Pollution gas concentration and diffusion model in shale gas flowback fluid
Authors: Cheng Xiao-Xiao , Liu Jian-Guo , Xu Liang , Xu Han-Yang , Jin Ling , Shu Sheng-Quan , Xue Ming ,   Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Science, Hefei 230031, China,   University of Science and Technology of China, Heifei 230026, China,   Safety and  Environmental Protection Technology Research Institute, CNPC, Beijing 102206, China
Published: June 2021

Aiming at the uncertainty of the concentration of the pollution gases and the spread of distribution in the process of shale gas development, the open optical path Fourier Transform infrared spectroscopy (FTIR) measurement system designed and built by ourselves is used to invert the concentration of pollution gases in the backflow, and the size of pollution source is calculated by the flow rate of backflow liquid and the size of pollution source. By combining with the field environment and establishing the reference coordinates, the Gaussian diffusion model is derived mathematically, and the non-point source diffusion model of pollution source is constructed and simulated. The result shows that the source intensity, distance, wind speed and atmospheric stability all affect gas concentration diffusion. The concentration of main pollution gas and the strength of non-point source are obtained by continuously measuring the backflow liquid for 80 h. The result shows that C<sub>3</sub>H<sub>8</sub>, C<sub>5</sub>H<sub>12</sub>, C<sub>3</sub>H<sub>6</sub>, CO and SO<sub>2</sub> are the main pollution gases of the backflow liquid, and their maximum concentrations are 4.689 mg/m<sup>3</sup>, 25.494 mg/m<sup>3</sup>, 30.324 mg/m<sup>3</sup>, 0.656 mg/m<sup>3</sup> and 4.620 mg/m<sup>3</sup>, respectively. The maximum strengths of non-point source are 1.9872 g/s, 10.9750 g/s, 12.8513 g/s, 0.2707 g/s and 1.9064 g/s, respectively. By combining the wind speed and daytime environmental conditions, the atmospheric stability is selected and the source strength is substituted into the non-point source diffusion model, the diffusion concentration of polluted gas is obtained, and the real-time monitoring of the concentration distribution of different polluted gases at different locations is realized. The traditional method is used to collect air samples in the field and determine them in the laboratory. It takes a long time and different sampling methods are affected by the preservation time of samples, etc., so there is a great difference between the measured values in the laboratory and the measured values in the field. The country has issued online monitoring standards for volatile organic compounds (VOCs), but most of them are for the determination of non-methane total hydrocarbon, which cannot realize in situ component analysis, and cannot implement the monitoring of VOC concentration distribution within the region. Compared with traditional measurement methods, the method of FTIR and non-point source diffusion model can not only realize the non-contact, long-distance, online measurement of pollution sources, but also set the dynamic factory boundary of the pollution area and determine the safe distribution area.