Use of Advanced Mud Gas Chromatography for Reservoir Quality Prediction While Drilling

Abstract

While drilling through a reservoir, a lot of valuable information can be obtained from mud logging to support formation evaluation. Field data will help wellsite geologists, petrophysicist and reservoir engineers to predict reservoir quality, fluid contacts and reservoir permeability based on formation gases detected while drilling. This study discusses some examples from exploratory wells that have recently been drilled in Kuwait. Gas readings were recorded while drilling through deep Jurassic formations to evaluate hydrocarbon content using Advanced Gas Chromatography. The primary components of the system utilized are: a constant volume gas extractor, a gas sample flow control system, and a high resolution chromatographic system. To interpret the findings Gas readings are monitored by a complex system which provides real-time continuous measurements of the concentration of formation gases from very light components such as methane, to heavy components such as C6, C7 and C8 hydrocarbon species, comprising n-hexane, n-heptane, n-octane, benzene and toluene. Formation gas is considered as the first indication of a reservoir's fluid characterization and reflects the extent of the productivity of the well. Geochemical ratios and equations can enhance the interpretation of field data and give the first indication of zones of interest that need further evaluation. Dedicated flowmeter sensors (Coriolis) utilized to evaluate formation fractures while drilling. The Delta Flow trend versus time is characteristic of the type of losses and consequently of the downhole fractures. Geochemical ratios are plotted against depth and lithology to determine fluid type, contacts, fractures and dolomitization. For this purpose Well-Site Geochemical Package services, including X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) workflow at rig site was also applied in of the wells. The analysis is based on measuring major and minor chemical mineralogy and elements of the drilled cuttings. This has helped in better quantitative assessment of different facies such as limestone, dolomite, shale etc., and providing a better control on geological model update while drilling the well. To take advantage of the field data, advanced mudlogging data are plotted on a depth log, which can be easily integrated with other data (post well data).