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PETRONAS Research has recently embarked on a study to determine and understand pressuresaturation variations in the Angsi field through the use of 4D seismic technology. Changes in hydrocarbon pressure and saturation due to production produce noticeable changes in amplitude response. 4D seismic, or the use of repeated 2D/3D seismic surveys as a function of calendar year enables detection of 4D signal indicative of pressure or saturation changes. The Angsi field is located in the Malay Basin approximately 106 miles offshore Peninsular Malaysia. Angsi is a joint venture development between PCSB and ExxonMobil Exploration & Production Malaysia (EMEPMI), with the field being operated by PCSB. The major oil bearing reservoirs are the I-35 and I-68 sands with several gas bearing reservoirs in the I and K sands. Determining pressure-saturation variations involve integrating several geophysical technologies working in close ties with the reservoir engineers, that is, supporting petroleum geo-engineering. These technologies include well-synthetic-seismic correlation, Rock Physics, Production Scenario Seismic Modeling at selected injector/producer wells, 2D/3D seismic modeling, 4D AVO Modelling/Interpretation and correlating seismic attributes with the engineering data, pressure history and saturation measurements. We are primarily motivated by other previous work which focuses on the use of Time-Lapse (4D) AVO data to decouple pressure and saturation variations taking note that AVO attributes of Intercept and Gradient may not be suitable, as they conflict with the small angle (θ) assumption in the data. We should study and use the seismic attributes which respond differently to reservoir changes in pressure and saturation. Figure 1 shows a rock physics representation in the Angsi field indicating the oil response separation from the background. An AVO modeling study was conducted to understand and compare the predicted AVO responses at the reservoir sands with the response observed on the base-monitor seismic data. A seismic inline showing the Angsi I35 main channel is correlated with the corresponding seismic channel model for a structural image analysis generated using actual acquisition parameters applied on the Angsi field. Figure 2 shows seismic attributes extraction results of the I35 reservoir channel. The extraction was performed on the base and monitor seismic data to study the resulting differences. These attributes and difference maps can then be correlated/integrated with pressure and saturation maps for interpretation of the 4D effects. We initially select the two (2) attributes of P- and S-Impedances that were earlier used by Tura and Lumley (1999). These attributes respond differently to reservoir changes, P-impedance changes being more sensitive to saturation changes, while S-impedance changes sensitive to pressure changes. These two AVO attributes are indeed very ‘robust’, being less sensitive to noise in the gather data, and could be computed by simple AVO inversion method. Improving reservoir monitoring through the effective use of 4D Seismic Pressure-Saturation discrimination methods will enable us to more accurately locate bypassed oil and therefore, increase reserves. In addition, 4D seismic will minimize costs due to infill drilling by optimally locating development wells. |
