A Refined Mud Gas Ratio Method for Fluid Identification While Drilling

Abstract Mud logging, a cost-effective surface measurement, offers a real-time and high-resolution mud gas data for monitoring drilling operations and identifying pay zones and trapped fluids in subsurface formations. However, traditional fluid typing methods of mud gas, such as Haworth ratios and Pixler cross-plot, yield inconsistent results with established petroleum engineering classification. This paper presents a refined approach that builds upon the traditional Haworth method, incorporating three key parameters: gas wetness (Wh), the differential between Wh and balance ratio (Bh), and the methane-to-total-hydrocarbon-gas ratio (C1R). This enhanced methodology enables the identification of eight distinct hydrocarbon fluid types and has been validated through a computer program that automates data quality checks, log visualization, and fluid type determination. The software has been tested on over 100 wells in a basin-scale study, establishing a set of cutoffs for fluid typing in the Paleozoic petroleum system. The predicted fluids agree with fluid samples, source rock maturity, and basin modeling predictions. Three well studies are selected to demonstrate the effectiveness of this approach in generating high-resolution fluid profiles, identifying fluids for multiple pay zones, and distinguishing between different gas types. This refined methodology establishes a robust link between mud gas ratios and fluid types, facilitating real-time fluid identification and supporting informed decisions regarding fluid and rock sampling, as well as optimizing well testing.