| Curde oil is the lifeblood of a country’s energy.By the end of 2018,China’s oil external dependence is approaching 70%,and its oil reserves are seriously insufficient.As oil security is facing a severe test,China is in urgent need of establishing a large-scale strategic petroleum reserve system.The underground salt cavern crude oil storage has the advantages of safety,stability,large scale,economy and high efficiency,which is the main form of oil reserves in developed countries.At present,there is no engineering practice of building underground crude oil storage salt cavern in China,and the field of underground salt cavern oil storage is far behind that of developed countries.Therefore,it is urgent to carry out relevant basic tests and theoretical research.The salt deposits in China are mostly lacustrine sedimentary layered structures with complex geological occurrence characteristics,which are different from the marine sedimentary huge thick salt layers or salt domes used in foreign countries.In particular,there are many non-salt intercalations in salt-bearing strata.The permeability and mechanical properties of these intercalations have an important impact on the long-term operation safety of oil storage salt cavern.However,the mechanism of mechanical and permeability evolution of intercalation under the attack of crude oil and brine is still unclear.Therefore,it is of great significance for China to promote the construction of large-scale salt cavern oil storage in China to carry out the analysis of surrounding rock permeability,mechanical characteristics and cavern safety.This paper conducts a pilot study on the oil storage in Jintan Salt Mine,Jiangsu Province,which is a candidate base for national strategic petroleum reserve(SPR).A hierarchical structure evaluation system of site selection and macroscopic suitability construction of salt cavern storage is established.The macroscopic suitability of oil storage in Jintan Salt Mine was evaluated.On the basis of satisfying the macroscopic suitability,the evolution rules of pore characteristics of surrounding rocks in oil storage reservoirs under different operating environments are further studied.The research focuses on the permeability and mechanical properties of the interlayers under oil and water erosion.Finally,combined with theoretical and experimental research results,with the help of numerical simulation measure,the long-term operation stability and tightness of the crude oil salt caverns are comprehensively analyzed.The main research results are as follows:(1)The site selection and macroscopic suitability construction evaluation of the crude oil salt caverns were carried out:Based on the analytic hierarchy process(AHP)theory,a hierarchical structure evaluation system for site selection and macro suitability of salt cavern oil storage was established.A suitability evaluation method for site selection and construction of oil storage in different salt rock structure regions was put forward.The macroscopic suitability of oil storage construction in this region was evaluated according to the macroscopic engineering geological characteristics and the occurrence of rock salt in Jintan Salt Mine.The result shows that the construction suitability of Jintan Salt Mine is P=8.6898,which can be used as the preferred storage site for crude oil storage salt cavern.(2)The microscopic pore structure characteristics of surrounding rock of salt cavern storage were revealed:By means of XRD and SEM,the microscopic physical characteristics of the surrounding rock of the salt cavern were analyzed.By studying the adsorption isotherm characteristics of surrounding rocks under different conditions,the pore structure and morphological characteristics of surrounding rocks were revealed.It was also found that the changes of pore structure caused by water-rock interaction and oil-rock interaction can not change the basic structure of pore.Through the combination of BJH method and mercury injection method,the full pore size distribution characteristics of surrounding rock under different conditions were studied.(3)The pore-permeability characteristics of the surrounding rock and the storage medium seepage law in the interlayers and cap rock were revealed:The pore-permeability characteristics of surrounding rock under different hydrostatic pressures were studied.It was found that the porosity and permeability of the surrounding rock is approximately equal to the value of the porosity and permeability measured when the geostress of the surrounding rock is at its maximum and has nothing to do with the change of internal pressure of the salt cavern.The pore-permeability characteristics of the interlayers and the cap rock after oil and water erosion were studied.It was found that the porosity and permeability of the samples eroded by crude oil decrease with the increase of erosion time,while the porosity and permeability of the samples eroded by brine increase significantly.The seepage model of intercalation and cap rock was established based on the single-phase liquid stable seepage theory.The basic equations characterizing the pressure distribution,seepage velocity and leakage amount of liquid in the interlayers and the cap rock were derived.(4)The degradation mechanism of mechanical properties of interlayers under crude oil and water erosion environment was revealed:The mechanical properties of the interlayer samples eroded by oil and water were studied by uniaxial and triaxial compression tests.The damage evolution law of the interlayers under water-rock interaction and oil-rock interaction was analyzed,and the influence of excess pore fluid pressure on the mechanical strength of the interlayers was discussed.The degradation mechanism of oil-rock interaction and water-rock interaction on the interlayers was analyzed by TAN detection under crude oil erosion and PH detection under brine erosion.It was found that the damage of the interlayers increased with the erosion of crude oil,but the permeability decreased,which was an abnormal characteristic.The reason of this phenomenon was revealed,that is,the adhesion and sealing effect of resins and asphaltenes in crude oil on rock pores.(5)The long-term operation safety analysis of the crude oil storage salt cavern was carried out:The stability of the cavity of Jintan proposed oil storage was analyzed through the evaluation indexes of the displacement and deformation of the surrounding rock,the change of the plastic zone around the cavern and the cavern volume shrinkage rate.Combining the risk factors of cavern instability and the safety risk and cost factors of surface equipment,the optimal initial wellhead pressure P_w of the crude oil storage salt caverns was 6 MPa.Through the evaluation index of seepage range,pore pressure change and seepage volume,the tightness of the crude oil storage salt caverns were analyzed.And the results of seepage calculation were compared with those of gas storage salt caverns.(6)It was verified that the crude oil storage salt caverns under the limit pillar width could meet the requirements of stabilities and tightness at the same time.It means that more crude oil storage salt cavern groups can be built in limited areas,which can significantly improve the utilization rate of salt rock resources and oil reserves and significantly reduce oil storage costs.Meanwhile,it provides a new application for the abandoned salt caverns which cannot meet the requirement of the gas storage salt caverns to try to rebuild used for crude oil storage. |
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