Study On Coal Failure Mechanism And Drilling Fluid Technology For Jurassic Formation In The Northern Kuqa

Tarim Oilfield is one of the main gas source areas of China's West-to-East Gas transmission.Its total exploration area is 56 X 104 km2,and the total proved oil and gas resources is 16 billion tons of oil equivalent.It is one of the main succession areas of China's strategic oil resources in the 21st century.The northern Kuqa structural belt is one of the high-yield blocks in Tarim oilfield,which has great potential for exploration and development.There are many large sections of coal seam in Jurassic.In the process of drilling,the instability of coal seam leads to many complex downhole accidents such as lost circulation,collapse and drilling tool sticking,which seriously restricts the economic and efficient drilling of oil and gas resources in this area.In order to effectively solve the borehole instability of Jurassic coal-bearing strata in northern Kuqa block and the complicated problems caused by it,this paper clarifies the important and difficult points of drilling in coal-bearing strata on the basis of statistical analysis of drilling completion data;secondly,through the physical and chemical properties of coal,micro-structure and rock mechanics experimental tests,the instability mechanism of coal seam is clarified;thirdly,the model of coal seam collapse pressure influenced by multi-cleats is established,the collapse pressure of coal seam is determined,and the optimum design of wellbore trajectory is carried out.Finally,based on the analysis results of microstructures of coal and rock,the material optimization,drilling fluid formulation optimization and field application of plugging coal and rock are carried out.The main conclusions of this paper are as follows:(1)The results of physicochemical properties and scanning electron microscopy analysis of coal show that there are almost no clay minerals in coal.The cleavage width of coal is over 90%with 1?10 ?m,and the coal are mechanically unstable.(2)The in-situ stress mechanism of the coal seam in the work area is strike-slip fault.Drilling coal seam along the direction of maximum horizontal principal stress has the best stability.The density of drilling fluid for stabilizing coal rock is 1.70 g/cm3?1.82 g/cm3.(3)The results of particle size analysis of commonly used coal-rock asphalt sealing materials show that the composite system of sulfonated asphalt,emulsified asphalt and 2200 mesh superfine calcium carbonate has the best sealing effect.The optimum additives dosage for coal core plugging test are:3%sulfonated asphalt+3%emulsified asphalt+3%ultrafine calcium carbonate.(4)The optimized anti-collapse drilling fluid system has excellent plugging inhibition,the rolling recovery of coal is more than 98%,API water loss is less than 2.3 ml,and HPHT water loss is only 2.4 ml.The anti-collapse drilling fluid has good field application effect and outstanding effect in stabilizing coal seam.The maximum diameter enlargement rate is only 9%.There is no downhole accident,and the drilling cycle of coal seam section is greatly shortened.It has broad application prospects in the area.The research results in this paper effectively improve the stability of Jurassic coal measures strata in the northern Kuqa structural belt,and provide a strong guarantee for the safe,economic and efficient development of oil and gas resources in the northern Kuqa working area of Tarim Oilfield.