Since exploration began in the Arctic Islands in 1969, various methods have been used to suspend wells drilled through the permafrost zone. Suspended gas wells and abandoned dry holes were left available for reentry by wireline equipment to run temperature and calipers surveys. Surveys were carried out on wells in which gas, cement, alcohol and diesel fuel were used as a non-freezing annular fluid through the permafrost zone and at wells where no attempt was made to displace the drilling mud to a non-freezing fluid. The casing has not collapsed in any of the wells where cement returns to surface here obtained on all strings, or where alcohol or diesel fuel was used to displace the drilling mud.
In order to evaluate the effectiveness of various fluids, including cement which might be used to fill the annular space between casing strings to prevent casing collapse in the permafrost interval. a number of abandoned dry holes and suspended wells were prepared for reentry prior to rig release. This was done in several ways. One way was to swab the inside of the last casing string dry from surface to a depth below the permafrost. This method worked well, however, temperature surveys were meaningless due to convective heat transfer inside the air filled casing. In order to reduce this effect some casing strings were top filled with diesel fuel following the swabbing operation.
Another way which wells were prepared for reentry was by reverse circulation to diesel fuel. It was reasoned that a more complete displacement to diesel would be obtained if the lighter density diesel was to be pumped down the annulus between the displacing string and the casing. Residual mud bypassed by the diesel would tend to fall through the diesel during displacement and would tend to move by gravity in the same direction as the displacing fluid.
The annulus between casing strings was prepared for displacement to a non-freezing fluid by ensuring good centralization between the tubulars. The casing strings were centralized throughout the permafrost at intervals of 120 feet using conventional spring type centralizers. In deviated or directionally drilled wells the frequency of centralization was increased and in a number of cases rigid bar centralizers were run instead of spring centralizers. However, the rigid centralizers were only run in casing to casing annuli.
If cement was used to displace the drilling mud in the annular space, no special cement preflush was used. In most cases the preflush was simply 20 bbls. of water ahead of the cement. Usually the cement was displaced in plug flow behind 9–5/8" and larger casing and in turbulent flow behind 7" And smaller casings. An attempt was made to maintain the cement density as uniform as possible while mixing. Normally gypsum cement blends were mixed to 14.6 – 15.2ppg and a high alumina cement were mixed to 16 ppg.
Note that the purpose of this paper permafrost is defined as the −2 ºC isotherm.
