A Study On Gas Flow Mechanism And Reservoir Characteristics Of Shale Gas Reservoir

In this paper, we systematically studied China Southern marine Silurian Longmaxi shale gas reservoir characteristics with experiments of pulse decay technique, high pressure mercury injection, liquid nitrogen adsorption and desorption and backscattered electron scanning microscope. Shale gas adsorption and desorption characteristics were obtained through shale gas adsoption and desorption isotherm experiments. Based on the interaction mechanical mechanism of methane molecules and pore wall molecules, shale gas flow mechanism and adsorption mechanism were studied; shale gas matrix desorption-diffusion-seepage coupling experiments were carried out to study shale gas macroscopic flow mechanism; the following key findings were made:(1) Zhao104 well and Qianjingl well both have a very high clay minerals, of which the mixed-layer of illite and montmorillonite and illite is dominant, and the brittleness mineral content is also high; shale porosity is low and mainly in the range of 1.0%-3.0%, and the permeability is mainly distributed in 0.1 mD-1 x 10-7 mD.Gas content has a positive relationship with TOC, the higher TOC and the higher gas content does the shale have. Shale pores can be divided into the following types:clay flocculation pores, intragranular pores, intergranular pores, organic pores and micro-cracks; shale has a relatively wide macropore distribution with a multimodal shape from tens of nanometers to hundreds of nanometers; shale mesopores is mainly between 4-6nm with a slit-like and ink bottle shape; pore volume and the specific surface area of shale were positively correlated with TOC content.(2) Shale adsorption curve and desorption curve is not coincident and the thermodynamic reason why desorption curve is in hysteresis is that the shale adsorption process isosteric heat greater than desorption process isosteric heat. Langmuir model and desorption model can describe the isothermal adsorption and desorption processes very well. Factors that influence shale gas adsorption and desorption isothermal curves are:temperature, moisture, TOC content, thermal maturity Ro and clay mineral content. Based on the interaction mechanical mechanism of methane molecules and shale pore wall molecules, methane occurrence state in shale pores is revealed and shale gas adsorption mechanism is revealed; under the condition of knowing the shale mineral composition, a shale adsorption isotherm curve prediction method which can predict adsorption isotherms at any temperature and pressure using SLD is proposed.(3) Taking gas occurrence state in nanopores and its influence on gas properties into consideration, a mathematical model of shale gas flow in nanoscale pores was established, and the factors affecting gas flow were analyzed. The total gas flow flux in nano-pores is the sum of viscous flow caused by the pressure gradient and Kn diffusion caused by gas concentration. Factors influencing gas flow flux in nano-pores are:temperature, pressure, pore size and pore type.(4) Based on shale gas desorption-Diffusion-seepage coupled physics experiments, shale gas desorption-diffusion-and seepage coupling mathematical model was derived and calculated. Adsorption gas will affect gas production and pressure wave propagation; the gas desorption constants will affect gas production, the greater Vd and pd and the greater gas production; permeability and diffusion coefficient also affects gas flow, the higher the permeability and diffusion coefficient and the greater gas production; shale matrix porosity affects free gas content, the higher porosity, the more free gas content, and the greater gas production.