Study on Cap Rock Stability during SAGD Production in Shallow Oil Sand in Block M,Canada

The stability of overlying cap rock and its effective sealing to the injected steam under high temperature are crucial for the successful development of shallow oil sand with steam assisted gravity drainage (SAGD).The mineral composition,microstructure,thermodynamic parameters and rock mechanics parameters of the core from cap rock in the Block M,Canada were obtained by X-ray diffraction analysis,CT scanning,steady-state plate method and tri-axial stress test.A mini-fracturing test was used to define the minimum in-situ stress and the formation fracture pressure gradients of the oil sand reservoir and its cap rock.The stress distribution of cap rock during SAGD production was analyzed by coupling a numerical simulation of reservoir fluid dynamics with geomechanics.The experimental and simulation results show that the content of clay minerals in the core of cap rock is high,while the proportion of illite-montmorillonite mixed layer is as high as 35%.The core may expand when it contacts with water,resulting in a decrease in strength.The cap rock has no cracks or micro-cracks,but has a weak surface area,and exhibits heterogeneity in its petrophysical properties.The minimum principal stress (along vertical direction) of cap rock is 3.65 MPa,which is beneficial for the effective sealing of injected steam.During the entire lifecycle of production,the maximum Mises stress is always lower than the minimum principal stress of cap rock.Under the designed SAGD steam injection temperature and pressure,the overlying cap rock of oil sand reservoir in the Block M has not undergone plastic deformation or damage,and reliable sealing can be maintained throughout the production period.Based on the research results,a method was established to evaluate the cap rock stability of SAGD production of oil sand,and the research provides a technical reference for the optimization design and implementation of steam injection thermal recovery in such reservoirs.