The quest for oil and natural gas reserves is taking the industry to areas where production was previously considered uneconomical. Floating liquefied natural gas (FLNG) is a technology that allows for the production, liquefaction, storage, and transfer of liquefied natural gas (LNG) at sea, removing the requirement for pipelines, reducing development costs, and unlocking new remote fields for production.
The Prelude natural gas field was discovered in the Browse Basin in Western Australia, by a major operator in 2007, with the additional Concerto field discovered nearby in 2009. The combined gas fields have around 3 trillion cubic feet of liquids-rich gas. The development strategy was the Prelude FLNG facility.
The Prelude FLNG facility is supplied with gas from 7 near horizontal wells, completed with 7-in. production tubing.
One of the well completion challenges faced was related to the sand face completion strategy. Formation evaluation studies determined that the optimum sand face completion for Prelude was cased and perforated. The challenge was perforating 600 m of near horizontal interval in underbalanced conditions in one trip, followed by an immediate well cleanup to the Mobile Offshore Drilling Unit (MODU) via a well test facility, thereby maximizing perforating tunnel cleanup efficiency and optimizing recovery of perforating debris during the well cleanup.
Various options were considered, from multiple coiled tubing runs to tubing-conveyed perforating (TCP) shoot and pull operations, but all had drawbacks and were not able to achieve the previously mentioned underbalanced perforating and well cleanup requirements.
This led to a perforating strategy whereby a 600 m TCP gun string was deployed across the reservoir and dropped off. The completion fluid at this stage was already a base oil, which would generate the desired underbalance during the perforating event later. Gun deployment was followed by installation of the 7-in. production tubing and completion assembly. Only after completion installation and testing were the guns pressure activated from surface and the well cleaned up immediately through the well test facility.
The 600 m spent gun string will remain in place across the reservoir section for the well lifecycle through to final abandonment. While leaving perforating gun strings downhole is not an uncommon occurrence in the industry, the mildly sour downhole environment on Prelude introduces a corrosion risk for traditional mild steel gun systems, which has the potential to lead to gun degradation with corrosion particles being produced up the well leading in turn to potential well and facility operability issues. To avoid this specific risk, the operator worked with the service company to develop the world's first corrosive hostile environment (CHE) TCP perforating gun system.