Trapped gas saturation is existing gas saturation after displacement by water and it is one of the key parameters for the estimation of gas recovery factor and gas production deliverability for the reservoir, especially gas reservoir under active water drive. This paper presents a case study of producing North Senoro gas field in Indonesia for series of experimental work of trapped gas saturation measurement and comparison with estimated trapped gas saturation using well-known correlation from various core experiments results. Effect of trapped gas saturation on gas recovery and gas production deliverability were also assessed in various sensitivity analysis using simulation model.

Trapped gas saturation varies with rock physical properties such as rock type, porosity, permeability and initial gas saturation and experiments method. Various core samples were chosen to cover range of rock properties in bulk of reservoir and we applied two experiment methods under spontaneous imbibition, Counter-Current Imbibition (CCI) and Co-Current Imbibition (COI). Measured trapped gas saturation had strongly dependency on permeability rather than porosity. The comparison between two methods illustrated that COI showed lower trapped gas saturation compared to CCI except the core plug with high permeability. Difference of measurement results between CCI and COI came from different boundary condition and flow regime between air and toluene during experiment. As co-currently flow regime by COI was closer to fluid behavior in reservoir condition compared to counter current flow regime by CCI, the results from COI was adopted for further analysis.

Both of CCI and COI results showed higher trapped gas saturation compared estimated one from Agarwal correlation which defined trapped gas saturation as a function of rock type, porosity, permeability and initial gas saturation. It indicated measurements for specific formation is necessary and critical, especially for complex carbonate formation instead of using correlation.

The measured data was utilized to analyze effect on recovery factor in North Senoro gas field as a case study. In simulation model, trapped gas saturation was defined as a function of permeability and initial gas saturation based on our experiment results and Land equation. Sensitivity analysis results concluded that effect of trapped gas saturation on gas recovery factor was limited without any aquifer drive meanwhile trapped gas saturation was critical under strong aquifer support because bulk of gas was trapped by water under high pressure and it kept abandonment pressure high at the end of production high.

In addition to accurate measurement of trapped gas saturation, clear understanding of aquifer strength is necessary for realistic gas recovery factor estimation and optimum development planning.

You can access this article if you purchase or spend a download.