Mixing in Underground Storage Reservoirs

ABSTRACT Underground storage fields developed on aquifers or converted from producing reservoirs frequently encounter problems related to molecular transport and mixing phenomena. The need for understanding flow and displacement processes as affected by mixing and dispersion in the porous matrix surrounding storage wells is important, both in design and operation of storage units. There are four specific occasions where mixing and dispersion takes on practical significance in storage: Total or partial conversion of working inventory from native to pipeline quality dry gas.Use of the storage unit for selective storage of gases having different compositions.Prospects of replacing part of the cushion gas by a lean or inert substitute.Use of chemical or radioactive tracers in investigating possible leak paths. In order to provide a capability to predict the field performance as affected by mixing and dispersion, a new methodology has been developed which emphasizes numerical simulation implemented by appropriate computer programms. The programms permit calculation and graphical tracking of the front separating the gases involved in the miscible displacement process as affected by heterogeneities, operating conditions and the physical dispersion. Identification of physical parameters necessary for appropriate modeling is included. The field data reported in the paper include one storage reservoir where an entire working inventory and about half of the cushion gas, have been displaced by cycling a gas of different characteristics. Similar conversions are currently being planned for two other storage reservoirs in the next few years. In an area which will have profound implications in economics of underground storage, interesting data have been collected by substituting inert gas for part of the cushion gas in two storage reservoirs. One of these substitutions has recently been completed while the other is in progress.