence of dolomite in
the formation. An ECS driven ELAN model shows an overall 5% increase
in porosity compared to the conventional ELAN without ECS. A
continuous geo-chemical permeability log has also been created with
the help of ECS channels and it matches well with the MDT mobility.
The results are shown below:
Forthcoming Events:
Editorial Committee:
1.
RR Tiwari, ONGC
2.
Kapil Seth,
Schlumberger
3.
S Mohanta, RIL
4.
AK Kakani, BG
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