Geosteering and Monitoring Methods for Underbalanced Coiled Tubing Operations in Gas Reservoirs

Abstract

Abstract Under-Balanced coiled tubing drilling (UBCTD) is one of the techniques used in developing low pressure gas reservoirs. However, a drawback in UBCTD is their limited geosteering and monitoring tools due to the coiled tubing size compatibility and hole dynamic condition. As such, we present a review of the current available geosteering and monitoring technologies that will help in achieving optimum geo-navigation through the gas sweet spot layers. In this work, a review of current geo-navigation and monitoring technologies for UBCTD operations will be demonstrated. After defining the associated challenges of steering with UBCTD, different technologies will be outlined based on their applicability and effectiveness in landing the UBCTD laterals in the sweet spot layers. These technologies are going to be in the form of downhole logging while drilling (LWD) tools (gamma ray and resistivity), real-time surface measurements, as well as cutting analysis like X-ray diffraction (XRD) and X-ray fluorescence (XRF). Based on a thorough analysis of the different measuring methodologies, each one has its advantages and disadvantages, as follows: (i) The gamma ray measurement helps determine the lithology through the naturally occurring elements of the layers, but its reading doesn't effectively detect gas bearing zones. (ii) The resistivity reading helps in distinguishing conductive with non-conductive; however, good resistivity contrast between the layers needs to be present in order to distinguish layers that cannot be detected from the gamma ray alone. (iii) Gas flowback while drilling is an excellent indicator of gas presence, yet it doesn't directly indicate productive zones for better steering. (iv) Utilization of cutting analysis is the closest direct measurement of the reservoir, but its issue comes from its subjectivity, sample quality, and inconsistency in cutting depth correlation. As a result of the comparisons, we concluded that a combination of all the techniques is essential in achieving optimum well placement with UBCTD, where some technology weaknesses can be tackled with the other strengths. This review represents a method of utilizing existing geosteering and monitoring technologies to advance UBCTD reservoir navigation capabilities by integrating different LWD, real-time measurements, and cutting analysis to achieve optimum lateral placement, yielding better field development and increased gas recovery.