Macroscopic properties of sandstone are commonly attributed to the degradation of its microstructure during heating treatment processes. However, few previous studies have focused on comprehensive observations on how the microstructure of sandstone changes with temperature. In this study, a kind of sandstone containing quartz, albite, calcite, and laumontite (little), was collected from Linyi (Shandong Province, China) to observe the microstructure degradation changes with temperature by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and thermo-gravimetric analyses (TGA). Firstly, 10 groups of sandstone samples were heated from 25 °C to 900 °C. Then, some core micro-parameters including lattice constant, full width at half maximum (FWHM), micro-strain, dislocation density, TGA curve changes and failure characteristic of the mineral were analyzed comprehensively. Finally, the underlying mechanism causing the microscopic thermal damage at different temperature intervals was also discussed. The results showed that: (1) quartz, the framework component of this sandstone, underwent an α- to β-phase change over the temperature range from 400 °C to 600 °C. This phenomenon caused the lattice constant, micro-strain, dislocation density and TGA curve to decrease sharply during this interval, leading to the microstructure deterioration of sandstone; (2) calcite underwent a decomposition reaction between 600 °C and 800 °C, and resulted in the XRD pattern peak, lattice constant, micro-strain and TGA curve dropping continuously. It destroyed further the internal microstructure of sandstone and produced numerous inter-granular cracks around quartz crystals; (3) further examination found that the decomposition reactions of minerals presented non-synchronized characteristics due to the different sensitivities of minerals to temperature, which led to thermal stress, thermal fracturing of minerals, and thermal reactions happening in different temperature intervals.

Although a multi-stage hydraulically fractured horizontal well in a shale reservoir initially produces gas at a high production rate, this production rate declines rapidly within a short period and the cumulative gas production is only a small fraction (20–30%) of the estimated gas in place. In order to maximize the gas recovery rate (GRR), this study proposes a multi-parameter optimization model for a typical multi-stage hydraulically fractured shale gas horizontal well. This is achieved by combining the response surface methodology (RSM) for the optimization of objective function with a fully coupled hydro-mechanical FEC-DPM for forward computation. The objective function is constructed with seven uncertain parameters ranging from matrix to hydraulic fracture. These parameters are optimized to achieve the GRR maximization in short-term and long-term gas productions, respectively. The key influential factors among these parameters are identified. It is established that the gas recovery rate can be enhanced by 10% in the short-term production and by 60% in the long-term production if the optimized parameters are used. Therefore, combining hydraulic fracturing with an auxiliary method to enhance the gas diffusion in matrix may be an effective alternative method for the economic development of shale gas.

Formation heat treatment (FHT) can be achieved by converting electromagnetic energy into heat energy (that is microwave heating or MWH). Experimental evidence shows that such FHT can significantly enhance oil and gas recovery. As relatively few research studies have been reported on microwave heating enhanced shale gas recovery (MWH-EGR), a fully coupled electromagnetic-thermo-hydro-mechanical (ETHM) model is developed for the MWH-EGR in the present study. In the ETHM model, a thermal-induced gas adsorption model is firstly proposed for shale gas adsorption and fitted by experimental data. This thermal-induced adsorption model considers the increase of matrix pore space due to the desorption of the adsorbed phase. Further, a thermal-induced fracture model in shale matrix is established and fitted by experimental data. Finally, this ETHM model is applied to a fractured shale gas reservoir to simulate gas production. Numerical results indicated that the thermal-induced fracturing and gas desorption make predominant contributions to the evolution of matrix porosity. The MWH can increase cumulative gas production by 44.9% after 31.7 years through promoting gas desorption and matrix diffusion. These outcomes can provide effective insights into shale gas recovery enhancement by microwave assistance.