Knowledge of the magnitude and distribution of water influx can be essential for managing water-drive gas fields. Geophysical fieldwide monitoring can give valuable information, particularly offshore where well control is sparse and observation wells are expensive. Advances in the accuracy of seafloor time-lapse gravimetry have made this method feasible. It can quantify which areas are flooded, providing information complementary to well-monitoring, production, and 4D seismic data. Gravimetric monitoring may aid material-balance calculations, which are vital for assessing reservoir-drive mechanism and estimating initial and remaining gas volumes. In addition, it can constrain reservoir simulation models. Our goal is to produce better physical insight into typical density changes occurring in water-drive gas fields and their associated surface-gravity response. It is feasible to monitor displacement of gas by water in reservoirs that are only a few meters thick. Gravimetric monitoring can detect edgewater encroachment in early stages. With current accuracy, the method is applicable for gas reservoirs of modest size ( in situ gas volume) at medium depths .