Hydrocarbon Compositional Gradient Revealed by In-Situ Optical Spectroscopy

摘要

This paper presents a case study of a North Sea appraisal well in which a vertical fluid-composition variation, missed by a conventional pressure-gradient-analysis method, was observed in situ in real time by a new fluid-composition analyzer using visible near-infrared (NIR) spectroscopy. For optimal oil production, assessing the spatial variation of fluid properties is as vital as assessing the spatial variation of formation properties. Conventional wireline triple-combination measurements showed that the interval of interest was uniform and free of noticeable impermeable layers. A resistivity log showed an approximate oil/water contact (OWC). Wireline pressure testing identified three different pressure gradients corresponding to gas, oil, and water, all in hydraulic communication. However, the pressure testing did not indicate a gradient in hydrocarbon composition. Fluid was sampled and analyzed in real time by a wireline fluid-sampling-analyzing tool string that included the fluid-composition analyzer. This tool analyzes petroleum fluid and gives concentrations for four group compositions (C_1, C_2-C_5, C_(6+), and CO_2), gas/oil ratio (GOR), and qualitative information regarding heavy-end content and stock-tank crude density. The analyzer showed that the hydrocarbon fluid in an oil-bearing zone was not vertically homogeneous but, instead, had a vertical variation. The samples captured by the wireline sampling tool were sent to a laboratory for compositional analysis that confirmed the variation determined by the downhole analysis. Both results identified the heterogeneity of hydrocarbon fluid in the interval. This paper also briefly covers the measurement principle of the analyzer and discusses the impacts and benefits brought about by the new technology. The concept of flexible fluid sampling is particularly important because it enables operators to make sampling decisions on the basis of real-time fluid-analysis results rather than a predetermined job plan.