SPE-191724-PA Exploiting Shallow Formation Strengths to Deepen Riserless Casing Seats
The paper reviews the science, and resulting advantages, of exploiting the deepwater phenomena of the early and progressive growth of the fracture gradient immediately below the seafloor in determining casing seat setting depths. This would improve the reliability, well integrity and economics of deepwater wells. This method would allow the subsea structural casing string, the first string in any deepwater well design, to have a dual purpose of supporting the subsea axial casing and riser bending loads while providing sufficient shoe strength for the subsequent casing string. This allows subsequent casing seats to be set deeper than current practice thereby reducing the number of casing strings in attaining well programmed depths.
The current deepwater well design uses the 1st casing to support the axial load of the subsequent string and initiate the drilling process of drilling the next section to set the 2nd casing string. This practice to “jet” the structural casing to depths of 200 to 300 ft below seafloor results in insufficient leak-off shoe strength to adequately mitigate any shallow hazards. These hazards can be shallow gas or water flow, near-surface active faulting or gas hydrates. The drilling process continues with a jetting inner drilling string to drill to the top of the first drilling hazard. Subsequent casing strings are generally set above every identified hazard. The current practice creates narrow drilling operating windows in the deeper well sections which results in high equivalent circulating densities (ECD) which can prevent drilling to planned depths. This causes, at a minimum, significant lost time, with the real cost being the loss of attaining the well objectives. This is a typical situation in the deepwater drilling environment. This process of hazard mitigation results in more casing strings compared to setting the casing seats according to prevailing pore pressure - fracture gradients using with the proven technology of casing drilling.
Many of the principles and practices used in deepwater have been adopted and adapted from shallow water experience with various level of success. However, the deepwater drilling industry has come to recognize the shortcomings in existing well designs with there having been few efforts in changing the current deepwater well design. Leading GOM drilling professionals have noted that deepwater well designs and execution practices need to be challenged to drive for improved well integrity and economics. Many attempts have been applied to improve this situation with use of technology such as MPD, expandable casings, stress caging, etc. without a fundamental change to the well design. There have been some successes but not without adding significant operational complications and without any general acceptance for routine utilization.
The proposed deepwater well design method could replace the practice of "jetting" in the structural casing to about 300 ft below the seafloor to drilling the structural casing to about 1500 ft below seafloor. This could be done without any significant modification to existing wellhead designs.
The result would be a dual purpose structural casing with the following advantages:
Increased well integrity: mitigation of shallow drilling hazards in the riderless sections & improved ECD management in the deeper narrow drilling operating windows
Decreased number of casing strings: less operational time to drill the well & less Capex in casing material
Larger casing across target formations to improve production rates & well economics
The concerns surrounding improving well integrity of deepwater wells for environmental issues of accidental well releases and the need for deepwater projects to compete for investment funding of oil and gas developments has made the Subsea Drive System a force for change, and investment opportunity.