In Tarim Oilfield the Ordovician carbonate reservoirs develop lots of cracks and caves. The formation pressure is very low(>6000m, less than65MPa), leading to the narrow mud weight window(MWW) problem. Serious lost circulation and reservoir damaged was encountered when we use the traditional overbalanced drilling method, also the oil and gas reservoirs cannot be discovered timely, therefore whole process underbalanced drilling technique must be employed. The underbalanced drilling technique has been applied in the narrow MWW formations, but there’s no report of the whole process underbalanced drilling technique (including the underbalanced drilling, underbalanced tripping, underbalanced logging and underbalanced completions, etc.) being used in such formations in China.

Based on the analysis of difficulties encountered by traditional overbalanced drilling methods such as serious lost circulation, horizontal wells cannot reach the design sections, serious damage to the reservoir, the feasibility analysis and construction plan of the whole process underbalanced drilling in Y block is proposed. The first whole process underbalanced drilling of horizontal wells in Tarim Oilfield was conducted and completed. Finally 6 layers of reservoirs are being discovered (2 layers are not discovered by the prospect well), the cumulative thickness is 16m, which increases the rate of oil and gas discoveries.

Compared with the on-site well, the efficiency of drilling is greatly improved, no lost circulation and no complex situation encountered; the ROP increases 74% compared to the adjacent wells in the same layers. It also created the deepest drilling depth record using the inflatable mud in China. The whole process underbalanced drilling program formed in this thesis provide an effective drilling and completion mode for the Y block in Tarim Oilfield, which greatly increased the rate of oil and gas discoveries, and reduce non-productive time, while this method has some reference significance for the low-pressure reservoir development in the other parts of the world.

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