Natural Fracture Systems in CBM Reservoirs of the Lorraine–Saar Coal Basin from the Standpoint of X-ray Computer Tomography

• Published in: Environmental Sciences Proceedings 2021-01, Vol.5 (1)
• Main Authors: Privalov, Vitaliy, Pironon, Jacques, de Donato, Philippe, Michels, Raymond, Morlot, Christophe, Izart, Alain
• Format: Article
• Language: English
• Subjects: Applied geology; Earth Sciences; Geochemistry; Petrography; Sciences of the Universe
• ISSN: 2673-4931
• DOI: 10.3390/IECG2020-08772

The Lorraine–Saar Basin is one of the largest geologically and commercially important Paleozoic coal-bearing basins in Western Europe and has considerable coal reserves in numerous coal beds. The basin stands out due to its sedimentary column of up to 6 km and its inversion, resulting in Paleozoic low-amplitude erosion at around 750 m (French part of the basin) and pre-Mesozoic (Permian) erosion between 1800 and 3000 m (the Saar coalfield or German part of the basin). Thermal maturation of organic matter in sedimentary clastic rocks and coal seams has led to the formation of prolific coalbed methane (CBM) plays in many domains throughout the Carboniferous Westphalian and Stephanian sequences. Coal mines here are no longer operated to produce coal; however, methane generated in “dry gas window” compartments at a depth exceeding 3.5 km has escaped here via several major faults and fracture corridors forming “sweet spot” sites. Faults and a dense network of tectonic fractures together with post-mining subsidence effects also increased the permeability of massive coal-bearing and provided pathways for the breathing of environmentally hazardous mine gases. Nearly all CBM plays can be classified as naturally fractured reservoirs. The Lorraine–Saar Basin is not excluded, indeed, because of the experience of geological surveys during extensive coal-mining in the past. The knowledge of geometrical features of fracture patterns is a crucial parameter for determining the absolute permeability of a resource play, its kinematics environment, and further reservoir simulation. The main focus of this contribution is to gain an insight into the style and structural trends of natural cleat patterns in the basin based on the results of X-ray computer tomography (CT) to ensure technical decisions for efficient exploration of CBM reservoirs. To explore the architecture of solid coal samples, we used X-ray CT of a coal specimen collected from Westphalian D coal from exploratory well Tritteling 1. The studied coal specimen and its subvolumes were inspected in three series of experiments. At different levels of CT resolutions, we identified two quasi-orthogonal cleat systems including a smooth-sided face cleat of tensile origin and a curvilinear shearing butt cleat. The inferred cleat patterns possess features of self-similarity and align with directional stresses. Results of the treatment of obtained cleat patterns in terms of their connectivity relationship allowed the presence