Since early 1990's, Downhole Fluid Analysis (DFA) has been developed to monitor mud filtrate contamination for Wireline Formation Tester downhole sampling. DFA can also provide accurate reservoir fluid information in real time such as hydrocarbon composition including CO2. However, DFA technology cannot measure Nitrogen because N2 has no absorption in the Near Infrared Region (NIR). Therefore, it cannot be directly detected with any spectrometer measurement downhole. This paper will present innovative methods that can be used to predict the amount of N2 in each reservoir. These new techniques can help many clients in the EAG and as well as other basins to accurately quantify N2 without the need to wait for PVT laboratory analysis which generally takes several months to complete.
Detection of non-hydrocarbon gases in oil and gas fluids, such as nitrogen gas, is very important for reservoir assessment and management. N2 content affects reserve estimation, especially in the area where reservoir fluids have high N2 contents. In our experience, there are several basins in Asia where N2 and CO2 coexist in the same reservoirs. N2 was charged into reservoirs from the source rock in the same geological time as Hydrocarbon (HC). The CO2 then later charged into the same reservoirs. Xu et al (2008) and Mullins (2019) suggested that the ratio of HC. and N2 are in proportional for each basin. However, the CO2 which was later charged are variable in each reservoir depending on CO2 source and charging area. The relationship between HC. and N2 can be used to predict amount of N2 using three proposed methods (1) Basin Base Method (2) Iteration Methods using DFA spectrometer and InSitu Density measurements., and (3) Equation of State (EOS) Method. This nitrogen prediction techniques were developed to better characterize reservoir fluids and overcome the limitation of the existing technology that's unable to detect and measure nitrogen at downhole conditions. This method can offer extra information, especially for our new Ora Intelligent Wireline Formation Tester technology where answer products will be expanded to tailor client objectives.
The N2 and HC. relationship from each basin are examined in detail from our DFA and PVT data base. The ratio of N2 and HC. were then recorded as initial value for Basin Base Method. Then the second N2 prediction technique that uses individual hydrocarbon compositions and downhole density measurements were conducted to calculate missing N2 mass from spectrometer measurements. A ternary diagram was prepared to visualize and determine correlation of the gas composition components. It was found that straight line can be obtained on the Ternary diagram between N2, HC., and CO2 for each reservoir. A detailed calculation based on fluid components and partial densities together with iteration process allows to estimate the mass percentage of nitrogen. The results were then compared with actual value from PVT lab. These nitrogen prediction techniques already have been tested and validated using various datasets from South East Asia and other. This technique can be extended to be part of Reservoir Fluid Geodynamic (RFG) to evaluate lateral reservoir connectivity and to better understand CO2 and N2 charge to reservoirs.