Flow of gel and oil in open and tight formation fractures

Abstract

Open fractures refer to fractures sufficiently separated to have a very large effective permeability and which permit fluid flow at appreciable velocities; consequently fluid pressures at the source can be maintained for such fractures only at high injection velocities. Tight or closed fractures refer to fractures which have a low to moderate effective permeability and which will not permit flow at appreciable velocities; consequently hydraulic pressures in such fractures become nearly equal to the injection pressure. The difference between the open and tight fractures is a matter of degree.Flow characteristics and relative pressure drops of different oils and gels, as would be encountered in open formation fractures, are analyzed by means of flow observed in three different small diameter tubes. Penetrabilities of a light oil and a gel, as would be injected into tight fractures and unfractured formations, are investigated with porous ceramic bricks. The flow mechanisms of the oils and gels are found to be markedly different at appreciable flow velocities, and resultant pressure losses are higher for oil than gel in open fractures. This is because oil flows largely as a ‘Newtonian’ fluid, while gel flows as a thtxotropic medium. By contrast penetrability in tight fractures and unfractured rocks is much greater for a light oil than gel under hydrostatic conditions. Some relative advantages of oil and gel in formation treatments are discussed.