Deciphering the formation period and geological implications of shale tectonic fractures: a mini review and forward-looking perspectives

In complex geological areas, the chronology of tectonic fracture formation is pivotal for the conservation and enhancement of shale gas reservoirs. These fractures, evolving over different geologic epochs, critically influence the modifications in hydraulic fracturing. The review sheds light on an integrated methodology that bridges conventional geological evaluations with experimental diagnostics to decipher the intricate evolution of such fractures in complex geological areas. Shale tectonic fractures, predominantly shear-induced, are delineated into four distinct levels (I, II, III, IV) based on observational scales. Understanding the geometric interplay across these scales provides insight into fracture distribution. Recognizing the constraints of isolated approaches, this study amalgamates macroscopic geological assessments, such as structural evolution and fault analysis, with microscopic techniques, including fluid inclusion studies, isotopic testing, rock AE experiments (U-Th)/He thermochronology, and AFT analysis, etc. This combined approach aids in accurately determining the tectonic fracture’s genesis and its geological time. Future research endeavors should refine this framework, with an emphasis on enhanced geochemical profiling of fracture fillings

Supercritical adsorption of CO2 and CH4 on shales and other porous media

Low natural gas recovery factors from shale reservoirs have stimulated interest in Enhanced Shale Gas Recovery (ESGR) using CO2 injection. This process seeks to exploit the preferential adsorption of CO2 in shale's nanometric pores, so as to enhance desorption of CH4 and to promote geological sequestration of CO2. To facilitate the design of this process, an integrated experimental and modelling workflow was developed and deployed on shale samples from the Longmaxi (China), Marcellus (USA) and Bowland (UK) formations to achieve the following: (i) high-resolution textural characterisation, (ii) supercritical adsorption measurements with CO2 and CH4, and (iii) their description by a novel mathematical model that predicts adsorption in chemically and morphologically heterogeneous materials. The results show that CO2 adsorbs more than CH4 at all pressures (2-3 times) and that both adsorption capacities and textural properties are strongly influenced by the shale mineralogy. The model developed in this work is based on the lattice Density Functional Theory and describes adsorption systems featuring both slit and cylindrical pores and accounts for the presence of energetically distinct organic- and clay-rich pore surfaces. The workflow was calibrated on three model adsorbents (mesoporous carbon, microporous activated carbon and micro/mesoporous zeolite) to reveal the distinct pore- filling mechanisms in micro- and meso-pores. The use of these model materials enabled the creation of a predictive modelling approach for the description of shale adsorption data, which only requires knowledge of the shale's composition. An equilibrium-based ESGR proxy reservoir model was also developed and demonstrated that a cyclic CO2 injection operation, which includes a so-called soaking stage, may be required to achieve sufficient recovery and secure CO2 storage. The practical workflow presented in this thesis can be used to quantify accurately the Gas-in-Place and CO2 storage potential of shale reservoirs at subsurface conditions and design an optimal CO2-ESGR process.Open Acces