Acid gas, a mixture of CO2 and H2S produced from sour gas reservoirs in Western Canada, has been injected into deep geological formations for close to 15 years with a good safety record. Injection currently takes place at 41 locations into depleted oil and gas reservoirs, and deep saline aquifers. From this point of view, the acid-gas injection operations in Western Canada constitute a commercial-scale analogue for CO2 geological storage. A major issue in geological injection of fluids is the integrity of the injection unit; i.e., avoidance of leakage through natural or induced fractures. Regulatory agencies in Western Canada impose safe limits on the injection pressure to maintain the pressure around the injection well below the fracturing threshold of the rocks. An evaluation of the stress regime at the acid-gas injection sites in Western Canada was performed to assess the relationship between the maximum allowed wellhead injection-pressures and the rock-fracturing thresholds.
The stress regime in the Alberta Basin has been established in this study on the basis of 1446 hydraulic tests and on density logs in selected wells. On this basis, the minimum horizontal stress and the vertical stress have been estimated at all acid-gas injection sites. Minimum horizontal stresses increase with depth with a basin-wide average gradient of 16.6 kPa/m. Maximum vertical stresses increase with depth with a basin-wide gradient of 23.8 kPa/m. Fracture pressures increase with depth with an average gradient of 19 kPa/m, and are at all the sites greater than the minimum horizontal stress, but smaller than the vertical stress. Maximum bottom hole injection pressures are safely below the minimum horizontal stress, hence lower than the fracture pressure. Thus, there is no danger of opening existing fractures, neither, obviously, of inducing new ones.
The study has also shown that, in the case of acid or greenhouse gas injection, prescribing the maximum wellhead injection pressure, according to general values established for water disposal, is not sufficient because the gas most likely will not have enough bottom-hole pressure to overcome the formation pressure and enter the injection unit. Thus, for acid and greenhouse gas injection in geological media, there is need to establish the maximum bottom hole and wellhead injection pressures on the basis of minimum horizontal stress to avoid opening of potential pre-existing fractures, and on the basis of gas properties at reservoir and wellhead conditions (pressure and temperature).
The current acid-gas injection operations in Western Canada meet the safety criteria imposed by the need to maintain the integrity of the injection unit. However, the wide range of variability in the ratio between minimum horizontal stresses and fracturing pressures points out to the need to perform hydraulic tests at each site, rather than estimate the fracturing pressure from basin-wide fracturing gradients or numerical models. Performing carefully conducted tests will also allow site-specific determination of the minimum horizontal stress, hence of a better upper limit for the bottom hole injection pressure, to ensure that pre-existing fractures, if present, will not be opened.
Bachu, S., Haug, K. and Michael, K. (2008): Stress regime at acid-gas injection operations in western Canada; Energy Resources Conservation Board, ERCB/AGS Special Report 94, 42 p.