Abstract
Oil in shale formation includes free oil in inorganic pores and ad-/absorbed oil within organic matter (OM). The latter is immobile without exerting extra forces due to the strong molecular interactions between oil and OM. In this study, we conducted newly designed two-core series oil flow experiments with shale and sandstone samples to reveal the mechanisms of mobilizing the oil in OM, and a new phenomenon in shale samples was observed. Analysis using the theory of shale compaction showed that the OM and oil within it are compressed by the inorganic matrix in the depletion process, raising the effective stress of OM and its pore pressure. When the effective stress of OM attains a threshold value, the oil in OM is squeezed out into the surrounding inorganic matrix. For the first time, a threshold stress change of OM is defined, and a model is proposed for describing the mechanisms of mobilizing the oil in OM validated by the experimental results. The values of threshold tress change τ increases from 0.14 MPa to 0.48 MPa with the saturation pressure increasing from 0.51 MPa to 1.47 MPa. Field scale modeling shows that the mobilization of oil in OM is greatly affected by the permeability of OM and inorganic matrix, the stiffness of inorganic matrix, pressure drawdown, and the matrix size.
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