Abstract

The heterogeneity within a reservoir is difficult to predict, but the subtle changes of the seismic wave shape within any sequence provide valuable information about the reservoir and are preserved in 3D seismic data. In order to optimize the understanding of the reservoir, it is necessary to integrate geophysical, geological, petrophysical, reservoir and production data. Diagnostic tests and studies such as production logging, reservoir saturation, build up, pressure and temperature are commonly studied at the time of reservoir management in a developing field but off late seismic attribute is being used as an additional tool for reservoir management. With the advent of sophisticated algorithms and faster machines, today it is possible to extract a number of seismic attributes from within a particular sequence. In a heterogeneous reservoir or in an area of poor well control, synergistic study of seismic attributes can help us investigate the lateral variations in the quality of the reservoir and improve the imaging of reservoir units, providing vital clues for placing infill wells and injectors.

In the present study an attempt has been made to decipher the reservoir behavior in one of the western offshore fields of India. The field in question is in production from carbonate reservoir over last six years. At places, poor reservoir facies have been observed within the pay leading to poor transmissibility in the area thus affecting the voidage compensation in the targeted zone. Using the conventional tools, it was difficult to map the lateral facies changes in between wells. Efforts have been made to calibrate seismic attributes with other relevant data in order to arrive at a good prediction of the reservoir quality.

Background

The field we are going to discuss is one of the western offshore fields of India in the Arabian Sea. The first two exploratory wells E-1 on south and E-2 on north were drilled based on 2D seismic data in 1987. The structures were mapped as two independent structural culminations. Both produced hydrocarbon in commercial quantity. Initially it was believed that these were two different pools. Subsequent drilling of another exploratory well E-3 in-between E-1 & E-2, which falls in a saddle, also produced hydrocarbon. The oil water contact from the well logs suggested that the pool is one.

Later on 3D seismic data was acquired, 13 exploratory wells were drilled and the field was put on production since 1991 -92. As on today there are as many as 11 producing platforms and two water injection platforms. The field reached its production peak of 3.81 MMPTA in 1994 -95 with GOR 240 and water cut 8.6%. Soon after, the production steadily declined and reached 1.92 MMPTA in 1997–98 with GOR 372 and water cut 45.5%. Early water break through was observed in many producers and some developed gas conning. Some of the water injectors did not come out well and failed to inject as expected.

Geological setting

Hydrocarbon in this field is contained in Eocene limestone within the top 70 m of the formation.Depositional processes mainly govern the reservoir facies with the influence of diagenesis and tectonic related fracturing. The northern part of the field is deposited in restricted low energy environment whereas the south is in open marine moderate energy environment. The western limit of the pool is bounded by a major NS trending fault and OWC on other sides.

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