Derivation of Relative Permeability Curves from Capillary Pressure Curves for Tight Sandstone Reservoir Based on Fractal Theory

Wang, Xiuyu (China University of Petroleum) | Wang, Xiaoqiu (China University of Petroleum) | Wang, Jianfu (China University of Petroleum) | Pu, Yu (China University of Petroleum) | Yang, Shenglai (China University of Petroleum)



Relative permeability has a significant impact on the understanding of migration and accumulation of hydrocarbons in tight formations as well as on their development. However, estimation of oil-water relative permeability curves through conventional coreflooding methods is difficult due to the low-permeability and high capillary pressure in tight rock. This study aims at providing important theory basis for researching on the development of tight oil and gas reservoir.

In this work, relative permeability curves in tight core were derived from capillary pressure curves using normalization and non-standardized computational methods. The relationship in tight rock yields high-quality correlation coefficients, typically of the order of 0.95. This is interpreted here to indicate good fractal behavior, particularly for nanopores. Fractal dimension D ranges from 2.5362 to 2.7399 in this study. The larger value of D represents smaller pore size and more complicated pore structures. The connate water saturation from theoretically derived relative permeability curves for tight cores can reach a value as high as 60-80% and the two-phase flow area is narrow. The total effective permeability is low and the two-phase flow process is complex.

The drainage capillary pressure curve actually reflects the process during which the non-wetting phase displaces the wetting phase, thus the derived relative permeability curve for oil-water system should be comparable to the results from coreflooding experiments of oil displacing water. The calculated permeability curves are in good agreement with data obtained from the laboratory experiments, and can be especially applied to reservoir simulation work of the migration and accumulation of hydrocarbon in tight rock.


Tight oil is an alternative resource to conventional resource, which is widely distributed in China and other countries in the world. Tight oil refers to the oil that accumulates in source rocks as free or adsorbed substance, or the oil that accumulates in tight sandstones and carbonates adjacent to source rocks. Generally, this tight oil accumulation does not necessarily experience migration [1]. Tight oil which is mostly accumulated in the nano-sized pore-throats exhibits complicated flow behavior and is mainly driven by abnormal high pressure. Tight oil distributes pervasively and continuously in tight strata of source and reservoir rocks.