Advances in drilling technology in response to a need for greater evaluation and exploitation of previously marginal resources are leading to increased usage of Controlled and Managed Pressure Drilling systems. Their application on High Pressure High Temperature wells is also increasing in relation to accurate flow and kick detection provision as well as pressure management. The benefits they bring in terms of hazard mitigation, reduced formation damage and improved drilling dynamics are allowing fields with tight operating windows to be drilled with greater safety margins and better economics. A consequence of using closed and pressured systems has been the impact and use of surface evaluation techniques, particularly surface gas logging.

Despite provision of real-time information relating to reservoir pressure, fracture gradient and fluid invasion as well as high resolution flow and fluid density variation, until recently there has still been a lack of meaningful surface gas information. This was due to the fact that all surface gas detection systems except for GC tracer currently rely on atmospheric conditions to extract gas from mud in air.

By utilising a semi-permeable membrane gas extraction process directly behind the choke manifold, prior to the Mud Gas Separator, the sampling point forms part of the enclosed system. This provides significant benefits in removing any potential surface gas loss. The application and deployment of such a technique is described in this paper. The responsive nature of membrane extraction, along with the repeatability and quantitative nature of the process, lends itself well to comparison with high resolution pressure and flow data generated by the Managed Pressure Drilling system and has the potential to be used to increase drilling efficiencies. The relationship between fluid composition and gas in mud volumes at surface, with derived reservoir pressure and surface parameters is examined in this paper.

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