Simulation Of Fracturing Fluid-Methane-Minerals Interactions During Hydraulic Fracturing Of Shale Gas

Shale gas, mainly occurs in the deep crust of shale, is a kind of environmental and efficient energy. Hydraulic fracturing is the main method of the exploitation of shale gas. The fracturing fluid used in the process of hydraulic fracturing contains various ingredients, which may cause pollution to the groundwater aquifer and underground strata.In order to explore the reaction mechanism of fracturing fluid and shale, simple composition of fracturing fluid was prepared. Choose minerals of higher contents in China’s main shale gas producing strata, and use methane gas as the gas phase, indoor reaction experiment of single mineral and mixed mineral was simulated under the condition of high temperature and high pressure reactor. PHREEQC, as a simulation software for temperature, pressure, ionic strength and ionic types, was also used. The conclusions are as follows:Under the conditions of high temperature, high pressure and methane gas phase, methane was not involved in the reaction with single mineral and fluid; due to the properties of supercritical fluid, the dissolution degree of the mineral under the condition of methane gas phase was lower about 10% than that of Nitrogen conditions.Several different mineral were mixed in a certain proportion, and reacted with fracturing fluid under the conditions of high temperature, high pressure and methane gas phase. Calcite occurred not only dissolution reaction, but also acid-base neutralization reaction with the acidic fracturing fluid in liquid phase, and the amount of dissolved calcite was greater than the other three kinds of minerals. When the p H value varied from the initial acidic to nearly neutral, the dissolution reaction of calcite began to slow, so that calcite played the role of controlling p H value in the system; the dissolution degree of montmorillonite was small, but when encountering with water, the contact area was increased after swelling, and when the mineral structure became loose and the force between molecules became weakened, the molecules were more likely to be leached or exchanged. At this time, the dissolved amount of montmorillonite will increase. The dissolution degree of kaolinite and quartz was small, and the concentration contributed by dissolution were low.According to the balanced simulation results of PHREEQC, the dissolution degree of methane were about 1.1 mmol/L, and the p H value of the system were between 9 and 10 after stability. The change of temperature, the concentration of Na Cl and other factors will affect the dissolution of the four kinds of mineral, the higher the temperature, the greater the change of the amount of mineral dissolution; in a certain range, higher concentration of Na Cl leaded to bigger change of mineral dissolution amount. The pressure of methane had little influence on mineral dissolution changes; and in different type of water, the dissolution of minerals was also different due to the common ion effect.