Experimental analysis of matrix moveable oil saturation in tight sandstone reservoirs of the south Ordos Basin, China

Abstract

Tight oil reservoirs in the south Ordos Basin are characterized by fractured, heterogeneous oil-bearing strata (an oil saturation of less than 55% on average), normal pressure (0.8±) and extra-low permeability (less than 0.3 mD). In the Chang 8 tight sandstone reservoir in Honghe oilfield, micro- and nano-pores, especially those with a pore-throat radius of less than 1 μm, account for more than 90%. Fluid flow in the matrix is non-linear and crude oil flow rates are very low under normal pressure gradients. An improved understanding of oil mobility in a tight matrix is key to further development of normal-pressure tight-oil resources in the continental basin. In this study, constant-velocity and high-pressure mercury injection experiments were conducted using samples of typical tight sandstone cores obtained from the south of Ordos Basin. A new method for reconstructing the full-scale pore-throat distribution characteristics of tight sandstone reservoirs was established successfully, based on which multistage centrifugal tests, tests of low-pressure differential displacement of oil by water, and nuclear magnetic resonance tests were conducted in order to obtain the distribution characteristics of moveable fluid in different pores. The moveable oil saturation (MOS) and degree of oil recovery (i.e. ratio of accumulative oil production to producing geologic reserves) of the core samples under different differential pressures for displacement were determined. As for the tight oil reservoirs in the south Ordos Basin, the moveable fluids are mainly stored in sub-micron (0.10–0.5 μm) pores. For Type Ⅰ reservoirs (k > 0.1 mD), the volume percentage of moveable fluid in pores with a radius larger than 0.5 μm is relatively high (greater than 40%). The degree of oil recovery of water flooding serves as the basis for forecasting recoverable reserves for tight oil reservoirs. Recoverable reserves under water flooding, mainly occur in pores with a radius greater than 0.5 μm. The contribution of Type Ⅰ reserves to oil production is observed to be greater than 60%, and the degree of oil recovery reaches up to 17.1%. These results help improve our understanding on the evaluation and classification of Chang 8 tight sandstone reservoirs in Honghe oilfield and serve as theoretical basis for pilot tests to explore effective injection media and development methods to improve the matrix-driven pressure differences and displacement efficiency for oil.