A New Ultraslim LWD Solution Unlocks Formation Evaluation and Geosteering for Underbalance Coiled Tubing Drilling

Abstract Underbalance coiled tubing drilling provides an effective way to produce hydrocarbon by minimizing reservoir damage while drilling. The ultra-slim small borehole size (3.5’’) however precludes the use of standard logging-while-drilling (LWD) technology which has been developed for "slim" boreholes starting from a 5.5’’ bit size. There was a concern that the lack of LWD measurements in underbalance coiled-tubing drilling was leading to drilling wells blindly. Sparse mud logging data and gamma ray were the only measurements available to make decisions about well placement to maximize the well net-to-gross ratio. To fill this gap, a unique workflow was designed to integrate the latest LWD technology with mud logging and offset wells data. The workflow provides formation evaluation answers and real-time geosteering information to place the borehole in the sweet reservoir spot. The ultra-slim LWD resistivity was recently introduced in the field for logging while drilling in wells with underbalance coiled tubing operations. It was designed to withstand the very harsh drilling environment, including high shocks and vibrations and high temperature. The tool is a compensated electro-magnetic propagation measurement, providing phase shift and attenuation at two different frequencies. The four resistivity measurements are combined with the gamma ray measurement into a model-compare-update workflow to take real-time decisions while drilling on the wellbore trajectory positioning. A near-real-time machine-learning workflow provides formation porosity and mineralogy using the medium and deep resistivity and gamma ray from offset wells and advanced mud logging. This workflow integrates x-ray fluorescence and diffraction measured on cuttings, although the data is very sparse owing to the challenging cuttings collection conditions. It is calibrated to dynamic well data. The LWD resistivity measurement was validated by comparison with offset wells resistivity logged in overbalanced condition with conventional wireline and LWD measurement in a test well and by comparison with a wireline run conveyed on coiled tubing in one well. The workflow was applied on multiple wells in clastic and carbonate environments. Based on the observed production data, the geosteering workflow was deemed successful. The ultra-slim LWD resistivity tool for coiled-tubing drilling brings key data in an extremely harsh environment where data is very sparse. The technology introduction in the field is a key component of a strategic vision to increase gas production through underbalance coiled-tubing drilling. The value of this approach currently contributes to the success of UBCTD development wells and is limitless possibility of worldwide untapped reserves. Moreover, the Coil Tubing allied with this workflow can reduce carbon footprint by minimizing non-productive footage drilled and optimizing well placement investment and return of investment