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 Cretaceous and 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.
The integration of the gas data along with the drilling parameters (ROP, ΔFlow) can be of valuable inputs to quantify the Rock properties such as porosity and permeability, this new approach can extend the utilization of gas data not only for formation evaluation and fluid characterization but also for formation petro-physical structure.
To take advantage of the field data, the gas readings are plotted on a depth log, which can be easily integrated with other data. Geochemical equations are plotted against depth and lithology to determine fluid type, contacts and evolution.