Damage Mechanism Of Gabbro And Granite Under High-Temperature Water-rock Interaction

Geothermal resources have great potential to reduce the exploitation and utilization of fossil energy and alleviate global environmental problems.In the process of mining and utilization,high temperature water-rock interaction occurs in geothermal strata and surrounding rocks which affects the stability of the underground rock formations and restricts the mining efficiency of resources.Therefore,it is meaningful to study the damage mechanism of rocks after high-temperature water-rock interaction,which is important to improve the utilization efficiency of geothermal resources.In this paper,taking gabbro and granite as the research objects,using laboratory tests as the main means,combined with theoretical analysis methods,the influence of two kinds of rocks under high temperature water-rock interaction was studied.By carrying out analysis tests on the characteristics of changes in rock microstructure and elements in solution,the changing rules of mineral elements dissolved from rock were revealed.Combined with mechanical tests,the change process and damage mechanism of rocks during high-temperature water-rock interaction were clarified.The main conclusions of this paper are as follows.（1）After high temperature water-rock interaction,the total porosity of the gabbro decreases,but the pore structure is always dominated by small pores.The porosity of granite increases,and the pore structure transitions from dominated by mesopores to dominated by macropores.The uniaxial compressive strength of gabbro increases,and starts to fluctuate after reaching the highest strength of 167.77 MPa at 325℃.The uniaxial compressive strength of granite decreases,and the fracture toughness decreased from 0.92 MPa×m^0.5 to 0.50 MPa×m^0.5 before 325℃,and then starts to fluctuate.With the increase of temperature,the fracture mode of granite changed from crack parallelism expansion changes to intersecting development.Part of the gabbro surface was covered with minerals precipitated from the solution,the pyroxene particles were oxidized,and a small number of irregular pits appeared on the surface of the sample.Some small-grained feldspars fall off on the granite surface after 250℃,and obvious dissolution traces appear on the surfaces of feldspar and quartz,and at 325℃,the hornblende on the rear surface shows oxidation marks.（2）Based on inductively coupled plasma mass spectrometry,the concentration of some mineral elements in the solution after high temperature water-rock interaction at different temperatures was studied,and the change law of mineral dissolution elements before and after treatment was revealed.The results show that the concentration of mineral elements in the solution of gabbro and granite increases significantly.In order of concentration,they are Si,Na,K,Ca,Al,Mg,of which the concentrations of Si,Na,and Mg are 600,16,11.4 μg/mL for gabbro;439,34.2,17.4 μg/mL for granite,Si is the most important element lost by the two rocks.The change of element concentration changed the physical properties of the solution.The pH and conductivity of gabbro increases with the increase of temperature,while the granite decreases.The element concentration of gabbro fluctuates greatly at different temperatures,while that of granite fluctuates little,and the concentration of most elements reaches its maximum at about 325℃.（3）Combined with the characteristics of macro and micro changes,the changing process of high-temperature water-rock interaction of gabbro and granite was analyzed,and the mechanism of microstructure and mineral composition changes affecting mechanical strength was expounded.The results show that gabbro and granite are affected by high temperature water-rock interaction in two aspects,one is the dissolution of minerals,under high temperature water environment,the rock surface first appears dissolution,with the increase of time,the solution infiltrates into the rock,the minerals inside the rock begin to dissolve,element water out,the strength decreases.The second is the volume expansion of the mineral heat,squeezing the surrounding minerals,intergranular cracks,when the two crystals squeeze each other,resulting in thermal stress concentration,coupled with dissolution to reduce the strength of mineral particles,the emergence of transgranular cracks,rock strength further reduced.