Experimental Study On Reservoir Characteristics And Seepage Law Of Ultra-Low Permeability Reservoirs In Changqing

By using rate-controlled mercury penetration technology, NMR technology, centrifuged laboratory experiment and micro displacement experiment, this paper studied the rock samples from ultra-low permeability sandstone reservoirs in Changqing oil field. Then, the experiment results of these samples are compared with that of extra-low permeability ones and tight ones also coming from Changqing so that the reservoir characteristics and seepage law of ultra-low permeability reservoirs can be discussed. The studies provide the following results:1. From the study of reservoir microcosmic pore structure, it can be known that the pore and throat development of ultra-low permeability reservoirs in Changqing is worse than that of extra-low permeability reservoirs in Changqing and better than that of tight reservoirs in Changqing. Compared with extra-low permeability reservoirs, the pore throat ration of ultra-low permeability reservoirs is bigger and its distribution range is broader; meanwhile, the large throats and the pore volume controlled by large throats in ultra-low permeability reservoirs are far less.2. From the study of reservoir movable fluid, it can be known that the average of movable fluid saturation of extra-low permeability reservoirs and ultra-low permeability reservoirs in Changqing is more than 40%. Even though the total movable fluid saturation in ultra-low permeability (K>0.3x10-3μm2) sample is close to that in extra-low permeability one, the movable fluid content controlled by large throats in ultra-low permeability (K>0.3x10-3μm2) sandstone sample is far less than that in extra-low permeability one. The movable fluid saturation of tight reservoirs is less than that of extra-low permeability or ultra-low permeability reservoirs and the movable fluid in tight reservoirs is nearly controlled by fine throats whose radius is less than 0.2μm. The average of T2 cutoff value of ultra-low permeability reservoirs in Changqing is suggested to be 15.22ms.3. From the study of reservoir monophasic fluid seepage law, it can be known that compared with extra-low permeability reservoirs in Changqing, it is harder to inject water for ultra-low permeability and tight reservoirs in Changqing. The threshold pressure gradient of ultra-low permeability reservoirs in Changqing whose permeability is less than 0.3x10-3μm2, is far higher than that of extra-low permeability reservoirs in Changqing and lower than that of tight reservoirs in Changqing. Permeability, valid throat radius and movable fluid saturation all affect threshold pressure gradient.