Value and Insights from Synthetic Seismic Validation of Reservoir Models in Carbonate Gas Fields, Offshore Sarawak, Malaysia
Kayes, Alexander D.; Baharaldin, Khairun Niza
Sarawak Shell Berhard, Miri, Malaysia.
From mid-2012, the Sarawak Gas Asset will comprise 20 producing fields accounting for more than 40 Tcf of gas initially in place and supplying more than 4 Bscf/d to the Malaysian LNG plant at Bintulu. A tool for performing quick and timely health checks of reservoir models is therefore invaluable for providing confidence in model-based volumetric estimates, production forecasting and optimum gas supply planning. In Shell, a synthetic seismic workflow is used for validating carbonate reservoir models against seismic data, effectively ‘closing the loop' in the integrated reservoir modeling process, which originates with the interpretation of the seismic data.
The underlying mechanics of the process is to convert the reservoir properties in the model (primarily porosity) to acoustic rock properties (Vp, Vs, and Density), based on water-wet rock property regressions derived from the available well data. Gassman fluid substitution is then used to convert the predicted acoustic properties to their gas-saturated state. As an initial QC of the model, the predictions of acoustic properties are compared to the measured acoustic logs at the well locations. Synthetics based on these models are subsequently generated by convolving the AI property model with a seismic wavelet extracted from the seismic dataset and compared back to the actual seismic data.
Developing a workflow for validating Sarawak carbonate reservoir models against seismic has come with many benefits. Field examples exist where the technique has been applied as an effective check for internal reservoir architecture. This extends to scrutiny of the lateral and vertical porosity variability away from wells. Furthermore, it is used for constraining the modelling and porosity enhancement that is assigned to karstified networks and in many cases, as a tool for testing dynamic simulations through the incorporation of 4D repeat seismic acquisition results. Given that seismic acoustic impedance drives both porosity and permeability models in the Sarawak carbonate reservoirs, it ultimately impacts the predicted dynamic behaviour of these reservoirs. As an early detection tool, this workflow can alert subsurface teams to issues inherent in their interpretation, depth conversion and modeling, which can be addressed in a timely manner to avoid surprises with respect to volumetric estimates, forecasting and well planning, resulting in more efficient management of the gas reserves of offshore Sarawak.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012