Abstract

Alpha 1 was recognized as the highest overbalanced well drilled in operator's history. A comprehensively designed superior mud system has enabled the well to be drilled successfully, meeting its objectives of producing shallow and intermediate intervals and able to appraise deeper 8-1/2" HPHT section. The HPHT section exhibits a marked pressure spike to pore pressure up to about 14,000psi with 310°F bottom hole temperature. The formation prognoses are very much uncertain due to limited data available from offsets. This considerable HPHT drilling challenges are further compounded by the high overbalance pressure - approximately 6,800psi observed while drilling with 18.4ppg mud weight.

Drilling mud designed for high overbalanced pressure well requires special attention particularly on the HPHT filtration control and mud rheology. In deeper section where wellbore pressure control is critical with approximately 1.5ppg drilling margin, the mud formulation and rheology was optimized in minimizing ECD. The components of the mud were heavily scrutinized to minimize risk of wellbore losses and differential sticking. Low fluid invasion system was introduced to provide wellbore shielding mechanics. Operator has made initiatives of conducting unconventional laboratory works in assuring the technical performance. Cost reduction initiative was also made by optimizing mud design with surplus mud and chemical inventory. These were incorporated into the mud design as to have low cost and fit for purpose mud formulation.

In actual operation, the mud rheology and ECD were deviated from the plan. Both major deviations demanded several adjustments on the mud formulation. Several lab tests were conducted and unconventional mud apparatus were brought offshore for further fine tuning on mud formulation. The 8-1/2" HPHT hole section was successfully drilled with no issue pertaining to losses, drag during tripping and differential sticking. The best practices and lessons learnt replicated from few HPHT wells drilled previously, in combination with the close collaboration on laboratory works with contractors has contributed greatly to the success of the well.

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