The composition models of local materials additive on light weight cement are very important on well completion design of borehole stability in producing procces. On Geothermal fields, the effect of temperature reservoir and steam are to changes of gel calcium silicate hydrate to alpha dicalcium silicate hydrate on cement model composition, it caused cement degradation and shrinkaged, as instability of borehole. On HTHP conditions caused the cement suspension was dehydration (partial of liquid loss), channeling or fuggy, shrinkage bulk volume, strength degradation, and increasing the permeability cement. For the anticipated cement degradation to added by silica flour 35% BWOS (Herianto 92) at upper temperature 110oC (Nelson 93). The methodologies of researches are makes some models composition of cement slurry to anticipated of strength degradation, shrinkaged volume, low permeability (impermeable), and light. They are to supported on stability of borehole completion. Local expanding additive to activate (grinding and vibration screen) on mining laboratory FIKTM ITB and burned on 1400 oC, retention time 8 - 10 hours at Balai Besar Keramik Bandung. The results of burned are Dolomite concentration neat 97.37% is pure periclase and fineness 2881 cm2/gr have density 29.16 ppg, and Calcite 88.79% is pure lime, fineness 3763 cm2/gr have density 21.32 ppg. After makes modeling composition researcher to new of mineral stability of borehole is called clyno tobermorite of ratio Cao and SiO2 is 0.74 on conditions optimum temperature 200oC and pressure 2000 psi on simulator curing chamber laboratories, density is 11.70 ppg, impermeable, thickening time on 70 Uc is 110 minutes, but the porosity is high.

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