Numerous studies show that the widely used diffusivity equation is a significant simplification of real fluid flow in reservoir. One of the underinvestigated phenomena is viscoelastic behaviour of liquid in porous media described by relaxation flow equations.

The objective of the work is to study features of viscoelastic flow. Choice of this topic was primarily determined by the fact that none of the existing non-linear flow models (with initial pressure gradient and permeability/pressure dependency) describes the observed decrease of absolute value of DD derivative compared to pressure derivative at shut down.

To solve the formulated problem, results of petrophysical and PTA studies were used; a number of viscoelastic flow methods were developed. The optimal method was chosen; its distinctive feature is simultaneous consideration of stress relaxation and deformation delay as well as taking into account deformation sign during loading (DD) or unloading (PBU). It was found that usually stress relaxation time is close to zero while deformation delay time ranges from half an hour to several hours.

The proposed model allows increasing interpretation accuracy of PBU and DD initial segments and long term pressure data, and also determining deformation delay time.

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