Borehole wall protection and efficiency enhancement technology for small-diameter drilling in thick loose strata

Abstract:

In thick loose overburden covered areas such as Fuyang and Bozhou in northern Anhui， the incidence of downhole accidents including borehole shrinkage and collapse during drilling exceeds 80%. By analyzing the characteristics of loose layers， this paper reveals the chain instability mechanism of borehole walls dominated by "hydration expansion-particle migration-mud cake failure". The key control indicators are proposed， including the filtration loss of flushing fluid ≤5 mL/30 min and mud cake cohesion>8 kPa， which can ensure the safety of long open-hole drilling for more than three months. A composite flushing fluid system consisting of KPAM， NH₄-HPAN， PAC-LV and ultrafine calcium carbonate is developed， and a technical process featuring "layered targeted treatment+plugging while-drilling+short tripping" is formed. The verification of four test boreholes shows that the borehole wall stability coefficient increases from 0.51 to 0.89， the accident handling time is reduced by 100%， the mechanical drilling rate is increased by 32.8%， and the monthly drilling efficiency of a single rig exceeds 800 m. The research results can provide technical support for drilling engineering in similar thick loose overburden layers.