| Title | Multi-Lateral Geothermal Wells in Volcanic-Hosted Reservoirs - Objectives, Challenges and Results |
|---|---|
| Authors | Jim Stimac, Marino Baroek, Dexter Pazziuagan, Ronald Vicedo |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | multi-lateral, drilling, fork, optimization, formation stability, rock strength |
| Abstract | Three multi-lateral (forked) wells have been drilled by Chevron in three different geothermal fields in Indonesia and the Philippines. Two of the wells (Nag-66 in Tiwi and Awi 20-1 in Salak) were drilled as injectors in areas with relatively low permeability. Multi-lateral completion was selected to enhance the overall injection capacity while minimizing capital investment. A third well was drilled as a deep producer to reduce steam costs. All three wells were successfully forked and remain operational. The two that have been put in service met or exceeded injection or steam rate targets (Nag-66, Bul-109) with some cost benefit. The third well (Awi 20-1) had low initial injectivity in both legs but has improved through hydraulic stimulation. All three wells were completed with an “openhole fork” design, where the second leg was sidetracked from the parent bore by means of a retrievable whipstock. The forked leg was completed with slotted liner up to near the fork junction, but a short section of openhole at the junction itself remained unsupported by casing or liner. We describe the methods used to assess the stability of the formation and pick the forking point. Openhole caliper logs and “Bingham” strength estimates can be used to evaluate formation stability to reduce costs, but sonic and density logs provide the most definitive information. Integration of data from offset wells, wireline logs, and correlation of these data with drilling parameters informed decisions and minimized the risk of junction collapse. On the drilling and completion side, the major challenges were related to acquiring the appropriate whipstock system, the whipstock placement including orientation and ensuring subsequent milling operations would place the kick-off point within the pipe body, the cut and pull operation for the liner in the forked leg, the whipstock retrieval operation, re-entry into the parent bore and cleanout of the bridge plug placed in the parent bore. |