Modeling Residual Gas Saturation to Aquifer Influx
Mark H. Holtz
Bureau of Economic Geology

With the U.S. moving to greater use of natural gas, the recovery efficiency of natural gas reservoirs is increasingly important. One of the major sources of lower recovery is residual gas. Residual gas saturation controls the volume of gas trapped in that portion of the reservoir that has experienced water encroachment. As water moves into a rock volume filled with gas, the water displacement of the gas is incomplete. The water fills pores and pore throats, causing capillary pressure and relative permeability effects to stop the flow of gas and allow only water to pass through the rock volume. This stoppage results in gas being trapped behind the encroaching waterfront as residual gas. The volume and location of the residual gas are controlled by the distribution of the petrophysical properties.

A method based on interrelationships between petrophysical properties creates a model for calculating maximum residual gas saturation (Sgrm). The model is developed as a function of porosity, permeability, capillary pressure, and initial water saturation. The input to the model and its results compare favorably with actual field data where aquifer encroachment is verified from well production history. The defined characteristics of residual gas saturation aid in defining wells that have drilled through aquifer-swept zones and in material balance calculations when building 3-D geocelluar models.