Unlock Large Potentials of Standard Mud Gas for Real-Time Fluid Typing

Standard mud gas data is part of the basic mudlogging service and is mainly used for safety. Although the data is available for all wells, it is not used for real-time fluid typing due to poor prediction accuracy. We developed a new method recently based on a large in-house reservoir fluid database and significantly improved the fluid typing accuracy from standard mud gas data. The new technology unlocks the large potential of utilizing standard mud gas data for thousands of wells. The standard mud gas data has limited gas components that can be detected confidently (usually from C1 to C3). In addition, they are raw data without recycling correction and extraction efficiency correction. Following the traditional geochemical analysis methods, some key parameters (C1/C2, C1/C3, Bernard ratio) have a universal threshold to distinguish gas and oil. The main reasons for the poor fluid typing accuracy are due to 1) lacking C1 to C3 composition correction for wells with oil-based mud; 2) geochemical parameters based on C1-C3 are field dependent. Based on the reservoir fluid database analysis, we divide the reservoir fluids from different fields into two categories. For Type I fields, there are large differences between C1 to C3 component ratios for oil and gas. When water-based mud is used, C1 to C3 component ratios from standard mud gas can be utilized directly to identify oil and gas. When oil-based mud is used, we developed a new method to achieve corrected standard mud gas composition for fluid typing using pseudo extraction efficiency correction based on Equation of State. For Type II fields, there are severe overlapping of C1 to C3 component ratios for oil and gas. The overlapping is the main reason for the poor fluid typing accuracy. We recommend utilizing a heated degasser when drilling into Type II fields to provide additional data of C4 and C5 for accurate fluid typing. Johan Castberg is a Type I field, and we achieved excellent fluid typing results for 14 wells using water-based mud and oil-based mud. There is no additional data acquisition cost for standard mud gas data, which is available for all wells. The new method makes accurate fluid typing possible for real-time well decisions like well placement, completion, and sidetrack. The innovation created significant business opportunities based on the standard mud gas, which has been regarded as not applicable data for accurate fluid typing for many decades.