Process of producing fibrillated nanocellulose with low energy consumption and a high drainability market pulp characterized by comprising the steps of: a) cooking and bleaching of biomass generating a mass rich in cellulosic and hemicellulosic polymer chains comprising very reduced amount of lignin and extractives; b) separating a fiber line from the mass of step a), wherein said fiber line has the following characteristics: b.1) average fiber length of 0.3 to 2.5 mm; and b.2) primary fine content of 3% to 30% of fines by mass; c) fractionating of the segregated fiber line of step b) by means of a fractionating system into at least two streams, a high-primary fines content material stream and a low primary fines content pulp stream, so called high drainage pulp; d) separating the high-primary fines content material stream obtained in step c), wherein said stream has a content of 10% to 90% of primary fines by mass and consistencies of 0,02% to 1%; e) thickening of the high-primary fines content material stream of step d) until its consistencies reach 20 to 15%; f) submitting the thickened material of step e) to a nanofibrillated cellulose production process, wherein it is subjected to mechanical refining energy, being the energy consumption used for the generation of the nanofibrillar cellulose smaller than in comparison with common cellulosic pulp starting material; g) separating the low primary fines content pulp stream of step c), wherein said pulp presents a massic amount of fines of about 3 to 80, with significant lower resistance to drainability and water retention value, wherein the °SR presents a reduction of 15% to 50% and the water retention value presents a reduction between 7% and 350; and h) drying of the low primary fines content pulp of step g) in drying pulp machines. a) cooking and bleaching of biomass generating a mass rich in cellulosic and hemicellulosic polymer chains comprising very reduced amount of lignin and extractives; b) separating a fiber line from the mass of step a), wherein said fiber line has the following characteristics: b.1) average fiber length of 0.3 to 2.5 mm; and b.2) primary fine content of 3% to 30% of fines by mass; c) fractionating of the segregated fiber line of step b) by means of a fractionating system into at least two streams, a high-primary fines content material stream and a low primary fines content pulp stream, so called high drainage pulp; d) separating the high-primary fines content material stream obtained in step c), wherein said stream has a content of 10% to 90% of primary fines by mass and consistencies of 0,02% to 1%; e) thickening of the high-primary fines content material stream of step d) until its consistencies reach 20 to 15%; f) submitting the thickened material of step e) to a nanofibrillated cellulose production process, wherein it is subjected to mechanical refining energy, being the energy consumption used for the generation of the nanofibrillar cellulose smaller than in comparison with common cellulosic pulp starting material; g) separating the low primary fines content pulp stream of step c), wherein said pulp presents a massic amount of fines of about 3 to 80, with significant lower resistance to drainability and water retention value, wherein the °SR presents a reduction of 15% to 50% and the water retention value presents a reduction between 7% and 350; and h) drying of the low primary fines content pulp of step g) in drying pulp machines. <EOS>