The formations in the Bayan block are complex in nature,mud losses,borehole wall collapse and pipe sticking etc.have long been problems hindering the drilling efficiency.Using laboratory methods such as X-Ray,electron microscopy and particle size analysis etc.,the lithology,micromorphology and particle size distribution of the rock samples taken from the unstable section of the formations were studied.The primary cause of wellbore instability is identified as the inadequate sealing capability of the drilling fluid concerning the micro-nano fractures within the formation,which leads to fluid invasion and results in the dissolution and detachment of salt and gypsum from the Linhe Formation.To deal with these problems,a powdered rigid micrometer and nanometer resin plugging agent XNZD and a flexible high molecular weight modified paraffin XNEP were introduced into the drilling fluid to improve its plugging capacity.The self-developed XNZD plugging agent has a particle size distribution that is compatible with the fractures'sizes of the unstable formations encountered in the Bayan block.Also introduced into the drilling fluid was a self-developed amine-based compound shale inhibitor XNYZ which was used in combination with potassium formate or other organic salts to improve the inhibitive capacity of the drilling fluid.In laboratory experiment with the optimized drilling fluid on a mud loss tester with fracture size of 0.03 mm,the amount of the drilling fluid lost was reduced by 98%because of the good plugging capacity of the drilling fluid.In core expansion test,the percent linear expansion of shale cores tested with the drilling fluid was reduced by 89.2%.These experimental results indicate that the drilling fluid has good plugging capacity and inhibitive capacity,and has provided an important technical support for the high efficiency drilling in the complex formations in the Bayan Hetao block.
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