STUDY OF THE INTERACTION BETWEEN MICROSTRUCTURE AND MACROSCOPIC BEHAVIOR FOR HYDRAULIC FRACTURING AREAS DURING SHALE GAS EXTRACTION

Abstract

Horizontal drilling is the major method of shale gas extraction, and hydraulic fracturing improves the gas production effectively. However, as the main gas migration channel, shale microstructures, such as the natural fractures and pores, are extremely complicated, and it is difficult to characterize them quantitatively by conventional approaches. Besides, the microstructural evolution of the hydraulic fracture (HF) area is more intricate. In order to quantitatively characterize the evolution of shale microstructure after hydraulic fracturing, and to investigate the contribution to the gas extraction process, a fully coupled fractal thermal–hydrological–mechanical permeability model is proposed. And the interaction of shale microstructure, thermal–hydrological–mechanical effects, and gas permeability in the HF area is also quantitatively investigated. Shale thermal conduction, gas pressure, and adsorption–desorption effect are defined as functions of the effective stress responsible for shale matrix deformation, and operate directly on shale porosity, which induces changes in shale microstructure, including the shale fractal dimension (for characterizing the density of natural fractures in shale) and the maximum fracture length. Multiphysical effects have a major impact on gas seepage, and directly affecting the permeability. Furthermore, compared to the widely used cubic permeability model, this model is significantly superior in analyzing the permeability evolution in the hydraulically fractured area, and the reliability is verified by the production data from the Marcellus Shale.