Method of Evaluating the Capacity of Gas Channeling Prevention of a Cement Slurry in Gelling Transition-State

Gas channeling in annular space is one of the technical difficulties encountered in oil/gas well cementing. Based on the theory of cement slurry weightlessness, the relationship among the effective fluid column pressure of the cement slurry, the internal structural resistance and the formation fluid pressure is analyzed to determine the timing of channeling detection. By continuously collecting data on the changes of the pore pressure in the cement slurry, it can be determined whether the gas has overcome the internal structural resistance of the cement slurry to cause gas channeling, and this leads to the invention of a new method for evaluating the anti-gas-channeling capacity of a cement slurry in transitional gelling state by direct observation. Using this method, the channeling of several cement slurries is studied. The study results show that: 1) bare cement slurry is almost unable to resist gas channeling; 2) by introducing appropriate amount of filter loss reducers, the anti-channeling capacity of a cement slurry can be slightly improved, with only limited effect; the cement slurry can only resist gas channeling pressure of 50 psi (0.344 MPa); 3) changes in the type and concentration of the anti-gas-channeling agent lead to different anti-channeling capacity of a cement slurry. Generally speaking, cement slurries treated with different anti-channeling agents can stand channeling pressures in a range of 100 – 200 psi (0.689–1.379 MPa). Some anti-channeling agents exhibit higher capacities of anti-channeling by increasing their concentrations in the cement slurries, with limits of performance based on their inherent characteristics. The method described in this paper is easy and simple, and has good practicality and repeatability. It can be used to compare the performances of different anti-channeling agents, and to provide reference to the design of anti-gas channeling cement slurries.