A laboratory study of microcracks variations in shale induced by temperature change

Abstract

When fracturing in shale gas reservoirs, the temperature distribution will change sharply due to the injection of a large amount of fracturing fluid. And then the temperature returns to the original formation temperature during shut-in period after fracturing. To investigate the influence of temperature variation, hydration, and temperature change rate, variations of microcracks were characterized by 3 sets of experiments. To simulate the fracturing and soaking process, another one set of experiments were conducted under the coupling effect of hydration and temperature. With digital image processing techniques, the change of cracks can be visualized and quantitatively analyzed. The reconstructed 3D cracks images show that temperature variation significantly affects the microstructure of shale. An interesting result was found that crack extension was not induced by hydration, but drying. So there is a possible additional effect of temperature on hydration experiments, which was neglected in previous hydration or spontaneous imbibition experimental studies. The comparison of gradual heating and rapid cooling shows that crack extension is also dominated by temperature change rate. The distribution variation of cracks and quantitative analysis show that the total area of crack walls could further increase after heating in the deionized water. A combination of hydration and heating is more likely to lead to the extension of cracks than individual effect. Based on experimental results, two suggestions were proposed to guide the fracturing and soaking process: (1) Increase the temperature difference between fracturing fluid and formation shale. (2) Shut-in and soaking for a period after fracturing.