Scale-Inclusive Micro-Memory Anomalies of Gas Flow and Diffusion in Naturally Fractured Shale Formations

Abstract:

Gas flow and diffusion in the highly complex pore structure of shale formations take place at different scales. This study focuses on the compound effect of natural fractures network and nonlocal process of gas desorption in the organic matter. These two processes can significantly influence the macroscale response of shale during the migration of gas and result in anomalous pressure behavior. Natural fractures are well recognized and considered as the main factor controlling gas producibility of shales. Complex micro-seismic event patterns associated with hydraulic fracture stimulation are stated as a result of natural fracture reactivation. The multimodal distribution of characterizing factors of natural fractures in shale, such as length and permeability, as well as the low permeable matrix consisting of nanopores and aggregated organic/inorganic material are two differentiating factors that limit the applicability of existing models for fractured formations. Thin fractures and low permeable matrix create a narrow zone around the matrix perturbed by pressure drop in fracture, which is different from the fully perturbed matrix and the linear flow assumption in the conventional fractured models. Dissolved gas diffuses through the organic matter and desorbs from the surface. The diffusion occurs as the classical molecular diffusion through single-phase matter, with very low diffusion coefficient. Gas desorption from the pore surface occurs instantaneously, while gas release from the deep zone of organic matter via diffusion is a slow process. The difference in the characteristic time of these mechanisms results in micro-memroy effects in gas desorption. A comprehensive scale-inclusive modeling of gas migration in naturally fractured shale is studied, which bridges the anomalous behavior of fluid at the nanoscale and the multimodal characteristics of shale at the micro/macro scale. An advanced model for gas desorption from the organic matter surface is considered, which includes gas desorption on the surface and the gas diffused to surface within the organic matter. The proposed study is to explain the diverse flow mechanisms happening at various scales in shales and efficiently addresses the anomalous rate behavior as well as the phenomena of long-range dependence and/or trapping events in shale plays.