Due to severe heterogeneity of thick positive rhythem reservoir and the gravity influence, the injected water channels and ineffectively cycles through its lower high permeability zones. For the whole reservoir, its sweep efficiency is relatively low and a huge amount of residual oil is enriched in its upper low permeable zones.The technique of in-depth fluid diversion by "gel dam" placed with horizontal well involves the injection of gel by horizontal or sidetracking wells drilled between the injection and production wells in the lower more permeable and strongly swept zones of thick positive rhythm reservoir, creating" gel dams". The injected water then will bypass the "gel dams"and be diverted into the upper low permeability zones where crude oil is relatively enriched. As a result, the sweep efficiency and oil recovery of the upper, low permeability zones can be both increased. In this paper, the EOR mechanism of the technique is studied by means of laboratory water flooding experiment combined with x-ray computer tomography(C T). The physical model, an artificial cementing heterogeneous model casted with epoxy resin on surface, is made based on the similarity criteria for water flooding process in 2D porous media.The water flooding processes in the physical model, before and after the "gel dam' is formed, are scanned by x-ray C T at different pore volume injected, which provides a visible means for studying the EOR mechanism of the technique, through observing the distribution feature of "gel dam', tracking the oil and water distribution, water flooding streamline’s change, fluid saturation distribution and its dynamic variation. The results of water flooding experiments indicate that, with this technique, the injected water is diverted into the upper low permeability layer to displace the remaining oil therein, and the sweep efficiency and oil recovery both increase significantly. After the "gel dam" formed, the ultimate oil recovery increases by 16.7%, its water cut falls to 62.8%, and its ascending velocity slowed down. With" gel dam"technique, gel can be placed in the deep reservoir zone to achieve in-depth fluid diversion.It will be a promising EOR method for thick positive rhythm reservoir.


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