Experimental Study On Glauberite With Water/Saturated Brine Fracturing

Glauberite has huge reserves and stable resources.It has a tendency to gradually replace glauberite and become an important source of industrial raw material sodium sulfate.Due to the problems of high labor intensity,low mining efficiency and serious environmental pollution in traditional mining methods(dry mining and chamber water solution mining),many scholars put forward in-situ solution leaching and fluid mining to mine glauberite.At present,the research on in-situ leaching and fluid mining of glauberite salt mine mainly focuses on the later water solution mining,and the research on the hydraulic fracturing process in the earlier stage is still very lacking.Based on this,this paper studies the mechanism of crack propagation and dissolution under the hydraulic fracturing of glauberite,provides reference for the fluid mining of glauberite seam,enriches the hydraulic fracturing theory,and carries out the mechanical characteristics test of glauberite respectively,as well as three stress conditions of axial pressure / confining pressure of 5/4 MPa,7/4 MPa and 9/4MPa,Hydraulic fracturing tests were carried out in two kinds of fracturing media: fresh water and saturated salt water,and the fracture morphology of the failure end face was scanned by3 D morphology scanner.The test data were calculated and analyzed by combining the characterization formula of crack roughness and the characterization method of internal geometric characteristics of fracture.The main conclusions are as follows(1)The compressive strength of glauberite rock sample(Pengshan glauberite mine,Sichuan)is 11.43-24.90 MPa,with an average of 16.86 MPa;The elastic modulus is2.10-3.45 GPa,with an average of 2.76 GPa.The density is 2.71-2.77 g/cm~3,74 g / cm~3;The Poisson’s ratio is 0.23-0.32,with an average of 0.26.The tensile strength ranges from 1.62 MPa to 2.59 MPa,with an average of 2.08 MPa.The internal friction angle is 23.44°,The cohesive force is 4.28 MPa.(2)Through the analysis of Glauber’s salt hydraulic fracturing process,the governing equation is introduced to describe it.Based on the governing equations,initial values and boundary conditions,a coupled mathematical model of hydrofracturing crack growth and dissolution for glauberite is established.(3)The fracturing curve of glauberite is divided into three stages: water pressure increasing stage,crack propagation stage and water pressure decreasing stage.The main difference between the two fracturing media is that: during the process of Glauber’s salt fracturing by clean water,there are many times of crack initiation,and the crack propagation process is accompanied by dissolution phenomenon,so in the crack propagation stage,the pressure fluctuates violently and lasts for a long time(370 s);In the process of Glauber’s salt cracking,the number of crack initiation is less,which tends to complete the crack initiation at one time.At the same time,the concentration of saturated salt water is higher,and there is almost no dissolution phenomenon in the crack propagation process,so the pressure fluctuation is small and the duration is short(250 s).(4)When the confining pressure is kept at 4MPa and the axial pressure is 5 MPa,7 MPa and 9Mpa,the crack initiation pressure of glauberite is 10.34 MPa,11.68 MPa and 12.73 MPa respectively;The crack initiation pressures of glauberite induced by saturated brine are10.71 MPa,11.96 MPa and 13.01 MPa respectively.The crack initiation pressure caused by saturated salt water is 3.5%,2.3% and 2.2% higher than that caused by clean water,respectively.The crack initiation pressure of glauberite caused by saturated salt water is higher.(5)Under the same in-situ stress condition,the crack propagation direction of glauberite is the same.When the confining pressure is kept at 4 MPa,with the increase of axial pressure,the crack propagation changes from parallel to vertical(parallel to the wellbore): when the axial pressure is 5 MPa or 7 MPa,the water / saturated salt water induced Glauber’s salt forms a transverse crack;when the axial pressure increases to 9 Mpa,the water / saturated salt water induced Glauber’s salt forms a longitudinal crack(with a certain angle).(6)The hydraulic fracturing process of glauberite is affected by both stress and dissolution.Under different stress states,the confining pressure remains unchanged at 4 MPa.With the increase of axial pressure,the roughness of crack end face increases,the crack opening increases,and the fracture contact area decreases.Under the same stress state,the roughness value of crack end face caused by glauberite water is greater than that caused by saturated salt water.The crack opening caused by fresh water is larger than that caused by saturated salt water,and the contact area is smaller than that caused by saturated salt water.