Deep Top Cementing Job-A Lesson Learnt from a Geothermal Well Drilling in Indonesia

摘要

A first geothermal exploration well had been drilled in Indonesia's Tulehu geothermal field located in Ambon Island in December 2010. As the drilling progresses soon after finishing the third casing i.e. the 9 5/8" casing size, losses of drilling circulation fluids were spotted at some depths. Although LCM material including Hi-Vis mud had been swept into the hole, however the lost could not be eliminated completely. This was not realized until after drilling personnel carried out cementing job. With the last casing to reach 618m, it was decided to perform two-stage cementing job; first by injecting cement slurry through the bit at the bottom and secondly squeezing second stage cement slurry via DSCC port opening slightly above the last 9 5/8" casing shoe. The first stage was to fill out annulus between 121/4"-borehole and 95/8" asing pipes, while the second stage was to fill up annulus between 13%" and 9 5/8" casing pipes. Although cement slurry injection were of the excess of 260% for the first stage and 100% for the second stage, however it was found later that the cement top could not reach top of the 13 3/8" and 9 5/8" annulus. Drilling personnel were on suspicions that cement slurry was not only lost in the open-hole interval but also incomplete first-stage cementing job making a hollow space above the first-stage cement top. Problem arises as the measurement showed that the top cement in the 13 3/8" and 9 5/8" annulus was at depth of more than 200m, an abnormal result of cementing jobs. The problem even more perilous as the annulus was filled with water and remaining mud that was displaced upward at the first stage cementing job. Drilling personnel were completely aware of the danger of water traps problem in geothermal wells. Thus they were attempting to assure that water should be drained before the deep top cementing job was carried out. Question arises on how to drain water from a l-7/8"-annulus-opening of 200m depth. After a careful tool design and action plan, the water was finally able to be drained by means of MACARONI pipes of 3/8"and 1/4" size that were installed from annulus top to the bottom of cement top. Using "capillary" principle, air from aerated drilling pump was injected through annulus opening. The pressure will force water to flow through the macaroni pipes out of the annulus. As the water was dried due to heating up of geothermal well, conventional procedure of top cementing job was able to perform from cementing facilities at ground surface.