Mixing in Underground Gas Storage

Abstract

Underground storage fields developed on aquifers or converted from producing reservoirs frequently encounter problems related to mixing phenomena. The need for understanding flow and displacement processes as affected by mixing in the porous matrix surrounding storage wells is important, both in design and operation of storage units.

There are five specific occasions where mixing and dispersion take on practical significance in storage

• 1. - Total or partial conversion of working inventory from native to pipeline quality gas,

• 2. - Use of the storage unit for selective storage of gases having different compositions,

• 3. - Prospects of replacing part of the cushion gas by a lean or inert substitute,

• 4. - Use of chemical or radioactive tracers in investigating possible leak paths,

• 5. - Compressed Air Energy Storage in depleted gas fields.

In order to provide a capability to predict the field performance as affected by mixing (or dispersion), a methodology has been developed which emphasizes numerical simulation implemented by appropriate computer. The programmes 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 have been successfully carried out.

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.