Effectiveness of Riserless Casing Drilling for Deepwater Hazards Mitigation: A Risk Assessment

Abstract

Abstract The benefits of casing drilling are decreased lost circulation, increased wellbore strength, and immediate dynamic kill capabilities. These all improve the mitigation of shallow flows and reduce wellbore instability and so increase the confidence in drilling the riserless section. Casing drilling is an established drilling process that has been used for decades in many environments with the notable exception of the riserless section of deepwater wells. The key to casing drilling improving mitigation of shallow drilling hazards is the rotation of the casing in the narrow annular space between the casing and the open hole. This strengthens the wellbore with plastering, or smearing, of the drill cuttings into the wellbore wall while simultaneously providing improved dynamic well control protection. In this single well barrier environment, Riserless Casing Drilling provides influx mitigation with dynamic kill capabilities, allowing for fewer casing strings, by deepening the structural/conductor casing, and also potentially deepening the high pressure wellhead housing (HPWH) surface string Riserless drilling is a common offshore and deepwater practice with the operational and economic benefits of not requiring a riser and blow-out preventer (BOP) stack. The use of a riser and BOP in the shallow subsea formation causes high bottom hole circulating pressure which often results in lost circulation. This makes riserless drilling the only practical method to drill these shallow sections to a depth where the formation strengths can support the hydrostatic forces of a long drilling fluid column ladened with drill cuttings. Riserless drilling has one well barrier for well control in contrast to the industry practice of two well barriers for all other sections. The standard two barriers are a primary fluid column barrier and a secondary barrier comprising of a BOP, casing string(s) and ancillary equipment. The single barrier situation has been managed via careful planning and execution. For the riserless section the industry has developed practices to minimize the potential hazards of operating with one well barrier. These involve geological evaluation to identify shallow hazards, i.e., the setting of casing strings near the top of these hazards, thus isolating the weaker formations. However, this practice adds casing strings in the riserless section and often the HPWH and surface casing is set higher than optimal. This situation can create small annuli below the riserless section which often causes sufficiently high equivalent circulating densities (ECDs) to cause the loss circulation – ballooning cycle. In deepwater narrow pore pressure – fracture gradient environments this often results in the well not reaching its planned total depth in addition to significant lost time attempting to cure the problem. Replacing the conventional jetting method with Riserless Casing Drilling for the installation of the structural casing eliminates contingency casing strings and allows the deepening of the HPWH surface casings. This decreases the well cost by reducing the number of casing strings while also increasing the likelihood of the well reaching its objectives. The application of Riserless Casing Drilling has been evaluated with a risk assessment which identified the hazards and quantified the risks by comparing the current riserless practice to the application of Riserless Casing Drilling. This risk assessment indicates that Riserless Casing Drilling reduces the risks of shallow water or gas influx and well bore instability with the smearing effect, and its geometry of small annulus, emulates pilot hole drilling while providing dynamic kill capabilities.