Pore Structure Characteristics of Ultra-Low Permeability Reservoirs

Pore structure characteristics of ultra-low permeability reservoirs are studied in this paper. The impact of pore structure characteristics on permeability and reservoir development effect is analyzed by conventional and constant-rate mercury penetration experiments. The results show that the frequency curve of pore radius distribution of ultra-low permeability reservoirs has little change with permeability. The average pore radius in ultra-low permeability reservoirs with different permeability levels had little difference, and its distribution ranges from 98.5 to 110.1 mu m. With decrease of permeability, the frequency distribution curves of throat radius and pore-throat ratio change significantly. The peak value of throat radius curve moves to the left and increases significantly, and the average throat-radius distribution decreases from 2.33 to 0.91 mu m. However, the impact of pore-throat ratio on permeability is contrary to that of throat radius. Both throat radius and pore-throat ratio has strong correlation with permeability, and they are in exponential relationship. With the in-depth study, it is found that seepage capacity of reservoirs is restricted mainly by throat radius and pore-throat ratio. When throat radius decreases to less than 1 mu m and pore-throat ratio increases more than 130, permeability decreases sharply and obviously. When permeability is less than 2 x 10(-3)mu m(2), the proportion of fine throats and medium throats under the condition of effective water injection is significantly lower, and the production of single well and the recovery efficiency by water flooding in ultra-low permeability reservoirs are low. The study explains effectively the fundamental reason for the poor efficiency of production and recovery of ultra-low permeability reservoirs from the mechanism.