Managed Pressure Drilling (MPD) and wireline logging under pressure were both successfully employed while drilling a pre salt, exploration well offshore Angola. The well was drilled with a 6th generation drillship using an integrated, Below Tension Ring (BTR) MPD package with the drilling contractor's Riser Gas Handler (RGH). Drilling exploration wells in pre-salt formations presents a plethora of challenges, with severe drilling fluid losses, often due to the natural fractures and/or vugular porosity encountered. This paper discusses the challenges faced drilling the desired hole section, managing severe losses, a downhole cross flow regime and high surface gas levels. Concern for exiting the salt section and taking total losses in a possible vugular carbonate is addressed and the contingent plans developed are discussed. Fluid loss management was critical and discussion is provided on the remediation methods used to limit losses. The paper describes how considerable gas was detected, handled and eventually controlled using the available surface equipment and methodology for preventing continuous gas influx and prolonged, off bottom circulating for removing the gas from the wellbore. Extensive wireline logging was completed, both at an intermediate stage and at total depth (TD). Most of the wireline logging was performed under minimal pressure with a logging adaptor installed in the rotating control device (RCD), 9 5/8" casing shooting nipple back to surface and wireline pressure control equipment (WPCE). The paper concludes with a series of key lessons learned that can be taken from this experience and considered when planning similar deep water, pre-salt exploration wells with the use of MPD equipment.
In the quest to improve reservoir management and maximize oil recovery, it is imperative to explore solutions that ensure the optimum benefit to cost ratio. While it is renowned as a reliable drilling technique tackling operational problems such as reduced Non Productive Time (NPT) and eliminating induced damage resulting from conventional drilling practice, modern Under- Balanced Drilling (UBD) applications have seen significant advancement enabling efficient reservoir re-development. Following the promising results and significant learnings yielded from the UBD pilots executed in onshore Abu Dhabi oil fields, wider application of UBD was considered for productivity enhancement in tight reservoir units in addition to maximizing reservoir knowledge through inflow data analysis while drilling. Consequently, a UBD campaign commenced by drilling two wells (X1 and X2) located in a challenging multi-layered tight reservoir with complex heterogeneities, including vertical/lateral permeability contrast and presence of faults/fractures. Overbalanced drilling in these zones causes formation damage resulting in limited contributing intervals within long horizontal drains, affecting communication and conformance between injectors and producers. Conventional stimulation techniques have proved inefficient in restoring well productivity/injectivity. The wells were drilled in stepping down trajectory maximizing reservoir contact and homogenizing inflow per subunit, with the laterals planned to cross existing reservoir features in order to evaluate their impact on flow contribution. Real time flow data were interpreted using service providers proprietary tool of Rate Transient Reservoir Characterization (RTRC). Current state of art RTRC method uses a rate integral productivity index (RIPI) that filters instantaneous PI while drilling in order to enable simultaneous evaluation of flow features and proper geo-steering to optimize well targets. In addition, multi rate tests at the end of each section were analyzed to establish a permeability/PI profile followed by mini build ups – when necessary – to confirm the reservoir pressure. Lessons learnt from the first well enabled improvent in the operational aspects of UBD control on the subsequent well through adequate design of UBD tools and conditions. RTRC analysis showed that the two wells were dominantly matrix producers with the well X2 indicated the presence of a secondary permeability attributed to fractures, which resulted in a higher well potential than expected. The acquired productivity baseline per sub layer will be useful in optimizing the completion strategy, considering smart completions and enhanced reservoir contact per sub layer. As a way forward, extending UBD application to other areas of the reservoir is also considered as an opportunity for future development. The case study presented herein, highlights the outcomes of RTRC in deploying strategic re-development options to improve reservoir performance and deliverability in the long term.
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