# Study on Elastoplastic Damage Coupling of Reservoir Mudstone Considering Permeability Change
## 1 Introduction  
Mudstone reservoirs are the new key research objectives of oilfield exploration and development. As a source rock solidified by mud, clay and gypsum, the mudstone minerals in the reservoir are microcrystalline, with a complex internal structure and a wide distribution of micropores and fissures and are rich in organic matter [1, 2]. Under the action of deep high ground stress, the micro-pores and fissures in mudstone are easy to expand and penetrate, and the damage evolution and plastic deformation of the structure occur. The mechanical properties deteriorate which leads to a significant decrease in the strength of the reservoir and plays a controlling role in casing damage [3, 4]. Therefore, mudstone is the key research object of oil and gas exploitation engineering in high buried depth environments. Damage evolution is the process of rock micro-defect expansion and coalescence under external load and the environment. It can reflect the degree of the deterioration of rockproperties [5] and is also the basis for the construction of models of damage mechanics. [PERSON] et al. [6] used the strain equivalence principle to consider the coupling relationship between plastic deformation and damage and proposed the thermodynamic model of plastic damage. [PERSON] et al. [7] developed an elastoplastic damage constitutive model with double yield surfaces based on the irreversible thermodynamics theory and damage mechanics theory for saturated soft rock. [PERSON] [8] developed a micro-mechanics-based model to investigate the micro-crack damage mechanism of the four stages of brittle rock under rotation of the principal stress axes. [PERSON] et al. [9] developed a damage-based constitutive formula for an overstress model which can be appropriately applied to the analysis of full dynamic stress-strain curves. [PERSON] et al. [10] presented a constitutive model for describing the non-linear mechanical behavior of different types of intact rock subjected to complex 3D stress states, which is formulated on the basis of a combination of plasticity theory and the theory of damage mechanics along the lines of a damage plastic model for concrete. [PERSON] et al. [11] proposed a damage constitutive model to describe the deformation and strength characteristics of intermittent jointed rocks under cyclic uniaxial compression, which can reflect the coupled damage induced by micro-flaws and macro-joints. [PERSON] et al. [12] developed a damage evolution model with respect to the volumetric strain and a differential elastoplastic constitutive equation based on the relationship between the evolving permeability and porosity and the volumetric strain. [PERSON] et al. [13] described damage evolution using the Kachanov damage theory and obtained the damage evolution equation and the constitutive model of rock mass under multi-stage creep load considering the initial damage. [PERSON] et al. [14] established a continuous damage constitutive model based on the framework of continuum damage mechanics and a strain equivalence hypothesis. The above achievements are mostly for brittle rocks and only consider the impact of elastic or plastic deformation on damage, without considering the coupling effect. The damage evolution of rock structure causes the friction sliding of fractures or joint surfaces, which forms the coupling effect of elastoplastic deformation and damage evolution. Therefore, it is of great theoretical significance to carry out the study of an elastoplastic damage coupling mechanical model. In oil and gas exploration and development engineering, the reservoir will generate stress concentration under outside construction disturbance, and then the reservoir rock will generate elastic-plastic deformation and damage evolution. The mechanical properties of high buried depth reservoirs have high particularity and complexity [15].