Prediction of sustained annular pressure and the pressure control measures for high pressure gas wells

The sustained annular pressure caused by channeling threatens the casing safety significantly. Cement mantle composite permeability and one-dimension unstable seepage were used to describe the channeling of high pressure gas based on the analysis of the structure of the cement mantle containing micro-fractures and the seepage process of high pressure gas. According to the mass conservation law and volume accordance principle, a model was built to predict and analyze the pressure rising process when gas invades into the trapped annulus, and then this model was used to study the impacts of different factors on the annular pressure. The pressure rising process can be divided into rapid rising stage and stable rising stage, and the factors affecting annular pressure include the gas solubility and compressibility of annular liquid, height of cement return, composite permeability of cement mantle and annular volume; the rising velocity of annular pressure declines with the increase of annular liquid compressibility; the ultimate value and rising velocity of annular pressure increase with the increase of cement return height; the higher the composite permeability of the cement mantle, the faster the annular pressure increase; the increase of annular volume can prolong the rising time of pressure. From the viewpoint of engineering, high pressure gas wells should take annular liquid with proper compressibility, self-repairing cement should be used in high pressure gas wells to improve cement quality, the composite permeability of cement mantle should be reduced by using proper measures if necessary, and annulus volume is properly increased to prevent the pressure from rising up too quickly.