Borehole Stability in Drilling the Paleogene System and Inner Buried Hill in Huabei Oilfeld

The lithology of the Paleogene system in the Huabei oilfeld is mainly sandstone and mudstone, intercalated occasionally with basalt and coal bed. Different formation pressure systems have been found in this oilfeld. Time required for coping with troubles during drilling accounted for 73.62% of the total time required for coping with troubles encountered in the whole process of drilling operation. The destabilization mechanism of the micro-fractured Paleogene system has been studied from mineral analysis, rock mechanics, formation stress measuring to mechanism of borehole wall destabilization. Large variations of cohesion (6-25 MPa) and angle of internal friction (26°-45°) of formation rocks and high formation stress cause micro fractures along the weak planes in rocks, resulting in borehole collapse and lost circulation. Abundant clays cause the formation to hydrate and swell, and after long time of soaking in water, network of fractures result. When pressure of mud column is greater than the collapse pressure of formation, the widths of the fractures increase exponentially, resulting in borehole wall sloughing. To stabilize borehole wall, Polyetheramine and a nano plugging agent were introduced into KCl drilling fluid. Polyetheramine inhibit osmotic hydration of clay through ether bonds and hydrogen bonds, and the unique action of amino group. This technology has been used in several wells such as Yangtan-1, Wen'an-101x and Antan-1x which penetrated the Paleogene system, no downhole troubles have occurred. The well Yangtan-1 successfully penetrated the long section shale formation which experienced severe borehole wall collapse in adjacent wells. The average hole enlargement of the well Yangtan-1 was only 1.8%, and the maximum hole enlargement, 14.82%. The well Antan-1x had maximum mud density of 1.50 g/cm³, 0.19 g/cm³ less than the mud density used in adjacent wells. Furthermore, excessive mud density is disadvantageous to borehole wall stabilization. It is suggested that the mud density should only be 15% higher than the equivalent density of collapse pressure, provided that there is no well control risk.