Investigation of Post-Abandonment Surface Subsidence in Steam-Assisted-Gravity-Drainage Operations

Abstract

Summary Steam-assisted gravity drainage (SAGD) has been extensively applied in thermal recovery from oil sands reservoirs in the Athabasca region of Northern Alberta, Canada. As the steam chambers associated with SAGD well pairs become mature, a form of abandonment is often applied that may include pressure maintenance in the depleted zone. Quantification of potential surface subsidence associated with SAGD abandonment becomes critical especially when the mature wells are in proximity to future developments. In addition, induced shear stresses should be estimated to fulfill well-integrity requirements. In the context of this case study, first, the development of a static geomechanical model (SGM) derived from a fine-tuned geomodel realization is discussed, which forms the basis for the iteratively coupled simulation model. The calibration work flow of the coupled reservoir/geomechanical simulation model to historical heave data is then reviewed and the effects of different parameters on calibration quality are investigated. Finally, the estimation of subsidence and the induced shear stresses in the nearby wells are discussed, and the magnitude of residual heave is quantified. The results of this study show that only a fraction (up to 38%) of surface heave is reversible (in form of subsidence) during the abandonment phase. Therefore, the magnitude of the surface subsidence and the associated shear stresses are small. The modeling study has also shown that a small magnitude of subsidence may be recorded even 10 years after abandonment. However, more than 50% of the surface subsidence is observed in the first 2 years after abandonment. Other important findings of this study include documenting the effects of thief-zone interaction and pseudoundrained loading as they relate to irreversibility of surface heave; documenting the effects of various geomechanical parameters on the quality of calibration against the historical heave data; observation of the relative effects of the isotropic unloading, thermal expansion, and shear dilatancy on the magnitude of heave; and quantification of incremental, yet small, shear stresses along the nearby horizontal wells.