Study On Wellbore Stability In Changqing Oilfiled Newly-developed Area

In this paper,fabric,physical,chemical and mechanical properties of the Changqing Oilfield Area Formation cores were analyzed,in the core,most of the non-clay mineral is quartz;the mainly clay mineral is illite,and the average content is 10%;the I/S interstratified mineral content is huge,and the average content is 19.62%,excluding montmorillonite,Gao135 well's I/S interstratified mineral content is lowest,while Zheng 70 has the highest content;The clay mineral content of Gao 135 and Luo 254 are lower,whose value are 0.89%and0.52%,the rest wells contain much more clay minerals.The kinds of rock are sandstone and oil shale,and the rock pores are poorly developed.Most of the clay minerals exit as the pore-filling manner in the rocks,and the cracks is filled by carbonate minerals.The porosity of rocks are less than 10%,what's more,all of the rocks's permeability are less than 1×10^-3?m².In a word,the reservoirs are low porosity and low permeability formations.Zheng 70 well has the biggest CEC among all the wells,and its number is11.24mmol/100g.The remaining wells's hydration expansion ability is very weak.Zheng 70's expansion rate is higher than the rest wells,and its dispersibility is medium,the rest wells's expansion rate and dispersibility are very weak,Zheng 70 well's SSI value is lower than any other well,and its value is 33.92 mm,while the rest wells have very high SSI value,and the average value is 95.83 mm.These values indicate that Zheng 70 possess the certain hydration expansion ability,and the remaining wells's hydration expansion capability are weak.Some oil wells's brittleness index are low,including Zheng 70?Wu 100?Zhang 22?Luo254?Gao 135 and Li 231.Wu 100?Zhang 22 and Li 231 have small friction angle,and their formations of mechanical properties are unstable,within the borehole,the well would turn out plastically deformed,causing necking.Li 211 is a brittleness formation,what's more,its friction angle is much more smaller.So,this well is most likely to cause clasping block and enlarge the caliper.Gao 135 and Bai 522's friction angle are more than 40°,what's more,their hydration expansion ability are weak,So during the drilling period,the downhole instability accident general does not occur,Zheng 70's friction angle is 41.46°,so its mechanical properties is stable,but its hydration expansion ability is strong,and it's the plastic formations,so it's important to prevent swelling caused by necking formation;Luo 254's brittleness index is much smaller than the other wells,and its friction angle is 33.55°,indicating that its mechanical properties is unstable,so after the long-term immersion fluids,it's crucial that preventing collapsing of the wall.The friction angle of Zhuang 233?Huan 317 and Ning 33 are33.47°?36.07°and 30.94°,and their mechanical properties and hydration expansion ability are weak,what's more,their brittleness index are high,so during the drilling period,it's important to avoid peeling off the block.The research has laid a good foundation for the further development of Changqing Oilfield newly-developed District.