In some of our sandstone reservoirs we have encountered apparent residual gas below the live gas column, and the quantification of the residual gas volume has been a challenge. The presence of such intervals of residual gas is related to the charge history of the field. If for any reason the trap was breached or tilted after gas charge, gas was replaced by water encroachment in the portion of the rock where the residual gas is now present, but the water displacement of gas was not complete. As water filled pores and pore throats, the free flow of gas stopped, allowing only water to pass through the pore system. This resulted in gas becoming trapped behind the encroaching waterfront as residual gas. The need to identify residual gas zones is important for several reasons, mainly, appropriate fluid corrections for porosity calculations and acknowledging that an absence of any well-defined water-leg will not cause an underestimation of water saturation by the incorrect use of Pickett plots. Appreciation of residual gas allows the use of imbibition saturation-height functions for both static and dynamic modelling. Quantification of residual gas saturations are complicated by sensitivity to porosity and cementation factor, ''m'', at low water saturations and to a lesser degree saturation exponent, n. A number of methods have been utilised in trying to quantify the range of in-situ residual gas saturations.
INTRODUCTION AND IMPORTANCE OFRESIDUAL GAS
We have now encountered many examples of residual gas. This paper represents the many learnings on how to identify, attempt to quantify and finally appropriately model the impacts of residual gas. The presence of residual gas is related to the charge history of the field. If for any reason the trap was breached or structure tilted after gas charge, gas will be replaced by water encroachment.