Research On Environmentally Friendly Water Based Anti-collapse Drilling Fluid Suitable For Offshore Drilling

The Weizhou 11-1 oilfield complex is located in the Weixinan Depression of the Beibu Gulf Basin in the South China Sea,and is rich in oil and gas resources with complex formation lithology and strong reservoir inhomogeneity.The analysis of the clay mineral content and pore size characteristics of rock samples from the Jiaowei,Weizhou and Liushagang formations of the We11-1 oilfield complex shows that the formation has a high clay mineral content and the nano-micron pore size and laminae are very well developed,which makes it easy for drilling fluid filtrate to intrude into the formation along the micro-fractures and laminae,leading to well wall instability.Traditional oil-based drilling fluids or synthetic-based drilling fluids are costly and not conducive to environmental protection,so there is an urgent need to develop new water-based environmental protection anti-collapse drilling fluids to replace oil-based drilling fluids.Based on the relationship between the molecular structure and environmental performance of drilling fluid treatment agents,an environmentally friendly nano-micron plugging and collapse inhibitor ZY-1 was successfully developed in this thesis.The results of the environmental protection,thermal stability,blocking and inhibition evaluation experiments show that the synthesized treatment agent is not only non-toxic and easily degradable,but also has good thermal stability and blocking and collapse inhibition properties.On this basis,the inhibition mechanism and blocking mechanism of ZY-1,an environmentally friendly nano-micron blocking and collapse inhibitor,were discussed.The response surface method was used to optimize the compounding dosage of different micron-level rigid plugging materials and ZY-1,and the effect of their interaction on the plugging rate was analyzed;the drilling fluid inhibitor,filter loss reduction agent and lubricant were selected indoors,and an environmentally friendly water-based anti-collapse drilling fluid system formulation for offshore drilling.And the drilling fluid performance evaluation results show that the drilling fluid system is not only non-toxic and easily degradable(EC₅₀ value of 44600mg-L and BOD₅/COD_cr value of27.04%),but also has the characteristics of high density,strong sealing and inhibiting properties,good lubrication and anti-pollution ability.Among them,the applicable upper limit of drilling fluid density reaches 2.2g/cm³,and the sealing rate of 10m D artificial core is as high as 92.3%,and the recovery rate of permeability can reach 87.8%;under high temperature and high pressure environment,the linear swelling rate of rock chips in Weisan section and Flow 2 section decreases by 35.4%and 13.3%respectively compared with that of clear water,and the rock chip rolling recovery rate increases by 89.36%and65.84%respectively compared with that of clear water.The extreme pressure lubrication factor of the drilling fluid at high density is 0.25 and the mud cake viscosity factor is 0.13,while it can resist 10%rock chip contamination,15%salt intrusion and 2.0%calcium intrusion.