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The shale formation in the Changning area is prone to complex engineering problems such as collapse of the wellbore, falling off the block and leakage during the drilling process. The reason is that the drilling fluid cannot effectively block the micro-cracks in the formation, and it is impossible to effectively suppress the internal hydration of the shale. In this paper, from the direction of improving the plugging and inhibition ability of the drilling fluid system, the strong plugging capacity plugging agent HXSZ-1 was added to the system to improve the plugging capacity of the system. Through the synergistic action of KCl, polyamine inhibitor and silicate, the inhibition performance of the system could be further improved. The indoor evaluation showed that the system had excellent plugging ability. The reduction rate of mud cake permeability at room temperature was 68.24%, and the HTHP static filtration loss was 2.7 mL, which was equivalent to the plugging performance of oil-based drilling fluid. The secondary recovery rate of shale was as high as 96.89% with the linear expansion rate of less than 4.8%, and the anti-pollution ability was strong. The EC50 value of the system was 11500 mg/L, which was non-toxic and could meet environmental protection requirements. The field application results showed that the system had stable performance during the drilling process. The average enlargement rate of well diameter was controlled within 4%, and the drilling cycle was shortened by 20%. The purpose of excellent and fast drilling was realized, and the field application effect was good. [ABSTRACT FROM AUTHOR]

China's shale is widely distributed, in which 70% of the strata are shale formations, and hard and brittle shale is more than 65%. Due to the structural properties of the hard brittle shale, the downhole complexes such as falling blocks, stuck drilling and wellbore instability are easily encountered during the drilling process, which seriously affects the progress of the oil and gas development process. Based on the mineral composition and microstructure of the shale, the shale hydration and the expansion of the shale under hydration are studied. It is concluded that the micro-cracks in the shale are extensively developed and expanded under hydration. It is the internal cause of rock mass destruction. Based on this, an evaluation method for the plugging effect of drilling fluid blocking agent on micro-cracks of hard and brittle mud shale is designed. Investigate the effect of plugging agent, addition amount and pressure difference on the cracking effect of micro-cracks, and propose the plugging mechanism of plugging agent in micro-cracks. The experimental results show that the combination of 4% CaCO3+2% EFD-2+4% NFA-25 in the drilling fluid system can greatly improve the plugging capacity of the system, effectively block the micro-cracks developed in the formation, and at the same time make the system A certain pressure bearing capacity. This has obvious effects on the improvement of the stability of the borehole wall, and provides new ideas and guidance for the design of the drilling fluid system. [ABSTRACT FROM AUTHOR]

As shale gas resources become more prominent in China's energy development plan, the problem of wellbore stability is one of the thorny issues affecting the development progress. It is necessary to effectively solve the problem of wellbore instability. To achieve efficient development of high-efficiency shale gas, the plugging performance of drilling fluid is an important basis for the stability of the borehole wall, and the plugging agent is an important agent that contributes to the good performance of drilling fluid. The mud shale formation has many microcracks and pores, or even nanoscale pores, making shale very low permeable. The traditional plugging agent has a large particle size, which makes it difficult to effectively block the micro-nano-scale pores in the formation. To solve this issue, a nano-blocking agent NFD-1 was synthesized in this paper. The experiment shows that the nano-blocking agent is nano-dispersed in water with a particle size of about 130 nm, which has little effect on the rheology of the system, and is also blocked by artificial mud cake. Compared with other nano-blocking agents, the plugging agent has strong plugging performance. When the concentration is 3%, the API fluid loss of the drilling fluid can be reduced from 6.3mL to 3.6mL. The HTHP fluid loss is determined by 13.0mL was reduced to 6.9mL, and the blocking ability of the micro-nano pore formation was close to 80% at 5% concentration. At the same time, the plugging mechanism of the plugging agent was studied, which provided technical support and application direction for efficient plugging of drilling fluid. [ABSTRACT FROM AUTHOR]