Practice and prospect of blowout preventer-assisted wire-line coring technology for offshore core drilling

Abstract:

Small-diameter core drilling plays an irreplaceable role in deep mineral resource exploration and marine geological surveys， however， wellbore stability control， harmful gas prevention， and full core quality assurance under complex marine environments and diverse strata conditions have long been technical bottlenecks in the industry. To address the engineering challenges such as difficult core preservation in unconsolidated soft plastic strata， easy core abrasion in hard strata， low core recovery in fractured coal-bearing strata， and deep hydrogen sulfide abnormal overflow in the CSDP-2 well （China Eastern Sea Area Scientific Drilling Well）， this paper systematically conducted research on improvements of the blowout preventer-assisted wire-line coring technology. By optimizing the drilling speed control strategy and establishing a pressure balance system for drilling fluid reinjection， the issues of wellbore collapse caused by tripping operations in shrinkage-prone strata and negative pressure suction during wire-line coring were resolved. Relying on the blowout preventer kill mainfold， a reverse circulation drilling fluid system was constructed， achieving multiple objectives including wellbore stability maintenance， harmful gas containment， and solid phase precipitation prevention. Combined with the adaptability modification of the "Tanhai 1" platform， a full coring technology system for complex shallow sea environments was formed. The improved technology achieved a 97.68% average core recovery rate across the entire 2843.18 m well section of the CSDP-2 well， setting a global record for full core recovery in continental shelf scientific drilling. The hydrogen sulfide overflow concentration was controlled below 0.5×10^-6， ensuring zero-accident safe construction. The engineering cost was significantly lower than conventional offshore oil and gas drilling， filling the gap in low-cost， high-efficiency drilling technology for domestic continental shelves. Combined with the industry development trend， the technology application prospects are discussed from the aspects of equipment integration， intelligent control， multi-scenario adaptation， etc. It provides a new technology scheme for the small-diameter deep hole drilling in complex strata and the ocean.