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Within the industry, there has existed a long-held perception that oil-based drilling fluids are the only option for landing wells fraught with downhole hazards such as severe loss-prone zones, highly reactive shales, and pressure variances. However, a recent development drilling campaign in North Kuwait has provided evidence that challenges this belief.
Wells constructed through the North Kuwait development program represent diverse sets of shale and sand sequences that pose distinctive and costly drilling challenges. At the start of the project, the use of conventional water-based drilling fluids to drill weak and highly depleted shale and sand sequences proved unsuccessful in maintaining wellbore stability. The operator experienced excessive non-productive time (NPT) and unforeseen costs due to downhole complications, including intermittent tight hole and wellbore enlargement, hole collapse, stuck pipe and logging tools, low-quality logs and poor primary cement jobs. Nevertheless, in keeping with a self-imposed edict to reduce its environmental footprint, the operator assigned drilling fluid designers the task of formulating a water-based mud (WBM) capable of drilling the extremely problematic wells without the fluids-related NPT and cost overruns encountered previously.
Taking into account the specific formation conditions, a wellbore stability study was initiated that resulted in the formulation of a customized high-performance water-based mud (HPWBM). The design incorporated an ultra-low fluid invasion (ULFI) additive to shield the micropores and microfractures found in sandstone, thereby reducing instability and improving hole integrity. This combination enabled the successful drilling of highly depleted, high-overbalance development wells with zero instances of differential sticking.
The subsequent application of the customized HPWBM in the Kuwait development drilling program demonstrated that highly troublesome wells can be drilled safely and cost-effectively, while still reducing the environmental impact.
Geological obstacle course
Operators have historically faced wellbore instability challenges in the reactive shales comprising the Zubair formation. However, before reaching the Zubair, operators must drill through the Shuaiba formation and its vugular limestone. The Shuaiba is predisposed to fracturing and losses so severe that some drilling programs in neighboring Iraq have been abandoned when NPT-related costs became unsustainable.