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.
The onshore Senoro field is an Indonesian carbonate pinnacle reef gas reservoir that so far has produced around 15% of the estimated original gas in place (OGIP). Twelve wells have been drilled consisting of ten production wells, one disposal well, and one monitoring well. All production wells were equipped with a permanent downhole gauge (PDG) to obtain near-bottom hole pressure data. A p/Z plot[1] of the pressure data exhibits a non-linear trend suggestive of pressure support from the aquifer or, perhaps, an adjacent gas reservoir in restricted communication with the main reservoir. If the non-linear trend is due to aquifer support, analysis assuming a weak water drive yields a bigger estimate of OGIP, and vice versa. Analysis of reservoir pressure and production data alone can result in unacceptably-high uncertainty related to drive mechanism and OGIP. This paper describes how cased-hole saturation logging from the gas reservoir across the contact and into the aquifer was integrated with other surveillance techniques based on pressure and production data to reduce the uncertainty and improve definition of drive mechanism and estimates of OGIP. Core data and pressure transient analysis in the Senoro field shows that the gas zone has a permeability in the range of 50 milidarcy (md) to 200 md, while the aquifer, which is mainly found in an underlying platform facies, has a much lower permeability in the range of 1 md to 40 md. The high permeability contrast between gas and water zone was expected to result in weak pressure support from the aquifer. Moreover, a tight streak is found in the middle of the aquifer which virtually eliminates bottom-drive aquifer support. Nevertheless, ongoing surveillance, including analysis of production data using flowing material balance and type curve analysis, indicates there is pressure support. Therefore, to address the uncertainty about aquifer support, it is necessary to evaluate the aquifer performance with other surveillance techniques to augment the PDG data. These other techniques included annual time-lapse saturation logging in the monitoring well, and static (wireline) bottomhole pressure (SBHP) surveys taken in select production wells and the monitoring well to estimate gas and aquifer pressure. After four years of production, the latest saturation log shows increased water saturation above the initial depth of the gas-water contact (GWC), indicating movement of the aquifer into the bottom of the gas column at the flank of the reservoir. The SBHP survey in the aquifer also shows that aquifer pressure has dropped by 330 psi compare to the initial pressure taken with a wireline formation tester. This compares to a pressure decrease of 370 psi in the overlying gas reservoir. Analysis of the current gas reservoir pressure trend, and the current aquifer trend, shows that the current depth of the GWC is about 50 feet shallower than the initial depth, which is consistent with the depth of the increased water saturation seen in saturation logging in the flank monitoring well. This result is consistent with the match from a simulation model with a weak water drive. Conversely, if a strong water drive is assumed, the decline in aquifer pressure will be smaller than observed and the GWC will be higher than observed. Sensitivity to combination of gas expansion energy and aquifer strength has been done to narrow the range of OGIP. The results described in this paper illustrate the benefit of integrating saturation and pressure measurements from the aquifer with other surveillance data to better measure aquifer performance, determine drive mechanism, estimate OGIP and recovery, forecast production, and anticipate problematic water production in a gas reservoir.
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