Analysis Of Coupling Mechanism Between Methane Absorption Characteristics And Shale Structure Evolution Based On Hydraulic Fracturing

Shale gas,as an unconventional energy source which can realize economic development and abundant reserves in the near future,has been paid more and more attention.The country has started a series of key projects on shale gas,and the exploration and development of shale gas are progressively advancing.The Long Maxi formation shale in Guizhou area is well developed and has good hydrocarbon generation conditions and development potential.In this paper,the coupling relationship between the pore and fracture structure evolution characteristics of shale and the methane desorption of shale before and after the hydraulic fracturing is analyzed.The main achievements are as follows: in this paper,the mian1 well in Guizhou area is studied.The original organic carbon content of the measured page 1 is 1.43%,the total organic carbon content is higher,the shale kerogen type is type II and the shale vitrinite reflectance(Ro)is 1.95%-2.29%,with an average of 2.12%,which has a good prospect for exploration and development of shale gas.The main samples of well Mae 1 are quartz,clay minerals and plagioclase,and the acidified shale is reduced by the reaction of acid and carbonate,such as calcite and other mineral components.The pressure of confining pressure,axial pressure and center fracturing pressure are increased alternately by constant pressure,and the pressure of the center hole continues to increase after the loading of the axial pressure and confining pressure to 16 MPa,and the fracture pressure of the untreated page rock is 22 MPa,and the fracture pressure of the page sample after acidification is 44 MPa.Through mercury injection test data,we can see that the apparent density and bulk density of shale in maye 1 well after acidification decreased.As the porosity increases from 1.9722% to 4.9668%,the effect of acid will dissolve the carbonate in the shale,reduce the total volume of shale,and increase the number of holes in the shale.The volume difference between the mercury intake curve and the mercury regression curve is larger,indicating that the shale samples have more open holes and better porosity,which is beneficial to the seepage of gas in shale.Through the experimental data of low temperature liquid nitrogen adsorption,it can be seen that the specific surface area,average pore diameter,unit mass and total pore volume of the shale increased after acidification.The BET surface area and the unit mass total pore volume of the shale samples in well Mae 1 well are relatively large,but the average pore diameter is small and at the nanometer level.It shows that the nanoscale pores are widely developed in the shale samples of well Mae 1 well,which is very favorable for the storage of shale gas.Under the action of hydraulic fracturing,the pore structure changes,the number of internal pores will increase,the porosity is generally increasing,and the porosity of the shale after acidification is also increasing.According to the T2 distribution map,the first and second peaks of the shale samples in well Mai 1 well have appeared,and the first peak is more obvious.It indicates that the proportion of small pores in the rock samples is large,the small pores are mostly,and there are also some large pores.The adsorption capacity of the adsorption curve in the low pressure stage will continue to rise with the increase of pressure.When the adsorption capacity reaches the peak,the adsorption capacity goes into the high pressure stage.The adsorption volume decreases with the increase of pressure,and the desorption curve also has the same law.After shale hydraulic fracturing,the adsorption of methane gas will be reduced.This is because the pore structure of the shale is changed to form a new large size pore.Large size pores,because of the larger pore size,are weak in the binding capacity to the methane molecules,making methane not easy to adsorb on the shale.After the acidizing treatment,the shale porosity will increase.In the process of shale gas exploitation,the appropriate proportion of acid will be added to the fracturing fluid to increase the acid proportion of the fracturing fluid.The seepage capacity of shale is increased by the use of acid to dissolve and corrode the pores of the cemented or rock layers in the shale reservoir and the mineral or blockage in the fissure.Combined with hydraulic fracturing,the increase of porosity will make the porosity of the inner pores in the shale better,and provide the channel for the diffusion and migration of shale gas,which is more conducive to shale gas flow in shale.