This paper will discuss the key focus areas in successfully delivering a slim well design as a Proof Of Concept (POC) for marginal fields and well cost optimization.

Well Tall-A is a Near Field Exploration (NFE) well targeting marginal reservoir which utilize the slim well concept; a 2-hole section well with 9-5/8" as the conductor. For a successful well execution, three (3) key focus areas were identified which are successful operation of 9-5/8" Casing While Drilling (CWD) to section TD, sustainability of 9-5/8" casing as conductor for the whole well life cycle and achievement of well objectives.

Tall-A recorded the longest and successful 9-5/8" CWD Level 2 (non-directional) for Asia Pacific with 1168m drilled footage as of year 2020. Lessons learnt from previous PCSB 9-5/8" CWD operation were incorporated for casing bit selection hence a heavy-set casing bit (8 bladed) which has been proven in drilling long hole interval in the Middle East (>1000m) was utilized. Continuous monitoring during execution is essential in ensuring the casing is set at the desired setting depth. Sustainability of the 9-5/8" casing as conductor for the whole well life cycle is critical for a slim well design concept. Several studies and extensive discussions between multiple parties has been incorporated to enable utilization of the 9-5/8" as conductor with required sufficient tension to sustain the exploration well lifecycle. A conductor study was performed which incorporated the Metocean data, rig data and connection Stress Concentration Fatigue (SCF) to qualify the 9-5/8" as conductor. To meet the primary and secondary targets; the 8-1/2" hole needs to be kicked-off early and build up to maximum 44 deg before maintain tangent to final TD at 2752m MDDF. Due to the long open hole (1475m) and well inclination within the avalanche hole cleaning regime (30 to 60 deg), the well is prone to hole cleaning problem and wellbore instability. Hence, it is critical to have good drilling practices and precise mud weight selection to ensure no hole problem encountered. The well was successfully drilled to TD, completed the well testing and P&A.

In summary, well Tall-A successfully maneuvered all challenges to deliver the well safely that resulted in Best In Class (BIC) performance. The slim well design concept has been proven achievable and serve as base design for future marginal wells.

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