Effect of depressurization on trapped saturations and fluid flow functions

Production below bubblepoint will generate free gas, first as discontinuous gas up to the critical gas saturation and thereafter free or mobile gas. The level of critical gas saturation is affected by pressure decline rate, interfacial tensions, pore structure, etc. The gas relative permeability is strongly reduced when trapped gas is present. Recent experimental studies have proved that gas relative permeability can be several orders of magnitude lower for an internal gas drive process (gas liberation during depressurization) than for an external gas drive process (gas injection). The critical gas saturation may indirectly influence gas breakthrough, gas cut, and also oil production. Network modeling has been used to investigate physical relations to factors influencing the formation of critical gas saturation and the corresponding flow functions. The rock matrix composition determines, together with irreducible water saturation, diffusion paths, and therefore the degree of supersaturation in the medium. This mechanism combined with depletion rates, describes how trapped and mobile gas saturation evolves and determines factors influencing critical gas saturation. We observe that bubble generation is strongly dependent on depletion rate, which in turn affects the critical gas saturation. The gas relative permeability is found to be two orders of magnitude lower than f8r gas injection even at relatively high gas saturations. We discuss the importance of including the physics of depressurization and advice on correct implementation of depressurization in reservoir simulations. The results show that lower coordination number leads to higher critical gas saturation. The variation of critical gas saturation with pore structure diminishes at higher depletion rate. The significance to field application of depletion is that near the production well the pore structure has little influence on the critical gas saturation, while at low depletion rate in the reservoir (far from the well) the pore structure may be an important factor for the critical gas saturation. Copyright 2008, Society of Petroleum Engineers.