As the deepwater Gulf of Mexico (GOM) drilling operations move into deeper water and well depths there has been a lack of consistent and sustained drilling performance improvement. This is an evaluation of the GOM deepwater wells in an attempt to understand the reason for this poor drilling performance and propose a solution to adapt the well designs for the specific challenges of deepwater drilling. Execution of these very expensive wells, which often fail to achieve objectives or worse, are lost, requires a step change in drilling performance. The complex deepwater drilling environment has pushed the typical offshore well construction design model to its limits. Many of the well design philosophies and the well equipment itself are not well suited for the unique deepwater drilling environment. This requires step changes in well design philosophy, and the understanding and acceptance of the associated risks of implementing new practices. The acceptance of change has been a monumental driver in our industry. The goal is to ensure that exploration and development of oil and gas continues to be feasible in this industry subject to volatile commodity prices and ever increasing costs. A paradigm shift in well design philosophy that involves managing the drilling risks in the shallow hole sections, where the well costs are minimum, rather than the current practice of incurring risks after significant investment has been made, is critical to future success and economic viability of deepwater drilling. The well design model presented uses the shallow and rapid growth of the pore pressure/fracture gradient (PP/FG) environment to optimize casing seats. Drilling with Casing (DwC) is an enabling technology that can be a mitigant for managing shallow hazards. The fundamental premise is to use this technology to set the first, and possibly the second casing strings, significantly deeper than current practice. The proven ability of DwC to mitigate many similar drilling hazards as those encountered in deepwater drilling would allow the casing seats to be based upon the prevailing PP/FG environments, rather than being influenced by the shallow hazards. This could allow for the following well design improvements: • Larger annuli below salt for improved drilling margin management. • Less total casing strings in the well. • More use of conventional casing strings sizes for drilling and geological contingencies. • Enhanced planning and use of solid expandable systems. • Decreasing the risk of not obtaining at least an 8-½ in. ID completion, essential for economic success in deepwater environments. • Batch drilling into salt, which optimizes horsepower as well as cost, utilizing smaller capacity rigs for the lighter hook load casing lifts.

Drilling 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.

Summary A 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.

This technical submission on Deepwater Casing Seat Optimization is a follow up to the submission of Riserless Drilling with Casing: A New Paradigm for Deepwater Well Design OTC-19914, presented in May 2009. This paper had discussed the need for a deepwater well design change for GOM due to the well documented poor performance of drilling complex wells in the GOM, and subsequently proposed a well design concept based upon setting the first two casing strings significantly deeper than the current practice. This submission continues the discussion the rational for setting the riserless casing strings deeper than current practices and reviews the feasibility and challenges of a well design to implement such a significant change in the current deepwater well design.Riserless Casing Seating Optimization is a design method for the installation of riserless casing strings in deepwater drilling environments. The conventional deepwater well design model fails to take advantage of the early and progressive growth of the subsurface fracture gradient immediately below the mud line, due to perceived requirement to set an another casing string above anticipated shallow drilling hazards. This results in the inability to supply sufficient and safe "leak-off" for the drilling of subsequent holes and casing installations, adequately mitigate shallow hazards, and avoid "wasting" casing diameters in the riserless hole sections and thus subsequent sections to total vertical well depth. This method provides for the installation of at the minimum two riserless casing strings, beginning with the structural conductor, which is conventionally installed by "jetting" operations. The proposal is for the dual purpose use of the structural conductor in that it is installed at deeper depth so that it takes advantage of the early growth of the fracture gradient, and then becomes a true structural casing being able to support the axial loads of the BOP's and subsequent casing strings. Furthermore this ensures sufficient leak-off protection for next string and the safe drilling of subsequent shallow hazards. This technical presentation suggests that drilling with casing could be the mitigating technology that will allow the hole sections depths to be drilled to depths corresponding to the pore pressure / fracture gradient environment. Drilling with casing, actually liner drilling due to the water depth, improves the near wellbore shallow fracture gradient while providing better dynamic ECD control of potential shallow water or gas flows. Furthermore there is a discussion on some of the key well design issues that pertain to this proposal.

The paper reviews the advantages of exploiting the deepwater phenomena of the early and progressive growth of the fracture gradient immediately below the mudline in determining casing seat setting depths. This would improve the reliability, well integrity and economics of deepwater wells. This method allows the subsea structural casing string, the first string in any deepwater well design, to have a dual purpose of supporting the required subsea axial loads while providing sufficient shoe strength for the subsequent casing string. This allows subsequent casing seats to be set deeper than current practice reducing the number of casing strings to attain well programmed depths. The conventional deepwater well design uses the criteria of the structural casing primarily to support the anticipated axial load of the subsequent string(s) for its setting depth. The practice is to jet the structural casing to depths of 200 to 300 ft below seafloor. This results in insufficient leak-off shoe strength to adequately mitigate any shallow hazards that may exist such as shallow gas, near-surface active faulting, shallow water flows and gas hydrates. Therefore, a casing string is generally set just above every identified hazard, adding rig time and increasing the number of casing strings in the well design. This can be detrimental to the well objectives by creating high equivalent circulating densities (ECD) in the lower well sections. These ECD's in narrow drilling windows can prevent continued drilling, or at a minimum cause significant lost time. This situation is a typical problem in the deepwater drilling environment. The deepwater drilling industry has had to recognize the shortcomings in existing well designs. Many of the principles and practices used in deepwater have been adopted and adapted from shallow water experience with various level of success. Leading GOM drilling professionals have noted that deepwater well designs and execution practices need to be challenged, especially in light of the BP Macondo incident, to drive for improved well integrity, and of course economics. The proposed deepwater well design method could replace the practice of "jetting" in the structural casing with drilling-in the casing to about 1500 ft below seafloor. This could be done without any modification to existing wellhead designs. The result would be: Increased well integrity: In the riserless section, mitigating shallow hazards with stronger casing shoes. Ensures structural casing is placed at optimum depth to provide maximum bending moment resistance. Below HPWH conductor, increase the drilling operating windows (larger annuli). Decreased well cost: Reduce at least one casing string. Minimize trouble time with narrow drilling windows and "junked" wells. Increased well objective reliability: Less casings and larger holes below the HPWH and its conductor allow additional casing strings for geological or mechanical sidetracks. Increases drilling operating window to reach programmed TD. The concerns surrounding the well integrity of deepwater wells with both the existing well design and the need for deepwater projects to reduce their costs to compete for investment funding has become the force for change.