Early production of gas reservoirs is usually associated with a volumetric gas driving mechanism with no water production. Aquifer activity is minimal as well during the early life of the reservoir. In this paper, we will discuss about the good engineering practices based on several shut-in pressure data to observe and maximize marginal gas field value. We will also discuss about the possibility of water drive behavior in this field. Shut-in pressure data plays an important role in determining the in-place and reservoir dynamics of the gas reservoir. High shut-in pressure usually indicates high gas reserves. On the other hand, it shows a very strong water drive existence. The study takes place on a sandstone gas reservoir with an abnormal pressure regime on it. Production performance was then analyzed using the rate transient analysis (RTA) to determine its properties and gas in place and crosschecked with shut-in pressure data. From these steps, we can determine the trend of both static and flowing material balance (FMB) analysis to predict the reservoir dynamics. During the early life of production, it is clear that volumetric reservoir plays an important role in the reservoir dynamics since it produces no reservoir water. However, after 1 year of production, it starts to produce reservoir water. Monitoring starts when the first shut-in pressure shows a quite unexpected value. It puts a sense of both high gas reserves and aquifer activity. After applying all the pressure and production data on FMB and p/Z plot, it shows that both high gas reserves and aquifer activity exist in this field. The results of this study change the development strategy of this field, preventing doing major investment on high capital expenditure (CAPEX) with low results due to high aquifer activity. We can conclude that good reservoir monitoring and analysis combining several analytical methods can enhance our insight into reservoir dynamics. Combining FMB and p/Z, geologist starts to compare aquifer volume based on geological data and found to be similar with the results coming from analytical data. 3D reservoir simulation also confirms similar results based on those analyses.