Big Size Bit Design Field Test for Gravel Layer in Piedmont Structure

South Tianshan exploration area in Tarim Basin is a piedmont structure,with very complex geological environment,where layers shallower than 3 000 m deep containing thick gravel layers are poor in drillability.On basis of analyzing physical property of gravel layer,660.4 mm 444.5 mm roller bits suitable for gravel layers and 444.5 mm PDC bit for microconglomerate layer have been designed.Drilling attackability,efficiency and stability of roller bit were enhanced by optimizing tooth projection and toe width,regulating tooth density,adopting screw wheel carrier,sliding and gliding complex bearing,center nozzle,and moderate length nozzle.PDC bit designed has 7 nozzles,and drilling efficiency was enhanced by adopting abrasion-proof and shock resistance compacts (19.0 mm),mid-low density cogging distribution,double-row cogging on shoulder,and enhanced gauge.Bits designed have been tested in the first and second drilling sections of Well Ku2.Compared with adjacent well (Well Ku1),penetration speed and average footage of 660.4 mm roller bit rose by 84.62% and 445.04%;penetration speed of 444.5 mm roller bit and 444.5 mm PDC bit rose by 46.15% and 128.85%.The bits have also been tested in Dabei and Keshen area,and results showed that 660.4 mm roller bit could enhance penetration speed and save drilling time significantly.On basis of field test,the result showed that drilling bits designed were suitable for drilling in gravel layer in piedmont structure,with significant increase in penetration rate and drilling efficiency.