A saturation modelling approach is presented for fields and reservoirs under complex hydrocarbon charging history. The model resolves saturation-height functions for the primary drainage, imbibition and secondary drainage equilibriums. As part of the approach, a method of evaluating the residual hydrocarbon saturation below the initial free water level is proposed.
The developed theory is based on the principle of capillary pressure-saturation hysteresis on the drainage-imbibition process in a water wet system. In this study, saturation-height models have been built with parameters determined by fitting the SCAL capillary pressure drainage and imbibition measurements. A novel approach is presented for evaluating the residual hydrocarbon saturation in relationship with the initial hydrocarbon saturation, integrating both core measurements and log saturation with a pressure depletion factor. An integrated process of modelling a primary drainage-imbibition-secondary drainage cycle based on both SCAL data and well logs is provided.
Parameters for the primary drainage, imbibition and secondary drainage curves are resolved respectively. Large variety of SCAL capillary pressure data and well logs from The Netherlands and North Sea fields are used in the study. The modified van Genuchten (VG) equation provides the best fit with the core imbibition curves and has subsequently been implemented for the reservoir and field models. Examples and case studies have successfully demonstrated the modelling of the hydrocarbon saturation under imbibition and secondary drainage equilibrium, which in extreme cases could co-exist in a field. The results of the saturation-height model in a 3D reservoir model using the fluid contacts information from log data, pressure and seismic are seen to very well align with the saturation log in wells. The residual gas saturation model built from the core measurements is compared with the in-situ residual gas saturation observations from logs. The mobility of residual or trapped gas at reservoir conditions is also discussed. From the examples and case studies discussed, it can be concluded that this new approach for hydrocarbon saturation modelling for fields under a complex fluid fill history of imbibition and/or secondary drainage equilibriums is robust and can be implemented in 3D static model and considered having high potential for dynamic reservoir simulation.