The development increasingly involves reservoirs with low and ultra-low permeability reservoir. "Traditional" welltest analysis technologies in such reservoirs lose their efficiency (Davletbaev A.Y., Zhylko E. Y. etc. 2015, Loznyuk O.A. etc. 2015) as they require long-time shut-in (about 1 month and more), which causes huge losses in production (Davletbaev A.Y., Islamov R. R etc. 2015). In such conditions, analysis of usual operation data analysis and decline analysis become relevant (Arps J.J. 1945, Fetkovich M.J. 1980, Ilk D., Blassingame T.A. 2013, Doublet D.E. etc. 1994). This method has a lot of limitations, in particular, it is necessary to know the value of the reservoir pressure at the moment of the reservoir operation for interpretation, which is not always possible in the conditions of the operation and maintenance phase. The present work offers the method permitting to increase the informational content of the decline analysis, as well as to determine the value of initial reservoir pressure by combining «short» build-up welltest analysis with decline analysis in the operating well.

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