Some Transport Properties For Low-Permeability Reservior Based On Fractal Theory

Because of the special structural characteristics of low permeability reservoir, fluid flow in low permeability reservoir is different from moderate and high permeability reservoirs, for example, flow does not follow classical Darcy law (i.e. Non-linear seepage), severe formation damage in reservoir development and spontaneous imbibition, etc. Formation damage may widely occur during drilling, coring operations, well completions, production, or subsequent injection of water for enhanced recovery, especially for low permeability reservoir, the damage is even more critical. Spontaneous imbibition displacement in oil reservoirs is an important recovery mechanism of low permeability fractured reservoirs. Research and analysis of the basic flow in low permeability reservoir have very important meaning to promote reservoir production.The flow and macroscopic transport in natural porous media, such as reservoir rock, have wide applications in many fields and have steadily received considerable attention. The natural porous media have extremely complex microstructures, which can be characterized by fractal theory, and many studies apply the fractal theory to analyze the fluid flow in porous media. However, studies of reservoir damage by extraneous fluids and the spontaneous imbibition in low permeability fractured reservoir, based on the fractal characters of porous media, are limited. More practical results may be obtained through combining the fractal characters of reservoir rocks to study the formation damage and spontaneous imbibition mechanism. This has very important significance to reduce effectively formation damage and increase the recovery rate by using the spontaneous imbibition.The focuses of the present thesis are as follows:The first chapter outlines the status of low permeability reservoir in the national economy, and the basic structures and physical parameters of reservoir media and introduces the fundamental concepts of fractal theory; the second chapter reviews the theory and mathematical background of fractal characteristics of porous media, including the basic characteristics of porous media and apparent two-phase fractal theory. In this chapter, an analytical model for the permeable maximum pore size of porous media is derived based on fractal technique. In addition, the fractal dimension for roughness height is introduced to characterize the roughness characters of particle surface and the specific surface area of porous media is presented, with the roughness characters of particle surface be considered; the third chapter briefly summarizes the damage mechanisms of low permeability reservoirs and derives the fractal model for the invasion depth of extraneous fluids in porous media with the effect of capillary pressure be considered. When the model is applied to polymer composites industries, a good agreement is also found; the forth chapter outlines the mechanism and current researches of spontaneous imbibition in porous media and analyzes the impact of extent of convolutedness of capillary on the capillary rise in a single tortuous capillary and reservoir media. Based on fractal theory, analytical expression for time evolution of the height/weight of capillary rise in a single tortuous capillary is obtained, and fractal model for the spontaneous co-current imbibition is obtained, a good consistence between model predictions and experimental results is found. In this chapter, experimental results that are not in accord with classical Lucas-Washburn (LW) law are also explained theoretically based on the fractal character of tortuous capillaries/streamtubes of porous media; the fifth chapter summarizes the main work and innovation of the present thesis, and some comments are also made on the potential research subjects and directions about reservoir impairment and spontaneous imbibition based on fractal theory.