Study On Macro And Meso Damage Characteristics And Stress Evolution Of Surrounding Rock During Oil Storage In Goaf Of Gypsum Mine

Gypsum ore resources in China have large reserves and wide distribution.A large number of mined-out areas are inevitably left after open-pit mining.At the same time,with the steady progress of the "ecological China" strategy,the mined-out areas of gypsum ore have become a major hidden danger in the field of mine disaster and ecological governance.Hence,using gypsum mine goaf to develop crude oil reserve can not only "transform waste into treasure" to control underground space,but also ensure national energy security.However,systematic theoretical support has not yet been formed in this field,let alone a typical demonstration project,and there is still a lack of basic research on how to safely and efficiently utilize the goaf to construct underground oil storage.Therefore,this paper takes Xiaxin Gypsum mine of Huangmei Longyuan Gypsum Co.,Ltd.as the engineering background,the macroscopic mechanical experiments of oil-immersed anhydrite rock were carried out and the deterioration phenomenon of oil-immersed anhydrite rock was revealed.Based on PFC2 D numerical simulation software,the damage mechanism of oil-immersed anhydrite rock is discussed from a microscopic perspective.Next the influence of oil immersion on acoustic emission characteristics of anhydrite rock was investigated,and a damage constitutive model based on cumulative acoustic emission counts was established.Finally,the order and stability of mining and injection for underground crude oil storage in goafs are preliminarily studied.The main research contents and achievements are as follows:(1)The stability of the goaf of the Xiaxin gypsum mine in Huangmei was evaluated,and the feasibility of using the mined-out area for the construction of underground oil storage was preliminarily demonstrated.Combined with the modified Mathews stability diagram method and the limit equilibrium method based on the Molcoulomb yield criterion,the stability of stope roof and pillars under actual mining conditions was calculated respectively.The results show that the stability of goaf is good when the size of room is 12 m,the size of strip pillar is 12 m,and the isolation pillar in the middle is 20 m.(2)The macro and micro damage characteristics of oil-immersed anhydrite rock were revealed by means of laboratory test and numerical simulation.Through the physical and mechanical tests of oil-immersed anhydrite rock,it was found that with the same confining pressure,the compressive strength and elastic modulus decreased with the increase of oil-immersed time.The effects of oil immersion on the physical and mechanical properties of anhydrite rock were quantitatively evaluated by using the normalization function,and a beneficial conclusion was drawn that the higher the confining pressure,the less the influence on the mechanical properties of oil-immersed anhydrite rock.Based on the establishment of particle flow models of anhydrite rock under different oil immersion time,the damage characteristics of anhydrite rock under oil immersion were studied from the perspectives of meso-bond gap,failure stress field and failure displacement field,and the damage mechanism was revealed.(3)The influence of oil immersion on acoustic emission characteristics of anhydrite rock is studied,and a statistical damage constitutive model is established.Through uniaxial and triaxial compression ae tests of anhydrite rock with different oil immersion time,the influence of oil immersion on the AE characteristics of anhydrite rock was analyzed from the aspects of stress-strain curve,ringing count and acoustic emission energy.It was found that "the longer the oil immersion time,the rock sample gradually transferred to plastic body;The larger the confining pressure is,the rock sample gradually transfers to elastomer ".The relationship between damage variables and damage constitutive model of anhydrite sample was established by acoustic emission ringing counting,and the applicability of the constitutive model was verified by nonlinear fitting results.(4)The spatial-temporal evolution characteristics of stress in surrounding rock of goaf under different oil storage modes and long-term oil storage conditions are studied.Through the fluid-structure coupling numerical simulation method,the stress state,deformation characteristics and seepage field changes of the surrounding rock of the mine are compared and studied.It is recommended that the oil storage in goaf should be carried out in the order of "first down and then up in the middle section,and then symmetrical stored from the center to both sides in the same middle".At the same time,after 6000 days operation,the goaf deformation and stress change is still small,and the long-term stability of goaf is good.The research reveal the damage characteristics of anhydrite rock caused by oil immersion and the stress evolution law of surrounding rock during oil storage.By controlling the reserve process and state of anhydrite mined-out area,the final goal of rational utilization of anhydrite mined-out area space is achieved.The results can provide theoretical support for fine treatment and rational utilization of mined-out area in anhydrite mine.