An integrated rock-physics modeling workflow is discussed that aims at predicting the effective elastic moduli and density of a producing reservoir in response to pressure, porosity, and fluid-saturation changes, and that is suitable for application to compacting chalk reservoirs. A simple extended form of Nur's modified Voigt model is used to build the dry-rock elastic modulus and porosity relationship. Granular medium contact theory is used to model pressure change, and Gassmann's equation is used to model the fluid effect. A dynamic-compaction model from reservoir simulation is used to model porosity change due to compaction. The rock-physics model is calibrated at well locations from the Ekofisk Field in the North Sea; however, the absence of time-lapse logging makes the 4D calibration challenging. Effective fluid properties are calculated dynamically with the Fluid Acoustics for Geophysics (FLAG) software program, using inputs from the reservoir simulator and PVT measurements. For low water saturation (< 20%) and high-porosity (> 30%) rocks, the model predictions show very good correlation with measured data. The model underpredicts the sonic velocities for rocks with low porosity and high water saturation.