Papers - - Production Engineering - Plug back Cementing Methods

During the past year considerable interest has been shown in cementing operations for shutting off bottom water, whipstocking, etc. A number of plug-back methods have been used, with various adaptations for special conditions, but those described in this paper are representative of the methods now in use. Conditioning the Well Various preliminary operations for improving the results from plug-back operations have been evolved. In one method, a nitroglycerin shot is discharged at the upper portion of the formation to be plugged off, the purpose being to make a wide place in the hole in order to give the cement a more secure anchor against thrust pressure of the water against the bottom of the cement plug after the well is again put on production. Another is intended to attain a similar result by under-reaming a wide place in the hole by means of a rotary wall scraper. In one well in which this device was used there was a water sand at the bottom of the well immediately below the cap rock above which the oil sand occurred. Before the plugging back operation, the water sand was underreamed in an attempt to get a square shoulder on the underside of the cap rock, in order to prevent the water from breaking in over the top of the cement plug after the well should be put on production, and also to get a more secure anchor for the cement plug against the thrust pressure of the water against it. As an aid in calculating the amount of cement required to fill a given amount of hole to be plugged back, a device has been developed for measuring and recording the irregularities and cavities in open holes, called an "open-hole recording caliper." If consists of a steel shell, 5 in. in diameter, which encloses a recording mechanism that receives impressions from four movable arms that make sliding contact with the wall of the hole. The arms operate a ratchet, which raises and lowers a recording stylus against a chart. Each arm registers its own radius. The device is run into a well on a steel measuring line. While the device is being lowered, the arms are folded inside the shell; they are