Riserless Casing Drilling for Mitigation of Shallow Hazards - A Paradigm Shift in Deepwater Well Construction

Abstract

AbstractDrilling hazards in deepwater riserless intervals include shallow water and gas flows, disassociating gas hydrates, lost circulation and formation instability all which are particularly challenging without a proper mitigation strategy. Unmitigated, these hazards can result in well control events, inability to get casing to planned depth, poor cement jobs, and a lack of structural integrity for subsequent BOP and wellhead installations and even the loss of the well. This paper introduces a paradigm shift in deepwater well construction where the structural casing is installed significantly deeper via a subsea riserless casing drilling system to setting depths based on established pore pressure and fracture gradients. This casing drilling system will replace the established method of jetting in structural casing and then drilling the subsequent hole section to above the next shallow drilling hazard and installing the next casing or liner. Casing drilling will enable the mitigation and isolation of a shallow hazards while running casing in a single trip. The practice of casing drilling is well established in land and shallow marine environments for drilling efficiency and drilling hazard mitigation. Its application into the deepwater environment requires the development of new technology. This well design approach and the use of casing drilling could eliminate multiple deepwater riserless strings-allowing the high-pressure wellhead housing and its conductor to be set significantly deeper.The jetting process is technically limited to a setting depth about 300-ft. below the mudline. This process uses mud motor technology without pipe rotation to "push" the conductor into the seafloor which can be insufficient when drilling harder sediment. The jetted structural casing requires a consolidation, or soaking, period for the soil to settle and strengthen after reaching its setting depth. This w+U- can require the jetted conductor to be held in place for a period of a few hours to as much as 48 hours to attain the required resistance force. Jetting practice, with its limited setting depth, had become an accepted practice for the vast majority of subsea drilling operations. It is this depth limitation combined with the offshore practice of designing well geometry from top-down that manifests the drilling problems for the narrow drilling operating windows in deepwater. The shallower the 36″ casing is set and the requirement to set casing strings above anticipated shallow drilling hazards causes a large number of casing strings of ever decreasing diameter to reach the programmed total depth of the well plan.Casing drilling in the riserless section allows mitigation of shallow drilling hazards to deepen the structural casing, the first casing, and subsequently set riserless casing strings according to the prevailing pore pressure and fracture gradient environment. The inherent benefits of casing drilling by providing drilling hazard mitigation and with the isolation of a shallow hazard zone(s) in a single trip without pipe tripping and its related effects is fundamental to the improvement of offshore and deepwater well designs and operations.