Annulus Pressure Analysis of a Shale Gas Well under Varied Temperatures and Pressures

With consideration of the severe problems related to annulus pressure in shale gas wells,mechanical analysis were performed to clarify the impact of changes in temperatures and pressures during fracturing and production on the annulus pressure in shale gas wells.According to elastic mechanical theories,a mechanical model of a double casing system was established.The formula in calculating the stress of each contact surface was derived based on a displacement continuity condition.In addition,the impact of casing internal pressures,temperatures and in-situ stresses on the sealing ability of a cement sheath were reviewed.Research results suggest that the increase in temperatures,internal pressures,in-situ stresses,and elastic modulus of cement are beneficial and may improve the sealing ability of the cement sheath.On the other hand,decreasing casing thicknesses can be damaging because it increases the radial stress on the interfaces.Radial stresses on interfaces may increase with well depths,so the key to relieving the annulus pressure in the wellhead is to improve the sealing ability of the cement in the lower section of the vertical well.The first and second interfaces are the dangerous positions and they are where there is maximum risk of cementing failure.Improving the cementing quality of the first and second interfaces is helpful in reducing the risk of annulus pressure generation.