Abstract: It is theoretically proved by this paper that: (1)Y2 decline curve is only a transformation of Arps decline curve without any improvement in theory and form; (2)Y2 decline curve can still not be applied to calculate the recoverable reserves when decline index of Arps n≥1 and n<0; (3) relationships between the primary decline rate D_i and decline index n of Arps formula and the parameters a and b of Y2 formula are as follows: n=1/b, a=1/nD_i . The two methods are applied to Romashikino oilfield of Russia for its decline data analyses, and the results indicate the validity of theoretical analysis in this paper.

Key words: Y2 decline curve, Arps decline curve, analysis