valve, letting methane through after achieving a set pressure. At P = 10 MPa, in 178 mm production casing, leaks could start at P = 8 MPa, while in ISS 178*245 mm the ICP could reach 2 MPa. This version is based on the experience of restoring integrity of the seals on 178-mm string in exploration well No. 1 of Ichemminskoe field and on hydrotesting the 178-mm string in some ICP-prone production wells at Vankor field. The tests consisted of brief opening of the leak-off valve (Nefteprommash) or release bolt (Corvet) in the bottom flange of the X-tree cross-member. This procedure revealed the presence of the gas between the wellhead seals of the string No.1, 178 mm diameter, thus indicating their partial leakage. The seals of the 178mm string started to leak over time because of poor landing of the string the on slip-type casing hanger, which happened due to a design flaw in casing heads manufactured by domestic producers of wellhead equipment with rubber seals on the strings. The flaw lies in the fact that, in the casing head, there is insufficient distance between the bores (conical and cylindrical) for the slip hanger (retaining device) and the cylindrical bore for the wellhead seal. This arrangement of the bores in casing head body – that is, without sufficient distance – requires perfect alignment of the string and immaculate surfaces of the slips, which is essentially impossible. Perfect alignment of supposedly perfectly round string relative to the body of the casing head is virtually impossible to achieve, especially if the string landing is done after the cement hardening, when cement reaches the mouth and misalignment of the string is fixed by the hardened cement.
Consequently, the slips in the body of the casing head are often installed higher than the mounting seat and their upper surface enters the zone of the casing head’s bore for the wellhead seal. As a result, after the string duly lands on the slips, the support ring of the wellhead seal gets onto the upper edge of the hanger slips, rather than onto the top of the cylindrical bore designed for it, leading consequently to an excess draft of the rubber elements of the seals during the broach of the flanges of string heads or the X-tree cross-member. Depending on how far the slips enter into the seal zone, rubber elements of the seal are pressed on and deformed by pressure ring and carrier ring differently, sometimes so much so that such elements stiffen and lose their elastic properties over time.
When operating the well, pressure, temperature and vibration of the string settle the slips into their proper conical seat. The drawdown of the slips reduces the pressure on the stiffened rubber seals of the string, until the support ring of the seal is firmly seated on its cylindrical bore in the casing head. The elastic properties of rubber are nonlinear and are of highly relaxational nature at large values of load and time. In this