PROBLEMS WITH WELLS DRILLED THROUGH MASSIVE SALT-SECTIONS
Long sections of salt deposits present difficult problems in well completions because they create excessive loads on casing. It is generally accepted that salt creep can generate very high wellbore pressures and that in an unsupported wellbore it takes place in three stages. Primary creep starts with a relatively high rate of deformation just after the salt formation is drilled. After a certain time, this rate falls and a period of essentially low rate of deformation persists which is known as secondary creep. It is in the final stage, however, that salt creep reaches its maximal value and if the pressure and temperature exceed 3.000 psi and ‘278 °F, respectively, salt creep can generate very high wellbore pressures. Typically, an abnormal pressure gradient ranging from 1.0 psi/ft to 1.48 psi/ft can be applied to the casing leading to its collapse (Marx and El-Sayed, 1985; El-Saved and Khalaf, 1987). Severe salt creep-related casing problems have been reported in
the Gulf of Suez (Pattillo and Rankin, 1981) and West Germany (Burkowsky et al., 1981).
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Fig. 4.16: Typical buildup rates for horizontal drainholes. (After Fincher, 1989.) |
Two principal methods have been adopted to overcome the problem of casing collapse: thick-wall (> 1 in.) casing (Ott and Schillinger, 1982) and cemented casing string (Marx and El-Sayed, 1984). The most effective solution seems to be to use cemented pipe-in-pipe casing (composite casing).