The Use Of Full Waveform Inversion And Other Advanced Seismic Processing Techniques To Improve An Already Good Dataset – Nong Yao Field, Gulf Of Thailand
The Nong Yao Field is located within Block G11/48 in the Gulf of Thailand, 168km northeast of Songkhla, around 350km south-southeast of the Jasmine Field. The Nong Yao Field is in the Narathiwat Ridge, between the southern tip of the Pattani Basin and the northern tip of North Malay Basin. The field is formed by a shallow, low relief structure and is comprised of amalgamated stacked channels of Early to Late Miocene age. The Nong Yao Field was discovered in 2009 and production commenced in June 2015. The field is produced primarily through horizontal production wells targeting thin oil columns. Individual reservoirs are fault bound but often include stratigraphic components. The Nong Yao 3D seismic survey was shot in 2009 after the Nong Yao-1 discovery and was pre-stack time migration (PSTM) processed in 2010 by PGS. A smaller area covering the Nong Yao Field was then reprocessed using pre-stack depth migration (PSDM) in 2011. Seismic data quality on this dataset is generally very good, although shallow gas is present in some parts of the survey, below which the seismic data quality is significantly reduced. The 2011 PSDM data was used as the basis for developing the field and by mid-2017 32 wells had been drilled as part of the development. The development was heavily seismic driven, utilizing the seismic and inversion products to predict sand presence with high confidence, which is critical in a highly channelized environment. However, there were still some key uncertainties remaining, which the 2011 PSDM seismic could not resolve. Firstly, the prevalent shallow gas remained a problem, heavily degrading the seismic data quality and making fluid and lithology prediction difficult in these areas. Depth prediction was also highly unreliable due to the overly smoothed velocity model, which translated to a high volume uncertainty due to the very subtle structure of the field. Secondly, many of the reservoirs were thin with the amplitudes heavily affected by tuning and interference. This made fluid prediction more difficult and added significant uncertainty to volume estimates. Consequently, in late 2016, a second PSDM reprocessing project was initiated. There have been enormous advances in seismic processing technology since the first PSDM in 2011 and it was hoped that the key issues identified with the 2011 seismic could be substantially improved. Key aspects of the 2017 reprocessing were joint de-ghosting, high frequency full waveform inversion (FWI) and FWI guided Q Tomography. The source-receiver de-ghosting broadened the frequency spectrum of the seismic data substantially reducing sidelobe effects thereby minimizing the effects of seismic interference and tuning. The FWI and Q Tomography tackled the shallow gas issue, by firstly building a much more accurate velocity utilizing FWI, and then using this velocity model to capture all of the significant shallow gas in the area. This model was used as an input to the Q Tomography allowing the attenuation to be properly modelled beneath the shallow gas. The reprocessed data was delivered in mid-2017 and it showed significant improvement compared to the 2011 PSDM data. Tuning effects are drastically decreased, with the frequency spectrum significantly enhanced on both the low and high end. The structure in existing reservoirs changed, particularly in areas affected by shallow gas and now better matches the well results. The new seismic was also used to update the prospect portfolio, with a much clearer picture provided by the new seismic. Ten wells were drilled in early 2018 based on the interpretation of the newly reprocessed seismic data. All ten of these wells encountered hydrocarbons and have added significant reserves to the Nong Yao area.
AAPG Datapages/Search and Discovery Article #90331©2018 AAPG Asia Pacific Region GTW, Back to the Future – The Past and Future of Oil and Gas Production in the Asia Pacific Region, Bangkok, Thailand, September 26-27, 2018