Enhancing Geosteering Accuracy in Deep, Thin, Structurally Complex Coalbed Methane Formations Using Deep Azimuthal Resistivity Mapping Technology

Abstract

The exploration of coalbed methane (CBM) in China faces mounting challenges as shallow CBM reserves are depleted, compelling the industry to target deeper, thinner seams. In the Sichuan Basin, Well X1H targets CBM layers at true vertical depths (TVD) exceeding 4,000 meters. These target layers are characterized by extremely thin coalbeds (1–2 m), frequent interbedded shale intervals, and highly intricate geological structures. Conventional azimuthal gamma-based geosteering tools have proven inadequate in such demanding environments, often resulting in imprecise bit placement, suboptimal well trajectory control, and increased need for sidetracking operations. To address these challenges, this project deployed an integrated solution combining a next-generation deep azimuthal resistivity (DAR) multilayer mapping-while-drilling service with a rotary steerable system (RSS). The DAR mapping service delivers real-time, high-resolution resistivity inversion with exceptional depth of detection around the borehole and a minimum resolvable thickness of just 0.2 meters. These capabilities enable precise mapping of reservoir boundaries and thin interlayers, facilitating proactive wellbore optimization and improved understanding of reservoir distribution and architecture. The approach leverages a novel directional resistivity tool with advanced sensors and measurements, coupled with an innovative inversion process, to overcome critical geosteering challenges in ultra-thin, structurally complex CBM targets. In Well X1H, this technology enabled precise placement of horizontal sections within exceptionally thin coal seams. As detailed in this paper, the target zone measured only 1-1.3 m in thickness and exhibited substantial structural dip changes. Despite these difficulties, the operator achieved a 1,000 m horizontal section with a net-to-gross (NTG) ratio of 81.8%, surpassing expectations. Accurate tracking and mapping of thin coal seams were accomplished even in highly complex structural settings. This integrated solution significantly enhanced geosteering accuracy, efficiency, and success rates in deep, thin CBM drilling operations. The results from this case study demonstrate that advanced DAR geosteering, when combined with RSS technology, can unlock developmental potential in deep, thin CBM reservoirs with complex geology in China. The improved mapping capability now enables operators to drill longer and more productive horizontal sections, ultimately boosting CBM production. The authors propose that this fit-for-purpose solution serves as a blueprint that can be adapted and expanded to analogous CBM projects confronting similar geological and operational challenges.