A Novel Isotopic Technique to Distinguish Nitrogen Source in Downhole Sampling of Natural Gas

Understanding the origin of non-hydrocarbons in natural gases is crucial in the exploration and production cycle. High nitrogen (N₂) contents are occasionally observed during bottom-hole samples collection. A novel geochemical approach, including gas compositions, and stable carbon and nitrogen isotopes, was used to assess the origin of high N₂ content collected from several bottom-hole samples. The results suggest that the measured samples are likely to be contaminated from the pressurized N₂ cylinder used for gas lifting and sampling. The values of the nitrogen isotopic compositions (δ¹⁵N) of the analyzed samples with high N₂ concentrations are similar to the pressurized N₂ cylinder used for gas lifting and sampling. The contamination in pressurized N₂ compensated cylinder is due to N₂ leakage from the N₂ side to the sample side of the chamber while conducting downhole fluid sampling. Our mass-balance calculation suggests that the contamination percentages (i.e., amount of leaked N₂) from the pressurized N₂ cylinder into the samples is up to 70 mol%. One of the major causes of high N₂ content in natural gas is the occurrence of thermochemical sulfate reduction (TSR). The measured chemical and carbon isotopic compositions of the hydrocarbons indicate the absence of thermochemical sulfate reduction in the analyzed samples. The findings of this study provide geochemical evidence that help to solve problems while sampling and reducing the investment risks encountered with high N₂ content during exploration and production cycles.