Successful Drilling High Pressure Well with Depleted Levels: Field Lessons from Reservoirs in Offshore Jack Up

Abstract

Nowadays many mature fields are drilled worldwide, alternating depleted and pressurized levels. Depletion phenomena, due to long term production, increase problems associated to all well operations such as drilling, completion and production optimization. On drilling activities events like lost circulation, stuck pipe and wellbore instability are often observed. This paper outlines successful experience in drilling reservoir showing depleted zones (0.76 kg/cm2 /10 m) alternated with high pressure formation levels (mud weight used 1.95 kg/l) and characterized by differential pressure values (difference between hydrostatic mud pressure and pore gradient pressure) around 480 kg/cm2. Such situation creates technical challenges and requires implementation of particular drilling strategies. Experience in mature field offshore combined to innovative technologies leads to successful drilling. Operative experience, use of Eni circulating device (e-cdTM), Eni NPT predictive tool (e-nptTM), proper mud type selection and optimized drilling BHA have been key factors in achieving the desired target. E-cdTM: is a Eni technology that allows to maintain uninterrupted flow of drilling fluid into the well while adding (or removing) joints of "drill pipe" to the drill string, thus avoiding severe fluctuations in annular pressure, maintaining hole stability, decreasing stress formation and the possibility of stuck pipe events. E-nptTM: is a Eni software to real time monitoring drilling parameters and predict, based on Eni case experience, phenomena of Circulation Loss, Stuck pipe or Fluid influx. Mud type selection: oil base mud to mitigate problems due to high differential pressure; high performance water base mud (polymeric inhibited and encapsulated) to avoid reservoir damage even if the differential pressure value reached would have suggested to use oil base mud. Well proved BHA design with automated rotary steerable: to produce less stress to the formation without creating instability to the hole; moreover the BHA is equipped with annulus pressure while drilling (APWD) to monitor real time annulus pressure acting against the formation during drilling and near bit resistivity to allow proper casing point identification. Operatively it has been implemented, in enlarged phases, to uniform the hole at same size to optimize cementing operation of the column This paper presents the successful Eni district South Europe experience in drilling, with minimal NPT, high pressure well alternating pressurized and depleted levels due to many years of production. Such achievement has been obtained combining the most recent technologies, both Eni properties and not, and operative experience gained over tens of years of activity.