Underbalanced MPD of an Exploratory Well in Tight and Extra-Low Permeability Reservoir

Tight and extra-low permeability reservoir was usually explored by use of normal underbalanced drilling if the wellbore is stable and the formation pressure is clear. However, precise underbalanced MPD is the optimal technical solution in case the borehole is unstable and the formation pressure is unclear. Moreover, the precise underbalanced MPD would be effective for reservoir protection, enhancement of hydrocarbon discovery, improvement of ROP as well as reduction of well control risk. No oil and gas show were found by previous conventional drilling in a tight granitic basement of Indonesia. The first exploratory well was planned to explore the broken belt of the basement to discover the oil and gas zone, increase drilling efficiency, prevent lost circulation at crushed zone of the basement and minimize drilling troubles by utilization of precise underbalanced MPD in the potential target zone. Underbalanced MPD was achieved by use of low density water-in-oil drilling fluid, and the bottomhole underbalanced pressure fluctuation was accurately and effectively controlled. Underbalanced MPD operation was smoothly completed. Drilling with ignition under balanced managed pressure, connecting triples under balanced managed pressure, and tripping under balanced managed pressure were implemented. Slight under balanced condition of the bottomhole pressure during MPD operation was realized. The backpressure of the wellhead was accurately controlled between 55-135psi during precise MPD operation. 57 times of total accumulate successful ignition lasted 240 hours, which accounts for 80% of the total drilling time. The rate of penetration in tight target granitic formation under balanced drilling was improved and reached 10.8ft/hr. Neither losses nor overflow were detected during underbalanced MPD. Safe and high efficient drilling was realized. Good oil and gas show were observed. Abundant natural gas produced during underbalanced MPD. The basement hydrocarbon reservoir has been obtained important discovery. Application of precise MPD technology could accomplish reservoir discovery and protection, wellbore stability and reduction of well control risk. It prevents ordinary underbalanced drilling from change over traditional overbalanced drilling due to unable to satisfy the safe drilling which would result in secondary damage to the reservoir so as to improve integrated drilling efficiency.