The conventional approach to gas extraction at the surface is often flawed because of static extraction efficiencies used during drilling operations; a dynamic approach has proven to be much more effective. A dynamic extraction efficiency correction captures inter- and intra-molecular interactions and physical changes in drilling fluid that are not captured by static corrections. To demonstrate the difference between dynamic and static extraction efficiency corrections, both methods were used to analyze multiple wells of various reservoir types.
For any extraction system, a dynamic set of extraction efficiency equations is used to adjust the chemical composition for variations in chemical concentrations, temperature, fluid properties, and extractor type. A vapor-liquid equilibrium (VLE) model was used to dynamically adjust the gas composition to reflect the extraction efficiency and to compare to a static extraction efficiency correction. These correction methods were applied to multiple wells to compare the dynamic and static methods in several types of reservoirs.
The use of the dynamic extraction efficiency method has enabled more than 25 organic and inorganic chemical species results to be provided in 10% of downhole pressure-volume-temperature (PVT) samples, without the need to calibrate vs. the drilling fluid system. In direct contrast, the static extraction efficiency method, which was calibrated vs. the drilling fluid, showed inconsistent results and more significant errors. These results clearly indicate that the dynamic extraction efficiency correction is the better solution for predicting formation fluid composition.
The ability to dynamically adjust a wide range of organic and inorganic chemical species with changing fluid conditions provided more accurate predictions of formation fluid composition than possible using static correction.
A primary challenge for gas detection at the rigsite is overcoming the issue of inefficient extraction. Not being able to extract all formation gas while drilling can lead to issues with gas ratio analysis. Because of bias associated with higher inefficiencies of heavier gas components, gas ratio analysis can tend to indicate a lighter gas in the reservoir or even miss a reservoir section. To overcome these issues, methods to correct formation gas extraction were developed.
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