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~i . <br />Ivot surprisingly, the grea concentration of apparently sheared • Its occurred in close proximity to <br />sinkholes and in the groin' wells used in the tunnel simulation ~ussed previously. <br />Once it was evident that cleanout tools had left the well and were drilling new hole, the crew <br />would replace the cleanout tools with a drilling bit and drill another 40 to 100 feet to verify that the <br />original wellbore had, in fact, been exited and would not likely be reentered by continued drilling. The <br />40- to 100-foot "rat hole" thus created would be logged with a collar locator log. In some cases, the <br />collar log would detect metal, and field personnel would conclude that the original wellbore was adjacent <br />to the rat hole. Carrier perforating guns, with deeper shot penetration capability, would be combined <br />in 60-degree phasing and lowered into the rat hole. The objective was to perforate into the original <br />wellbore from the outside and establish a pathway. After perforating, a retrievable packer or cement <br />basket with tubing would be set and the rat hole would be squeeze cemented. Results were mixed, with <br />wells taking variable amounts of cement. Some wells accepted far more than the calculated volume of <br />the onginal wellbore below the shear zone. It was impossible to control or determine the distribution <br />of cement outside the rat hole; prospective results with this technique range from successful sealing of <br />the original wellbore to loss of all cement into lost circulation zones. <br />The "shoot-back-into-and-squeeze" method was discontinued after attempted on a few wells <br />because of inability to evaluate the uncertain results and because efforts to cement displaced wellbores <br />were not required by the plugging agreement between the Division and AlliedSignal. Subsequent sheared <br />wells were plugged, after cleaning out and drilling of a 40-foot rat hole, by the "assembly-line" <br />squeezing method detailed above. <br />Losr progress caused 6v cm•ing: pons! plugs before casing removal.--Removal of uncemented <br />casing is essential to assuring the best seal, but carries the inherent risk of allowing exposed formations <br />or annular debris to cave into the wellbore. Such caving could prevent reentry to the previously achieved <br />cleanout depth. <br />Surface casing would be left in the well to prevent caving before dowThole work (cleaning to <br />deepest attainable depth) was completed. Pulling the surface casing immediately after cleanout would <br />incur the risk of losing the hole and having to repeat the cleanout step. The alternative procedure was <br />to place cement in the bottom portion of the hole before pulling the upper casing. <br />The following paragraphs describe the advantages and disadvantages of setting a partial plug <br />before pulling casing. <br />Advcnirages 1) The presence of surface casing in the wellbore facilitates successful <br />reentry to depth with cementing tools. <br />2) Pipe between lost circulation zones and the wellbore prevents loss <br />of cement when the bottom-hole plug is set. <br />3) The step of cleaning the well to the deepest attainable depth is only <br />performed once, decreasing rig time and expense. <br />Is <br />?25 <br />