Development of Spontaneous Imbibition in Tight Rocks

Spontaneous imbibition of a wetting liquid in a tight reservoir is an important driving mechanism for enhancing oil recovery and providing satisfactory flow-back efficiency. In this paper, the authors experimentally study the characteristics of spontaneous imbibition in the tight sandy samples. The changes of imbibition mass with time in the samples with a different initial distribution of porosity are measured and analyzed. The nuclear magnetic resonance (NMR) method is applied to measure the development of imbibition in the pores of different diameters. The results show that the imbibition process is divided into three stages, depending on the pore size distribution in the samples. In the first stage, imbibition satisfies the Lucas and Washburn equation. In most cases, imbibition first develops rapidly in the small pores, and then the wetting phase migrates into the large pores, which is reflected by the peak shift in the T2 spectrum. The variations in the T2 spectra also reflect the formation of new fractures and the collapse of pores. The development of spontaneous imbibition is controlled by the pore size distribution, the formation of fractures, and boundary conditions. To simulate the imbibition process, the authors propose a simple model based on the pore size dostribution and capillary bundles. The model is used to demonstrate the three stages of imbibition in the porous media.