A novel workflow is presented where logging-while-drilling downhole fluid analysis (LWD-DFA) data is combined with surface mud logging to identify and quantify asphaltene distribution in the development phase of a North Sea field known to have complex fluid variations and compartmentalization. Data from two pilot wells was used for calibration and validation of optical densities from LWD-DFA and Wireline downhole fluid analysis (WL-DFA) acquired in wells drilled during the appraisal phase versus historical laboratory measured asphaltene data. Subsequently, three horizontal producer wells were drilled and the new validated/calibrated LWD-DFA data was used for fluid characterization and asphaltene distribution mapping in real time enabling decisions for drilling and completion design.
To de-risk and resolve this fluid variability and compartmentalization in the three horizontal production wells in the field development plan, a digitally enabled workflow coupling surface logging techniques with LWD-DFA was implemented. Identification of the presence of heavy hydrocarbon components (asphaltenes) and their mobility was of paramount importance. Continuous composition measurements were obtained via Advanced Mud Gas Chromatography to identify the heavier components and were further supplemented by Fast Field Total Organic Carbon (TOC) analysis at discrete depth intervals. This rich dataset, when merged with petrophysical LWD data, allowed fluid analysis station depths to be optimally selected and analyzed with LWD-DFA based optical spectrometry.
Historical laboratory data and Wireline DFA data were used to build correlations between optical densities at different wavelengths and the asphaltene content in wt%. These correlations were first validated while drilling two pilot wells and then utilized while drilling the three horizontal producers. Using these correlations, LWD-DFA optical densities (ODs) obtained at each selected fluid analysis depth were used to predict asphaltene distribution along the well trajectories while drilling.
Dynamic parameters during the pumping/clean-up phase of the LWD-DFA stations such as pressure drawdown, GOR, fluid composition, optical densities, fluid temperature were monitored and controlled in real time to reach the targeted level of contamination and collect high-quality single-phase samples for further fluid analysis in the laboratory.
The asphaltene real time data became of critical importance during the drilling of the producer wells and, ultimately, in making completion decisions. High quality fluid composition data as well as physical single phase hydrocarbon samples were acquired in the reservoir section of each producer well through the integration of surface logging, advanced petrophysical measurements and downhole fluid analysis. Moreover, the newly acquired data was integrated with previously acquired Wireline DFA measurements in neighboring wells to evaluate variations in fluid properties measured in-situ; in particular, to investigate vertical and lateral connectivity by modelling asphaltene gradients with the Flory-Huggins-Zuo (FHZ) equation of state. The clear demarcation of the various compartments observed in the appraisal wells was similarly observed in the production wells resulting in a substantial derisking of the drilling and completions operations.
The examples presented herein illustrate the significance of applying LWD-DFA to characterize fluid distributions in horizontal well trajectories and to complement existing well placement workflows to optimize reservoir exposure, all performed while drilling. Field development decisions are enabled in the while-drilling phase to optimize well and completion design and serve to further refine subsequent well placement.
