Pressure Build-up and Decay in Acid Gas Injection Operations in Reefs in the Zama Field, Canada, and Implications for CO2Storage

Abstract

The objective of this paper is to examinereasons for pressure rise in the Zama X2X pool in northwestern Alberta, Canada, that was used foracid gas disposal, and whether subsequent pressure decay was a result of pressure dissipation into a larger aquifer. The Zama X2X pool, approximately 1 km2 in size, is connected to four other nearby poolsthrough a common underlying aquifer. Pressure analysis for all the pools indicates that they are in goodhydraulic communication. Initial pressure in the Zama X2X pool was approximately 15 MPa. Pressuredeclined first during oil production, stabilizing at around 10 MPa in the early 1970s, after which started toincrease such that it reached 26 MPa in 1986. Subsequently, pressure declined reaching 22 MPa by 1995just prior to starting injection of acid gas (80% CO2 and 20% H2 S). The operator injected acid gas at lowerrates and wellhead pressures than those licensed by the regulatory agency. Despite significant production ofwater and hydrocarbons, the pressure in the Zama X2X pool continued to be higher than the initial reservoirpressure by more than 5 MPa, such that disposal operations were suspended in late 1998. Oil productioncontinued all this time until 2002. Numerical simulations using CMG-IMEXTM and corresponding sensitivity studies reported in this papershow that disposal of more than 1 million m3 of water between 1970 and 1988 and again in 1992-1993 inthe adjacent Zama YY pool, which is in good hydrodynamic communication with the Zama X2X poolthrough the aquifer below the oil column, is the main reason for the high pressures observed in the ZamaX2X pool. Sensitivity studies indicate that pressure decay in the X2X pool was due to fluid production.The study indicates that while pressure rise has been caused by hydraulic communication between theX2X and YY pools through the common aquifer, the aquifer was not of large volume to allow dissipationof the pressure. In addition to the case study, the implications of pressure communication to geological storage of CO2 in aquifers are briefly discussed.