Review of failures for wells used for CO2 and acid gas injection in Alberta, Canada

Wells have been identified as posing a greater risk for leakage from CO2 storage sites than geological features such as faults and fractures, particularly in mature sedimentary basins with high well density such as those onshore in North America. A commonly-held belief is that CO2 injection wells will pose a lesser risk than wells drilled for other purposes because greater care would be taken in regard to their completion and operation. The existence of CO2 and acid gas injection operations in Alberta, Canada, provided the opportunity to test this hypothesis. Currently in Alberta there are 31 wells used for CO2 injection and 48 wells used for the disposal of produced acid gas (a mixture of CO2 and H2S that is separated from sour gas to meet pipeline and market specifications). Only 22 wells were drilled specifically with the purpose of injecting CO2 or acid gas; all others are previous wells that have been subsequently converted to injection wells. Well failures include: surface casing vent flow, casing failure, tubing failure, packer failure, and zonal isolation failure. The incidence of well failure is greater in the case of converted wells than in the case of wells drilled and completed for injection purposes. Most failures are not caused by injection; they are due to general causes encountered in the general well population. Failures due to injection are mostly tubing and packer failures, which are monitored by regulation and are easily detected and repaired. The incidence of well failure is greater for wells drilled prior to the advent of regulations in 1994 regarding drilling and completion of injection wells. While the incidence of failure in the CO2 and acid gas injection wells in Alberta is comparable to that in the general well population, the analysis indicates that injection wells drilled for purpose and under a proper regulatory regime have a lesser incidence of failure than the general well population. A proper regulatory framework for CO2 injection wells is essential for reducing and preventing well failures. (C) 2008 Elsevier Ltd. All rights reserved.