Deepti Tiwari CEC-OG, ONGC Ltd, Delhi; R.V. Marathe, N.K.Patel,
K.P.Ramachandran, C.R.Maurya, and P.K.Tewari, Institute of Reservoir Studies,
ONGC Ltd, Ahmedabad
Twenty two years of history of polymer flood in KS-III sand in Sanand field,
Gujarat State, India, proves the efficacy of the process in a reservoir of 850
C temperature. KS-III sand of Middle Eocene age, is one of the few successful
cases of polymer flooding in such high reservoir temperature. Reservoir is
acting under partial edge water drive. Oil viscosity of 20 centi poise led to
adverse mobility ratio which resulted in cusping of water in structurally
higher wells. Hence polymer flood was considered the best option in this
reservoir for improving mobility ratio of oil and overall areal and volumetric
sweep efficiency which was also supported by the laboratory studies on native
cores of the field. Polyacrylamide polymer was selected for injection.
Polymer flood process in Sanand field has undergone many stages of
implementation. After initiation of polymer pilot in an inverted five spot
pattern in 1985 with one injector, 4 producers and one monitoring well, it was
extended to four inverted five spot patterns in 1993 with nine producers and
four injectors. Effect of the injection was visible in terms of reduction in
water cut, improvement in oil production and increase in reservoir pressure.
Success of pilot projects encouraged to full scale field implementation in
1996-97. Polymer injection was done in peripheral pattern in the main part of
the field. Subsequent reviews have been carried out from time to time and mid
course corrections have been taken accordingly.
Current paper discusses different stages of development in polymer flood, the
steps taken from time to time, lessons learnt and conclusions drawn on the
basis of practical experience and reservoir simulation.
KS-III sand in Sanand field in Ahmedabad-Mehsana tectonic block of Cambay Basin
is a sandstone reservoir in Kalol formation of Middle Eocene age. The field was
discovered in 1962 and was put on commercial production in May 1969. KS-III
sand is the major hydrocarbon bearing sand in the field with 64% of proved
oil-in-place and 95% of total oil production. Till date, the sand has produced
about 23% of proved oil-in place.
Geological, Reservoir & Fluid Properties
Sanand field is located at the western margin in the southern part of the
Ahmedabad-Mehsana tectonic block of Cambay basin which is a north-south
trending intracratonic rift basin. Structure is NNW-SSE trending elongated
doubly plunging anticline(Fig.1). Sanand is a multi-layered reservoir in Kalol
sands but KS-III is the main reservoir. Three NNWSSE trending longitudinal
faults separate the oil bearing area of main block from western gas bearing
blocks and the nonreservoir part in the west. One transverse fault, in the
southern part, makes the southern boundary of main block, KS-III. Another
transverse fault separates block of S-17 from the main block based on fluid
contacts. OWC in the main block is at 1327 mts MSL and in the block of S-17, it
is at 1347 mts MSL on the eastern part. A shale layer of varying thickness of
0.5 to 2.5 mts separates the KS-III reservoir into 2 units. This reservoir is
operating under partial aquifer support.
KS-III sands are a combination of shale, silt, sand and coal deposited under a
combination of marine, coastal marsh and deltaic plain environment. Reservoir
sand appears consistent laterally with the main portion of the field but
vertical heterogeniety is present which can be seen on logs as well as cores
visible in the correlation profile (Fig 2). Maps made on the basis of well
data, indicate the deposition of the reservoir facies in few depocentres (Fig.