Completing well in a carbonate formation is often a big challenge due to occurrence of severe losses in highly fractured limestone formations. In some cases, total losses might lead to prolonged operation time and/or well abandonment with attendant cost implications. Traditionally, Loss Circulation Materials (LCMs) are used to combat losses before completion operation commences. However, there have been limited success with this approach especially in this case where Karst was encountered.
Originated under Managed Pressure Drilling (MPD) system, Pressurized Mud Cap Drilling (PMCD) with continuous seawater injection was introduced in XT Carbonate Oil Development Project. Due to the subnormal formation pressure, seawater was injected continuously down the annulus to maintain the hydrostatic pressure of the hole while ensuring that no formation fluid or gas entered the well bore. By injecting at a rate higher than the loss rate, an artificially induced surface pressure allows for wellbore monitoring while completion assemblies are being deployed.
Prior to running in Lower Completion, a composite bridge plug was set at deepest possible location in production casing instead of the conventional Downhole Deployment Valve (DDV) resulting in the savings of about USD 1 Million. Some modifications were made on initial completion string whereby a sacrificial motor and a bit were included in Lower Completion string design. With the well in static mode, this modified lower completion string which consists of mud motor, drill bit and pre-drilled liner was used to drill out the bridge plug effortlessly. PMCD mode with continuous sea water injection was activated once the plug has been drilled out and while Lower Completion string was run to Total Depth (TD). After setting the Liner Hanger Packer and closing Fluid Isolation Valve, the well was back to static mode and ready for wellbore cleanup and upper completion runs.
Five (5) high temperature wells have been designed employing the above technique which resulted in significant savings in rig time and cost as well as formation damage reduction and increased operating efficiency. This paper summarizes the practical experiences gained during the planning and deployment of lower completion string with perforated liners in a total losses condition with PMCD .