Damage Characteristics And Permeability Evolution Mechanism Of Mudstone Under Multi-cycle Alternating Stress

Due to the engineering background of strong injection and production in the actual operation of the gas storage,the sealing capacity evaluation of the cap rock has become the focus of the research object,and the sealing capacity evaluation runs through the site selection evaluation,feasibility analysis and operation monitoring of the gas storage construction.Through a series of investigations on the factors affecting the sealing capacity of the gas storage caprock,the failure mechanism of the sealing capacity was discussed.The failure mechanism of the sealing capacity of the gas storage caprock was generally summarized as follows: 1.Capillary sealing mechanism,breakthrough pressure as a quantitative evaluation of capillary sealing ability index,is the most intuitive evaluation parameter;2.Mechanical sealing mechanism.The root cause of sealing failure of gas storage caprock lies in the geomechanical effect under the coupling action of alternating load and in-situ stress.3.Deformation and failure mechanism under alternating load.The essence of deformation and failure of caprock under alternating action of injection and production is fatigue failure;4.Permeability evolution mechanism under alternating load,under the action of a series of geomechanical effects caused by injection and production alternation,the porosity and permeability of cap rock will change,among which the permeability changes most obviously.As the parameter with the highest correlation with breakthrough pressure,the evolution law of permeability has a significant influence on the sealing capacity of caprock.This study take S gas storage reconstructed from depleted gas reservoir as engineering background,combining the literature research,mathematical statistics,theory analysis,laboratory test and finite element numerical simulation method,focuses on the evolution law of damage,deformation and failure,permeability and breakthrough pressure of cap mudstone,and systematically establishes the sealing evaluation system of gas storage caprock,which provides a scientific basis for site selection and long-term monitoring of gas storage.The main ideas of this study can be summarized as follows:1.The existing damage definition methods and the constitutive models of rock and soil materials considering damage were analyzed.Based on the laboratory tests of actual drilling cores(conventional triaxial tests and graded alternating load tests),a damage constitutive model considering the number of cycles was established.2.The experimental law of permeability evolution of rock and soil materials was analyzed,and a permeability evolution model considering damage was established based on theoretical derivation,and a hydro-mechanical coupling model considering damage was established with damage parameters as intermediate variables.The breakthrough pressure evolution tests of outcropping mudstone and oil well cement under alternating loads were carried out,and a gradual deterioration model of the breakthrough pressure with alternating times was established.3.The parameters that affect the sealing performance of the cover under alternating loads were analyzed statistically.Based on the analytic hierarchy process(AHP),Three,statistical analysis of the influence of cap rock sealing under alternating load of various parameters,based on the analytic hierarchy process(ahp)to establish gas storage caprock sealing evaluation system and the weighted amplitude,further based on the actual grade of relying on the engineering geological condition evaluation,the results show that the gas reservoir caprock quality is good,suitable for reconstruction of gas storage,but should strengthen gas storage in the gas storage operation stages of sealing monitoring.Gas storage caprock sealing evaluation system and the weighted amplitude were established.Based on the actual geological conditions,sealing capacity of the support project was evaluated.The results show that the gas reservoir has good caprock quality and is suitable for rebuilding the gas storage,but the sealing monitoring of the gas storage should be strengthened in the operation stage of the gas storage.4.Based on the actual formation conditions of gas storage S,the finite element numerical simulation at engineering scale was carried out,and the influence of upper injection pressure and reservoir permeability on the deformation and failure of cap rock was discussed through conceptual model.In addition,considering the engineering background of aquifer reconstruction of gas storage,a one-dimensional conceptual model of gas-water two-phase is established,and the influence of the breakthrough pressure and the thickness of the capock on the sealing capacity of the caprock were discussed.