Comprehensive Speed-Up Technology for Safe and Efficient Drilling Through Ultra-HTHP Huge Mudstone Formation in the South China Sea

How to drill through the huge mudstone formation safely and quickly in the Ying-Qiong basin, western South China Sea has always been a significant problem in deep-water drilling. Under the action of the hydrostatic pressure of high-density drilling fluid and confining pressure of deep strata, ultra-HTHP huge mudstone presents viscoelastic and severe plastic characteristics. It is difficult for the bit to penetration the strata and slow down the rate of penetration. Although the thickness of mudstone is less than 30% of the total footage, the pure drilling time accounts for more than 75% of the total drilling time, which is difficult to meet the demand for economic and efficient drilling. In order to solve the problem of low ROP, the deformation and failure characteristics of huge plastic mudstone are found through the experimental test. The critical confining pressure of brittle plastic transition of mudstone is analyzed. The evaluation method of brittle plastic transition of mudstone and its influence on drillability is proposed. The personalized design and selection template of drill bits for plastic mudstone is established, and the personalized bits and drilling acceleration tools are optimized. The design scheme of safe drilling fluid density is put forward, which considers both wellbore safety and elimination of chip hold down effect. Considering the engineering and geological characteristics, the integrated drilling speed-up technology with high-efficiency rock breaking and wellbore stability is formed. Compared with field drilling, the ROP is increased by 82.6%. This technology can improve the drilling efficiency and significantly reduce the drilling cost, which provides a reference for speeding up drilling in similar formations.