Utilization of Mud Logs for Oil Fraction Quantification in Transition Zones and Low Resistivity Reservoirs

Abstract

Abstract Accurate quantification of fluid fractions in transition zones and low resistivity pay reservoirs often poses a challenge to the interpreter. Dry oil production from low resistivity intervals, mixed flow production, or disparity between formation evaluation analyses and surface tests can lead to lower confidence in the downhole measurements or interpretation and increase the uncertainty of the calculated hydrocarbon volume in place. A variety of measurements may be exploited to mitigate these concerns and, in this paper, the authors present an assessment of the relationship between normalized hydrocarbon levels while drilling, and computed oil fraction from wireline logging of logging-while-drilling (LWD) measurements based on a comprehensive dataset of various surface and downhole measurements. The study investigated the validity of advanced mud logging data for oil fraction quantification for particular borehole and reservoir settings and found a consistent trend across the range of bulk oil proportions and normalized gas values. The results of the obtained regression, and alternative cluster-based models are presented. The sources of uncertainties and possible correction approaches are discussed. The findings suggest that in consistent environmental and reservoir settings, advanced mud logging data can be used for hydrocarbon fraction estimation while drilling to assist in pay zones delineation and identification of fluid contacts. Potential was also seen to identify intervals of fractional flow production in transition zone reservoirs, low permeability formations, and dry oil production in low resistivity pay cases. In addition, developing such relationships and models is useful for correct identification of mud log artifacts, such as produced hydrocarbons while drilling, gases from circulation, and zones of overpressure. Another application is to assist in defining petrophysical model parameters, aiding or replacing other supporting information.