Exploiting Shallow Formation Strengths to Deepen Riserless Casing Seats

Abstract

SummaryA significant improvement in deepwater well integrity can be accomplished by deepening the structural casing to have a dual functionality. This exploits the rapid growth of formation strength in the shallow first 1,000–2,000 ft below the seafloor. This first casing string in the deepwater well design would, firstly, support the axial and bending loads of the wellhead, blow out preventers, riser, and subsequent casing strings, as is the current practice, and secondly, provide sufficient casing shoe strength to mitigate the shallow drilling hazards. The basis for this recommended well design change has been the sporadic drilling performance in the execution of deepwater drilling operations, especially for exploration and appraisal wells, which has included some significant catastrophic well failures.The placement of the structural casing significantly deeper than current practice allows the well design to have larger casing diameters in the deeper well sections. This significantly improves deepwater well integrity by decreasing circulating friction. The current practice in the riserless section is to place casing seats above the identified shallow drilling hazards. The study reviews and evaluates the feasibility of setting the subsequent riserless casing strings according to the pore pressure and fracture gradient environment. This requires fewer casing strings to reach the planned well depths, which results in larger casing annuli across the deeper narrow pore pressure/fracture gradient (PP/FG) environment than in current deepwater well designs. This increase in annular space reduces the circulating friction across these sections, decreasing the loss of circulating/well kick cycles that are problematic and can prevent drilling from continuing to planned well depths.This study evaluates the effect of deepening the structural casing for the improvement of well integrity. The feasibility of various drilling methods and technologies required to deepen the structural casing, including conventional drilling, jetting, casing drilling, and reaming, are reviewed and evaluated. The method proposed for this deepening is the application of casing drilling technology. Its principles and merits are reviewed as it would be applied in a subsea environment in mitigating shallow drilling hazards and facilitating the deepening of the structural casing. Finally, the value of this proposal is evaluated in terms of meeting well objectives, improving well integrity, and reducing well construction time.