Exploring the influence of effective stress sensitivity on oil shale: implications for porosity and permeability

Abstract

Changes in the porosity and permeability of oil shale under the overburden pressure often have a significant impact on the subsequent development of reservoirs. The authors of this article investigated the over burden porosity and permeability characteristics of tight oil reservoirs in two regions in China: the Fushun West Open-pit Mine in Liaoning and the Jimsar shale oilfield in Xinjiang. Overburden porosity and permeability experiments were conducted on oil shale cores. Three-dimensional visualization and quantitative analysis of the micropore structure of cores under different effective stresses were used to model the relationship between oil shale porosity, permeability, and effective stress. In addition, the stress sensitivity of oil shale reservoirs was analyzed, using the damage rates of permeability and stress sensitivity coefficients. The results indicated that in both regions, oil shale porosity and permeability exhibited a decreasing trend with increasing effective stress, following a negative exponential function. When the effective stress was less than 8 MPa, the permeability stress sensitivity coefficient decreased sharply. Once the effective stress exceeded 8 MPa, the stress sensitivity of permeability in both regions weakened, maintaining a range between 0.2 and 0.4 MPa–1. The results of three-dimensional visualization simulations were the same as the experimental results. Taken together, the results showed that the porosity of different-sized pores decreased with increased stress, which reflected the synergistic effect of different-sized pores on porosity in shale. This study has practical significance for revealing the variation in pore size in shale reservoirs and establishing the physical characteristics of oil shale reservoirs.

Keywords

• oil shale
• porosity
• permeability
• stress sensitivity