Mercury and Hydrogen Sulfide Sampling Case Studies: Accurately Quantify The Low-Level Content Of These Impurities Using Wireline Formation Testers

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Abstract In the past ten years, companies have started to show more interest in accurately quantifying nonhydrocarbon components (impurities) such as carbon dioxide (CO2), mercury, and hydrogen sulfide (H2S), to help design their surface facilities and separators at an early design stage, in addition to meeting the requirements related to various gas sale agreements and regulations on health, safety, and environment (HSE). Simultaneously, it has been a challenging to accurately estimate these impurities, especially at low-level contents. In this paper we will discuss the complete process of downhole sampling techniques using an advanced wireline formation tester (FT), from pre-job planning, real time monitoring, and sampling, including results comparison between pressure-volume-temperature (PVT) laboratory analysis and testing at a larger scale. The objectives are to obtain a high-quality PVT sample, and accurately quantify low contents of CO2, H2S (around or less than 10 ppm), and mercury (around or less than 5 µg/m3) from the collected reservoir fluids. This paper presents three case studies from Southeast Asia region. Downhole reservoir fluid sampling method using FT is not a new subject in the oil and gas industry. However, a breakthrough in the FT sampling technique came about in 2005, when a focused sampling probe was introduced that significantly lowered the contamination values achievable in downhole sampling. In addition, the ability of sensor advancements to do real time monitoring to achieve low-level contamination during downhole reservoir sampling also played an important role in ensuring high-quality samples were collected. A close to 0% contamination from mud filtrate (oil or water-based mud) is now a target when obtaining a representative reservoir fluid sample to properly quantify these low-level impurities content. Coupled with the latest development in downhole fluid analyzer (DFA), reservoir fluid properties such as the gas/oil ratio (GOR), in-situ density, and composition (C1, C2, C3-5, C6+, and CO2) can also be measured at downhole conditions in real time. These have the added benefit of optimizing operational decision-making, thus minimizing the sample time per station, leading to rig time-savings. In this paper we will use examples from several fields in Southeast Asia region. The prejob planning consisting of numerical simulation will present the best probe type, expected pumping time, and volume, and selection on DFA tool to help monitoring RT sampling operation to achieve downhole fluid sampling objectives. The challenges to sample each impurity component, best practices, lessons learned, and comparison of real time results to actual PVT analyses; will also be discussed. Finally, there are limited resources or papers that have been published in the industry that fully describes this end-to-end process, including showing a combination of different cases for multiple impurities. We provide insight into the process of accurately obtaining representative high-quality PVT samples including their impurities for better reservoir characterization.

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