A Tale of Three DST: Evolution of operational and interpretation strategies to characterize a complex, HPHT, offshore carbonate gas reservoir

Abstract Well testing through the Drill Stem Test (DST) plays an important role in reservoir characterization, particularly for complex field developments. Lang Lebah field is a HPHT carbonate reservoir located in Central Luconia Basin, offshore Sarawak, Malaysia, has a very challenging development condition and therefore relies heavily on the data from DST to plan an optimal development strategy (Escobar, 2017). The first DST in the field was carried out in 2019 in the discovery well of the field. This was followed by two more wells in 2021 and 2024 where DST were conducted to adequately characterize the reservoir before proceeding to development. This paper presents the unique challenges faced with both operation and Pressure Transient Analysis (PTA) interpretation, and how the techniques and strategies were refined during this period to achieve an adequate level of reservoir assessment which is sufficient to move to FID in a multi-billion USD development. In the first well (2019), operational problems resulted in suboptimal DST acquisition and PTA data were uninterpretable. Extensive root cause analysis followed, and possible areas of improvement (both operational and acquisition strategy) identified. In the second DST in 2021, the data quality improved, but the interpretation was still a challenge as the well never came out of spherical flow regime. Interpretation could still be carried out to make development decisions. In the third DST in 2024, a different zone was tested where the refined strategy could provide very good quality data. New interpretation challenges were observed requiring an unconventional approach. Although this paper talks about this specific field only, most of the lessons can be applied for any other fields too. Also, there are key takeaways in terms of planning for reservoir heterogeneity in a carbonate reservoir which can impact overall job planning and contingency strategies.