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"Assembly-line" plug consisted of using a large servic~ to consecutively clean out and <br />remove casing from several wells. Once several wells were ready for logging and cementing, a smaller <br />hydraulic rig would perform that work. Yarosz (] 995) cites the following advantages and disadvantages <br />of the "assembly-line" approach: <br />Advmttages: 1) The rig used for cleanout and casing removal could be quickly <br />moved off the well after performing its part of the work, rather <br />than remaining on-site until logging and cementing services were <br />completed. With one "assembly-line" in progress, expenses <br />associated with waiting time were significantly reduced. <br />2) Efficiency and cost-effectiveness of sen•ice company visits were <br />increased with up to five wells logged and cemented at a time. <br />Disodvcrntages: 1) With one set of two rigs at work, supen~ision by company <br />personnel and the State inspector was manageable. However, when <br />multiple "assembly-lines" were in progress, ~~ith up to four rigs at <br />work on eight wells in a single day, effective supervision of <br />concurrent activities became difficult or impossible. <br />2) Because of the unpredictable nature of the work, sequencing of <br />operations and scheduling of equipment and service crews to avoid <br />excessive standby time were difticult when more than one <br />"assembly-line" was in progress. <br />3) Use of the "assembly-line" approach lengthened the time between <br />casing removal and cementing, thereby increasing the chances of <br />wellbore caving. Loss of progress resulting from these <br />circumstances had a particularly onerous and costly impact on one <br />well that had been cleaned out relatively deep. Subsequent <br />attempts to reclaim progress resulted in the loss of an entire <br />downhole tool assembly, the bottom-hole plug, and 5-1/2 days. <br />Because of the potential for this type of occurrence, the on-site <br />inspector recommends use of the "zssembl}-line" procedure only <br />for shallow cleanouts. <br />Sheared wellbores: "shoot-back-into-cn[d-squeeze"n: erhod.--As prevtousl}• mentioned, subsidence <br />affected some wells in the brine field by displacing wellbores so that [he lo«•er portion could not be <br />accessed from the surface. Condition of the displaced wellbore could not be Bete: mined. The shear zone <br />was usually in shale, often less than 25 feet into bedrock. An experimental method for placing cement <br />into the displaced lower wellbore was attempted at several of these wells. <br />Yarosz (1994) described the manifestation of wellbore shearing "It ma}• be oversimplistic to <br />contend that the wells were actually chopped in half, perhaps repeatedly, be; the effect on do«~nhole <br />work is as though that is the case.... Evidence provided by means of the aopeatance of the do«nhole <br />ends of recovered casing pieces does suggest that some of the wells were in fact completely sheared." <br />17 <br />224 <br />A <br />