This paper describes the first successful attempt in the UK Continental Shelf (UKCS) to map CO2 in real time while logging during Centrica's recently concluded four well drilling campaign in the East Irish Sea, offshore north-west England. The East Irish Sea basin is characterised by a range of source rock types, prolonged history of hydrocarbon generation, and Tertiary igneous intrusion. As a result, reservoirs contain varying proportions of carbon dioxide (CO2), nitrogen (N2), and hydrogen sulfide (H2S) in addition to oil and methane. Two of these wells will develop the Rhyl gas field that was discovered in 2009 and found to contain high concentration of methane and CO2 and some N2. An early and accurate understanding of gas composition and variation in such newly discovered fields is critical to reserves estimates, and scheduling and optimizing gas processing at the onshore North Morecambe Terminal.

CO2 had been recognised as a risk prior to drilling the Rhyl exploration well due to the relatively low CO2 content in the neighboring North Morecambe field. During logging and testing several qualitative indications of a higher CO2 content were identified: a high gas density measured at the separator and difficulty in sustaining a flare. Traditionally, the presence of CO2 has been assessed and quantified via lab analysis on samples acquired by both wireline formation testing tools and well tests. Reliable quantification of CO2 from reservoir fluid samples can be difficult due to mud filtrate contamination and because it readily dissolves in produced formation water.

In many CO2 rich reservoirs, compositional gradients are observed with higher CO2 concentrations at the base of the gas column. The two Rhyl development wells provide an opportunity to improve our understanding of the CO2 distribution throughout the field. The latest generation of Downhole Fluid Analysis (DFA) technologies was deployed with the Wireline Formation Testing (WFT) tool. Such technology uses a dual spectrometer system (filter and grating) to accurately measure CO2 content downhole, prior to scavenging. A dedicated channel for the CO2 absorption peak is complemented with dual baseline channels on either side to subtract the overlapping spectrum of hydrocarbon and small amounts of water. The DFA sensor was used at each sampling station during the WFT run of the first Rhyl development well measuring an almost constant vertical CO2 distribution. DFA data was complemented by pressure and pressure gradient analysis and integrated with high definition fullbore microresistivity images, allowing a better understanding of the gas composition across the Rhyl field, hence removing a significant project risk.

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