The conventional oil-based mud used in the second member of Weizhou formation in Beibu Gulf Oilfield is of character of low oil-water ratio and electric voltage, which makes the water phase activity in mud unable to be adjusted during drilling operation to balance the water activity in collapse-prone formation, and causes the shale to hydrate and disperse. At the same time, the sealing material and mechanism are relatively simple, making it difficult to fill the irregular micro-crack in the collapse-prone formation, and downhole problem for wellbore instability, resulting in poor reservoir protection effect. In order to solve the aforementioned problems, the collapse mechanism of hard brittle gray mudstone in the second member of Weizhou formation was analyzed. The oil-water ratio was improved on the basis of the oil-based mud formula used in the collapse-prone formation of Weizhou formation in the early stage, and the electric voltage of mud was improved by the compoound emulsifier. Also with the optimization of composite particle size plugging materials and with compound plugging mechanism, a novel oil-based mud system was established. The performance evaluation shows that the optimized oil-based mud is of high oil-water ratio (95:5 or higher). The electric voltage is greater than 2000V, and the plastic viscosity is small (PV: 36-50) and less amount of filtration (1mL). The sealing pressure is outstanding (the compressive strength of mud cake is greater than 20MPa), and the water/soil pollution resistance is promising. In the aspect of reservoir protection effect, the reservoir core permeability recovery rate reaches 98.70%. Moreover, in the aspect of inhibition performance evaluation, the shale hot-rolling recovery rate is up to 99.10%. And the cuttings remain intact after a long period of soaking for rock samples. Field application shows that novel oil-based mud system can solve the problem of wellbore instability in the collapse-prone formation of the second member of Weizhou formation. The borehole remained stable, and the casing was run smoothly after drilling. At the same time, the reservoir protection effect is good, with production rate three times higher than estimation. The successful application of this system can effectively solve the problem of drilling and reservoir protection in the collapse-prone Weizhou formation of Beibu Gulf.

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