Study On Mechanical And Microscopic Properties Of Gritstone Under High Temperature And High Pressure Of Water

The problem of geothermal tail water reinjection in sandstone has always restricted the development and utilization of geothermal resources.The utilization of geothermal resources involves the exploitation of groundwater resources,which will destroy the original stress state of underground rock mass.Therefore,it needs to meet the principle of fixed production and reinjection.When the water and heat consumption is completed,the tail water needs to be reinjected into the deep rock mass.The reinjection of geothermal tail water can not only increase the water level of geothermal wells,but also further extend the service life of geothermal fields.But at the same time,the environment of underground rock changes,which will inevitably lead to the change of rock mechanical properties,resulting in the use of deep geothermal resources is limited by the problem of tail water reinjection.Especially when the rock body of the geothermal system is gritstone,it is more difficult to implement geothermal tail water reinjection.So the continuous development and utilization of gritstone geothermal system has great challenges,so it is of great significance to further study the properties of gritstone.Geothermal tail water reinjection can not only increase the water level of geothermal wells,but also further extend the service life of geothermal fields.However,in the development of geothermal resources,the environment of gritstone is bound to change,and the change of temperature and water pressure will affect the physical and mechanical properties of rock and the structure of rock mass.Therefore,it is necessary to understand the mechanical properties of gritstone under high temperature and high pressure water environment,so as to improve the overall efficiency and flexibility of the whole system,which is of great significance to the development of geothermal resources.When the local hot bed is gritstone,the hydrothermal environment of gritstone will change in the whole process of geothermal development.At first,the gritstone is in the environment of high temperature and high pressure water.With the exploitation of geothermal water,the change of the surrounding temperature and water pressure of gritstone has an i MPact on the physical and mechanical properties of rock and the structure of rock mass.Therefore,it is necessary to understand the mechanical properties of gritstone under high temperature and high pressure water environment,so as to improve the overall efficiency and flexibility of the whole gritstone geothermal water storage system,which is of great significance for the development of gritstone geothermal resources.The authors highly appreciate the the Research Fund for Talents of Guizhou University(Grant No.2019.01)and Special Research Funds of Guizhou University(Grant No.2019.03).Taking the samples of coarse sandstone in the inland strata of Jiangsu Province as the research object.two kinds of standard cylindrical gritstone samples(φ 50 mm × 100 mm and φ 50 mm × 20mm)were prepared,which were treated with high temperature and high pressure water.Then,the triaxial compression seepage experiment and Brazil split experiment were carried out.The main purpose of this paper is to discuss the law and reason of the change of the mechanical strength of gritstone under the condition of high temperature and high pressure water.The main research contents are as follows:(1)The influence of reservoir environment change on physical and mechanical properties of gritstone is simulated under hydrothermal condition.The samples were treated by different high temperature and high pressure water heat treatment and cooling treatment.The conventional triaxial compression seepage experiment was carried out on the sample of size 1(φ 50 mm × 100mm).The results show that in the hydrothermal treatment group,when the treatment temperature is the same,the peak strength and elastic modulus of gritstone sample decrease with the increase of water pressure,and the permeability of gritstone increases with the increase of water pressure.In the cooling treatment group,the peak compressive strength of 50 MPa water-cooled sample is the smallest,the modulus of elasticity of the direct water-cooled sample is the largest,and the permeability coefficient of the sample has little change.(2)The Brazilian splitting test was carried out on the specimen of size two(φ 50mm× 20mm).The results show that the load displacement curves of gritstone specimens under different water pressure and cooling treatment all go through four stages:densification,elasticity,yield and failure.In the hydrothermal group,with the increase of water pressure,the densification stage of the sample is prolonged,but the influence on the strength and Young’s modulus of the sample is reduced,and the static splitting tensile strength also decreases with the increase of water pressure.In the cooling group,the tensile strength of WP2 is the lowest.Using digital image correlation method,it is found that the failure modes of gritstone samples in hydrothermal group and cooling group are the same.The maximum principal strains are first concentrated at the bottom or top of the gritstone specimen at the beginning of the crack.The small jump of local strain means the initiation and propagation of crack,while the fracture of strain gauge leads to the sudden change of strain..(3)Scanning electron microscopy(SEM)was used to analyze the microscopic characteristics of gritstone samples,revealing the propagation of microcracks in gritstone under the action of high temperature and high pressure water.It was found that the microcracks of gritstone samples grew and widened with the increase of water pressure in hydrothermal group,while in cooling,the length and width of cracks of samples cooled directly by water decreased,while in 50 MPa water cooled group,the microcracks grew and widened with the increase of water pressure The width and length of crack increase sharply.Some representative samples were selected for X-ray diffraction experiment,and it was found that the main mineral components of gritstone samples were K-feldspar,albite,calcium feldspar,quartz,muscovite,limestone.After high temperature and high pressure water treatment,the content of mineral components changed obviously.