Routine coring in tight shale of the Lower Silurian Longmaxi Formation exhibits low rate of penetration(ROP).While some new coring technologies by screw tools may achieve an improved ROP by more than 50%.However,they features low feasibility due to frequent core fall and big difference in each coring footage,which impedes their promotion.Thus,taken difficulties in high-speed screw coring into account,not only rock properties were analyzed for every member of Longmaxi Formation but technological char-acteristics were discussed.And both screw selection and coring parameters were optimized.Additionally,one technique of long-barrel screw coring was built.Results show that(ⅰ)screw tools are selected properly and a method to govern coring displacement is opti-mized in individual member to further bring core diameter under control and prevent cores from barrels during core cutting;and(ⅱ)cores are cut with pump off and the circulation displacement is optimized to avoid cores falling.This built technique was applied to the Longmaxi Formation in Z301 and Z221 wells,and totally 7 barrels were achieved with the coring footage of 174.50 m,the length of 173.73 m,and the recovery of 99.76%,respectively.A remarkable increase in this recovery signals breakthrough made in screw in-novation.And this technique may solve low coring ROP in shale,sandstone,limestone,aluminaceous mudstone even with poor drilla-bility.
Early SilurianScrew coringRecoveryCoring footageRate of penetration
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