The author states that in order to solve the problem of behind-the-casing flows of liquids and gases, more information is needed on the plugging properties of cement slurries (PPCS) used in casing cementing. While waiting-on-cement time the hydrostatic pressure in the annular space decreases. This leads to the inflow of reservoir fluid into the well, the destruction of the still weak structure of the cement slurry and channeling formation. A list of PPCS criteria is proposed which are necessary and sufficient for assessing the intensity of filtration-suffusion flows and the state of integrity of the cement slurry structure. As a criterion of filtration properties, the so-called initial gradient is considered; it characterizes the conditions for the beginning of the pore fluid movement in the cement slurry. The second criterion is the characteristic of the beginning of the destruction of cement slurry solid skeleton by the filtration flow and the beginning of the channeling formation. Analysis of the methods for experimental evaluation of the indicated criteria has been carried out. It is shown that due to the instability of cement slurries properties it is fundamentally important to link the PPCS criteria to some characteristic time point. As such time point, the author proposes to use the end of repression on the formation - the beginning of the fluid inflow into the well. A description is given of several methods for determining the repression period. The most informative among these methods is the one that uses the dynamics of the pore pressure of cement slurries. A technique for obtaining pore pressure curves of cement slurries is recommended. This technique is suitable for use, including in industrial laboratories. A method for assessing the quality of reservoir isolation during the waiting-on-cement time and some methods for preventing behind-the-casing flows are presented.
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