The primary cementing of a high pressure well occurs a gas - flow quite often in the annulus due to the loss of hydrostatic pressure during the hydration and setting of cement, thus causes the hydrostatic pressure in the annulus to be unable to balance the formation pressure. The result can hardly be estimated of its severity.
This paper presents a study on the mechanism of hydrostatic pressure loss of cement slurry after placement. The experimental study shows that the loss of pressure could be divided into four steps. Furthermore, it is also found that hydrostatic pressure loss will continue even after the cement loses its own weight completely. This phenomenon was evidenced by the fact that the hydrostatic pressure decreases rapidly in the simulated test cylinder and it is named the "negative effect". Study on the process of gas migration into the cement is also presented in this paper. It is obvious that when the formation pressure is higher than the hydrostatic pressure in the annulus, gas will not migrate into the cement and gas flow does not happen either. Because the cement slurry has a certain resistance to the migration or percolation of gas into it, but when the resistance is lower than the gas pressure, the gas will percolate into the cement and result in a gas flow consequently. In order to prevent the gas percolation, it is necessary to increase the internal resistance of the cement until it is equal to or in excess of the formation pressure. Based upon the foregoing understanding, an impermeable cement additive G60 has been developed.
JZ 20-2-10 is a high pressure exploratory well in an oil field in Liaodong Bay of Bohai, China. During drilling, 840 cubic meters mud was lost into the payzone and from it gas kick came several times. When the well was circulated with 1.66 kg/1 mud, the pay zone took mud, but soon after the pump was shut down, the well started to flow. This paper describes the application of a cement additive G60 in cementing the 7″ liner, and the cement bond log and variable density log analyses are given. The cement job was indeed a great success.