A New Method For Calculating Relative Permeability Of Tight Rock By Spontaneous Imbibition

Relative permeability is of fundamental significance in soil science, petroleum engineering, and many other industries. There are two frequently used methods to obtain relative permeability. One is direct experimental measurement and another is theoretical calculation from capillary pressure data. However, these methods may suffer limitations in core samples with extremely low permeability. Experimental measurement of both relative permeability and capillary pressure can be quite difficult and not reliable in such tight rocks, especially in the tight rocks with both matrix system and fracture system, which have more complex physical properties and abnormal percolation patterns.Spontaneous water imbibition is a percolation process and an important part of water-flooding process, because the data are significant in forecasting the production performance. There have been many reports and papers on this subject. However, less attention has been paid to the research on calculating relative permeability using spontaneous water imbibition data.In this paper, a new method for calculating relative permeability of tight rocks was proposed by combining spontaneous water imbibition and numerical simulation. It has been found experimentally that the relationship between the imbibition rate and the reciprocal of recovery by spontaneous water imbibition has two linear parts in rocks with fractures but has only one linear part in rocks without fractures. One of the two linear parts may represent the fracture property and the other may represent the matrix or the set of smaller fractures. On the basis of this theoretical assumption, the effective water permeability of both matrix and fracture at Swf (water saturation behind imbibition front) were computed by using spontaneous imbibition data and establishing dual-media model. The results demonstrated that the new method is a substantial and practical solution for obtaining relative permeability of tight rocks. Relative permeability curves of both matrix system and fracture system in tight rocks can be inferred simultaneously using the new method.