| Popis: |
In 2016 PETRONAS embarked on the challenge of drilling and completing high temperature, highly deviated fractured carbonate development wells using Pressurized Mud Cap Drilling (PMCD) techniques. With equivalent formation pressure gradients of 13ppg, H2S levels of 140ppm and formations with potential for massive losses, but also very high gas productivity, severe challenges were presented to the drilling engineering and operations teams and to the logistical infrastructure; these challenges were successfully overcome, ensuring that the wells were delivered safely using the planned Tender Assisted Drilling Rig. Maintaining well control integrity throughout all drilling and completion operations is always crucial to ensuring safe well delivery. In wells with total losses, drilled through highly productive carbonate formations, there is a high risk that any loss of the primary drilling fluid barrier may result in the well flipping out to gas very rapidly. This risk is highest during trips out of the hole, when the Rotating Control Device (RCD) pressure seal must be removed to retrieve the drilling BHA and run the lower completion. For wells with total losses, options to control the well during trips have traditionally involved a combination of methods including setting gunk plugs, setting composite bridge plugs, or using a preinstalled Downhole Isolation Valve. The purpose of this paper is to present a fourth option, the top-kill method using a dual-density fluid system. During the 2015-2016 PETRONAS development drilling campaign, following thorough risk assessments and detailed planning and logistical evaluation, this method was proven to be a reliable and safe means of controlling the well during trips. A total of five (5) wells were drilled from the development platform, using a surface BOP stack and an RCD. Top kill operations were engineered by pumping two types of fluid into the annulus at surface; a slightly underbalanced Light Annular Mud and a heavier density Kill Mud. By combining these fluids, the well was put into an engineered overbalanced state, with the loss rate controlled via a pre-determined pumping rate. Once the RCD seal element was re-installed, the well reverted back to pumping Light Annular Mud into the annulus as per normal PMCD procedures. This paper highlights the meticulous planning, execution, training and continuous learning that was evident in these challenging operations. This paper also discusses critical HSE concerns and makes recommendations for future PMCD projects. |
