Investigation of Post-Abandonment Surface Deformation in SAGD Operations

Abstract

Steam-Assisted-Gravity-Drainage (SAGD) has been extensively applied in thermal recovery from oilsands reservoirs in the Athabasca region of Northern Alberta. As the steam chambers associated with SAGD well pairs become mature, a form of abandonment is often applied for 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 of future developments. In addition, induced shear stresses should also be estimated to fulfill well integrity requirements. In this case study, a coupled reservoir -geomechanical simulation model is developed, calibrated and utilized to estimate the magnitude of post-abandonment subsidence and the induced shear stresses in Surmont SAGD Pilot Project. In the context of this study, first, the development of a simplified 2D static geomechanical model using an original geomodel realization as well as geomechanical properties and stress profiles is discussed, which forms the basis for the coupled simulation model. The calibration workflow of the coupled reservoir-geomechanical simulation model to historical heave data is then reviewed and the impacts of different parameters on calibration quality is investigated. Finally, the estimation of subsidence and the induced shear stresses in the nearby wells is discussed, and the magnitude of residual heave is quantified. The results of this study show that only a fraction (up to 40%) 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 quite 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 two years post-abandonment. Other important findings of this study includes (1) documenting the effects of thief zone interaction as it relates to irreversibility of surface heave; (2) documenting the impacts of various geomechanical parameters on the quality of calibration against the historical heave data; (3) observation of the relative impacts of the pressure and temperature fields on the magnitude of heave; and (4) quantification of incremental, yet small, shear stresses along the nearby horizontal wells.