RAVVA

Celebrating 16 Years of Technical Excellence

Innovating Development | OBC 3D Seismic Data

Ravva | Innovating Development

Ravva OBC 3D Seismic Data Ravva Field was discovered in 1987 based on 2D seismic data which delineated the broad structural geometry of the field. The total permit area is spread over 331.26 km2, which includes 24.9 km2 of onshore area in the northern part of the block. To further bring out the finer structural aspects of the discovered field, 3D seismic data was acquired in streamer mode in 1990. The data set had inherent feathering related data quality issues coupled with less signal to noise ratio. The 3D data was used for initial field development by installing 7 offshore platforms, oil production and water injection pipelines. A comprehensive knowledge about the structural evolution vis-à-vis reservoir characterisation was still needed, in terms of reservoir heterogeneity, which required finer sampling with good quality seismic data. This necessitated acquisition of better seismic data for imaging the subsurface in the life of the field development. In 2000, 3D seismic data was acquired using Ocean Bottom Cable (OBC) Technology over marine and transitional area to cover the potential subsurface structures interpreted to be spread over to the on-shore part of the permit area. In this method of seismic acquisition, the receivers are laid on the ocean floor and acquisition is carried out. The technology provides an opportunity to merge both the geophone and hydrophone signals, which gives a better result than the marine steamer mode seismic acquisition done in 1990. Additionally, OBC eliminates feathering problems inherently associated with the steamer mode of acquisition. The OBC method of acquisition was best suited for Ravva compared to towed steamer since the field was developed by then with 7 platforms and pipelines across the main part of the producing field. The OBC 3D, comprising transition and marine areas, has better quality with improved velocity analysis and proper migration. This is due to wide azimuths and considerable large offset (4500m) acquired in the OBC survey, which yielded in improved subsurface imaging and continuity of reflectors. Maximum offsets recorded in the OBC survey was 4500m as compared to 2500m in streamer data. This helped in generating the far (25o-40o) and ultra far angle stacks (35o-50o) apart from characterising the amplitude variations with offsets, which has given immense insights for placing the infill locations for development of middle Miocene reservoirs. The data set is being used for generating rock property and porosity volumes, which are then used for high resolution reservoir characterisation and static and dynamic modeling. The improvement in computing technology has brought in many advancements to the seismic processing and acquisition activity. These advancements provide an opportunity to capture spatial and temporal variations in the field. Therefore, fewer seismic surveys are required in the life of a producing field. Similarly, in Ravva we have carried out also 3D steamer and OBC 3D seismic surveys and the data was reprocessed from time to time. In 2010 a new OBC 3D data is proposed to be acquired for capturing the production related oil saturation changes and for identification of bypassed oil locales in the producing field.