Qatar's North Field is the largest non-associated gas field in the world. The North Field is being developed primarily to supply feedstock gas for a number of LNG (Liquefied Natural Gas) and GTL (Gas-to-Liquid) plants. Accurate determination of the amount of H2S and CO2 in the feedstock gas is critical for the proper design of the LNG and GTL plants. Due to long lead time required for the design and the construction of these processing plants, it is necessary to establish the feedstock gas specifications as early as possible during the FEED (Front End Engineering & Design) process. Moreover, once the plant design specifications are set, early estimation of H2S and CO2 content of Khuff reservoirs at the development well locations is necessary to design a well completion scheme to meet the plant design specification for the feedstock gas.
In 2003, RasGas decided to evaluate several sampling techniques for early estimation of the non-hydrocarbon (NHCG) gas content from Khuff reservoirs. After a thorough study of available options for making reliable measurements of H2S and CO2 content of reservoir fluids using down hole tools, it was decided to test Wireline Formation Testers (WFT), both open-hole (OH-WFT) and cased-hole (CH-WFT) tools, in an appraisal well as a "proof of concept" of these technologies for estimation of H2S and CO2 contents. The results from both OH-WFT and CH-WFT techniques were compared with normal DST surface recombined samples obtained from the same well.
In this paper, the following aspects of OH-WFT and CH-WFT sampling techniques for determining H2S and CO2 contents are discussed and the results/analysis presented:
Optimal tool / Sampling bottles / flow line configuration for reducing H2S scavenging effect.
Comparative analysis on a relative basis between OH-WFT & CH-WFT samples, both on-site and in laboratory.
Evaluation of OH-WFT, CH-WFT and DST sample compositional results on a relative basis.
Conclusions / Recommendations.