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Disadvantage ~) Setting a partial plug before ~~g removal and then a final plug <br />after castng removal necessil~l~s t+vo v[s[ts by the cementing <br />company for one well, increasing service company costs. phis was <br />particularly bothersome for AlliedSignal ++•hen it was sometimes <br />found that, for various reasons, significant lengths of surface casing <br />could not be pulled and the +vell had to be cemented through <br />perforations. In these cases, the period of time between the two <br />cement company visits did not yield any progress toward plugging <br />objectives. <br />Se+~ere lost circulation: stage cemerrrin?and 6ridQina techniques.--Lost circulation was a constant <br />problem, whether cementing an open hole or through perforations. Successful circulation was very rarely <br />achieved on the first attempt, and wells were very rarely cemented from total depth to surface in a single <br />stake. Hundreds of sacks of cement could disappear through a single set of perforations. Even with lost <br />circulation materials mixed into the cement, casing fill-up ++as often less than half of what it should have <br />been based on the quantity pumped (Yarosz, 1994). <br />As field personnel gained experience ++ith the lost circulation problems in Tully V211ey, emphasis <br />of the cementing process shifted to the use bridging materials in conjunction ++ith stage cementing in <br />order to reduce the quantity of cement needed to fill wellbores to the surface (Yarosz, 1994). Cementing <br />in increasingly sma]{er stages as the project progressed allowed crews to identify problem zones v+ith <br />less waste of cement (Yarosz, 1995). In addition, cementing engineers adopted the practice of ceasing <br />to pump when it became apparent that fill-up was not occurring. Once a lost circulation zone was <br />identified, cement placed below the zone +vould be allowed to set. Pea gravel would then be dumped <br />from the surface to fill the wellbore to a level across the perforations or open hole interval identified as <br />the problem. Six cementing stages and over 1,000 sacks of cement were necessary for some wells <br />(Yarosz, 1994). <br />Slcnited cosines: excavation angled entry reduced tool sine mtd drill string rigidirt~.--~lrellbore <br />deviations from vertical were visible at the surface at eight wells. Four wells were estimated to be <br />slanted more than 20 degrees. Yarosz (1990 describes the follo++ing methods used to attempt to gain <br />downhole access in slanted wells. Success using either of these methods w25 very limited. <br /> <br />r <br />1) Excavation This method involved excavating around [he tilted cz_sing to either <br />locate the hole at the bedrock interface 2nd reset the pipe vertically, <br />or to bend the pipe back to vertical ++ith a bulldozer and backfill <br />around it. In one case, the conductor pipe simply fell over as <br />bedrock ++•as approached and further excavation failed to locate the <br />wellbore. <br />?) ~l'orkine at an ansle Attempts to enter angled wells were made both ++ith a tilted rig and <br />a hydraulic po+ver s++ivel. The power s++lvel was preferred because <br />of safen• concerns with the tilted rig. .4t angles approaching 20 <br />degrees, however, connections were difficult and progress was slow <br />as the s++ivel banged in[o the rig or hung a+vay at an unworkable <br />angle (Yarosz, 1995) <br />19 <br />?26 <br />