Exploiting Shallow Formation Strengths to Deepen Riserless Casing Seats

Abstract

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.