Experimental Study On Thermomechanical Coupling Mechanism Of Hard Brittle Shale

Shale gas of China reserves rank first in the world,and the country attaches great importance to shale gas development.Shale gas reservoirs are usually relatively dense,and hydraulic fracturing is often required to transform the reservoir before exploitation.The flowback rate of hydraulic fracturing is low,and a large amount of fracturing fluid remains in the reservoir,resulting in hydration and expansion of shale gas reservoir rocks,the original pore compression becomes smaller,weakening the hydraulic fracturing effect.In order to solve the above problems,a method of heat treatment of hydrated reservoir after fracturing is proposed.The physical and mechanical properties of mud shale under different pretreatments such as hydration heating are tested through thermal disintegration experiment,pore-penetration measurement,splitting experiment and uniaxial compression experiment,and the change mechanism is analyzed;and by establishing mathematical modulus,the the temperature is related to the cracking of water-containing pores and cracks in the rock,and it is inferred that the influence of rock temperature and pore shape on the stress state of the water-containing pores and cracks is inferred.The results achieved are as follows:(1)Hydration will affect the thermal collapse of shale.The longer the hydration time,the lower the thermal collapse temperature of the shale,and the more fragmented products will be.When the hydration time is large enough,Fluctuate temperature is about 370℃.(2)After the hydration heat treatment of mud shale,the porosity and tensile strength have been changed.With the increase of heating temperature,duration of constant temperature,heating rate and hydration time,the porosity of mud shale is increased steadily,and the permeability has been changed abruptly following the percolation model.During the mutation,the corresponding porosity is between 9.21%-9.78%,and the tensile strength is steadily decreased,and the shale porosity is inversely related to the tensile strength.(3)Compared with the original mud shale,the hydration mud shale has lower elastic modulus,yield limit and compressive strength.The heat treatment hydration mud shale has a slightly lower compressive strength than the hydration mud shale.The elastic modulus and yield limit have been increased,but stell less than the original shale.(4)A prediction model of porosity after heat treatment of water-containing rock is established,through which the porosity after heat treatment of water-bearing rock can be calculated quickly and easily.Analysis of the model shows that the stress intensity factor at the end of high-temperature water-containing cracks has a positive correlation with temperature and is proportional to the root of the crack length.The model is extensible,and the effects of constant temperature duration and heating rate can be extended to the model to predict the porosity.