Study On Anti-collapse Drilling Fluid Technology Of Qingshankou Formation In Qian’an Area

Qingshankou Formation in Qian ’an area is prone to collapse during drilling,which leads to a series of complex underground conditions,which affects the construction safety and the completion of exploration work.Aiming at the problem of wellbore instability in this formation,this paper carries out research on anti-collapsing drilling fluid technology of Qingshankou Formation.Based on the analysis of the fabric and physical and chemical properties of the rocks in this formation,it is found that the clay minerals of the rock samples in this formation are mainly illite and almost contain no montmorillonite,belonging to medium-hard to hard brittle mud shale.There are micro-nano-scale cracks on the surface of the rock samples,the rolling recovery rate of clean water is between 32.4% and 35.6%,and the expansion rate is between 5.19% and 7.21%.There is poor activity between the rock sample and drilling fluid,which is weak to medium expansion and strong dispersion formation.Under the dual action of hydraulic potential and chemical potential,the filtrate of drilling fluid is easy to invade the shale along micro-cracks and react physically and chemically with clay minerals,resulting in the hydration and expansion of clay,resulting in the spillage of mud shale from the well wall.Meanwhile,with the intrusion of drilling fluid filtrate,the pore pressure near the well wall rises,making the pore pressure approximately equal to the liquid column pressure.The liquid column pressure could not provide effective support for the shaft wall,and the broken blocks began to fall.Therefore,effective plugging and hydration inhibition are the key issues that should be considered in the design of anti-collapsing drilling fluid.Based on the mechanism that osmotic pressure affects pore pressure and thus formation stress distribution,a method for calculating the safe salt concentration window was established,and on this basis,a brine anti-collapsing drilling fluid system was developed.Combined with the anti-collapsing countermeasures for Qingshankou Formation,the plugging agent and inhibitor are mainly optimized.The single agent optimization and compound experiment of plugging agent are carried out by using the established plugging evaluation method.The optimal combination of plugging agent is 2% sulfonated asphalt ZT-1+3% fine calcium carbonate GX-1+2% nano plugging agent NQ.Combined with inhibitor evaluation experiment,the optimal concentration of amine polyol inhibitor was 1%.The anti-collapsing drilling fluid system suitable for Qingshankou formation was formed through the optimization study of two key treatment agents.The anti-collapsing drilling fluid formula was salt water +4% bentonite +1%PAM+2% anti-salt copolymer STQ+1% fluid loss control agent SPKY+1% amine polyol inhibitor + multiple plugging agent.The laboratory evaluation results show that the anti-collapsing drilling fluid has good inhibition and plugging effect.The anti-collapsing drilling fluid system developed was applied in Dabai Pping 1-2 well,and it was proved that the anti-collapsing drilling fluid system can effectively reduce the underground complex situation caused by the instability of borehole wall and ensure the drilling safety of Qingshankou Formation.