DISCUSS ON CEMENTING TECHNOLOGY IN LUNPOLA AREA OF TIBET

Lunpola area in Xizang is 4700m above sea level with a harsh climate and the annual average temperature is 70o or so. The formation is mainly made up of shale with strong water sensitivity and developed fracture, its pore pressure is generally 0. 92~0. 97g/cm3 and the pressure of most intervals behaved negative; the geothermal gradient is 4. 0~4. 8℃/100m, the average is 4. 26℃/100m, the temperature is of abnormality. The frequent leakage occurred during drilling and cement slurry leakage occurred during cementing,the cement slurry did not return to the required depth with 56%~76% qualified rate and 38%~47% high-grade rate of cementing. Regarding to the problems of cementing leakage and cement consolidation with bad quality in this area, the true bottomhole and pressurewere simulated to optimize the addition agent suitable for low-density cement slurry in terms of improving lightweight cement strength. And then the cement slurry formulations with low-density, high early strength and flow pattern index can be obtained. No leakage occurred in practices of three wells, the qualified rate of cementing for west well Lun-5 is 98%, and high-grade rate is 89%.