The Importance of Saturation History for Tight Gas Deliverability

摘要

Contemporary thinking on the origin of “basin centered” gas highlights the importance of proximity of mature source rocks to many low permeability gas reservoirs. The quality of gas charge determines the efficiency of the system; this is largely dependent upon source rock characteristics (oil versus gas prone) as well as the time sequence associated with hydrocarbon migration and reservoir unroofing. Examples exist where tight gas accumulations exhibit production of both condensed water and formation water. This can occur where low permeability reservoirs exist downdip or updip of pervasive gas saturation, where the source rock is not in proximity to the reservoir or where source rocks are lean or limited relative to the capacity of the reservoir rocks. Water production can also occur where the reservoir system has been breached, buried, or tilted and water has been imbibed to partially replace gas, thereby trapping a residual gas phase. Dynamic rock typing “focuses on using petrophysical properties including rock type, porosity and effective permeability at reservoir conditions to divide the reservoir into flow units” (Liu et al, 2012); however, this should be done within the context of their saturation histories. Given the dynamic nature of some gas accumulations over a geologic timescale, it is important to model the relative permeability controls on gas and water deliverability using both drainage and imbibition models. A comparison of drainage versus imbibition models highlights the importance of saturation history, since minor variations in saturation can result in large changes in fractional flow and deliverability - depending upon whether drainage or imbibition conditions apply. Tight gas petrophysical studies must go beyond volumetrics and should consider both static (storage) and dynamic (flow) properties within the context of the petroleum system and the evolution of the current day pore geometry and fluid saturation distribution.