Research On The Damage And Fracture Mechanism Of Wellbore Stability

It is a big problem in petroleum industry for the wellbore to lose stability in the process of drilling. It exists everywhere and troubles the petroleum industry: According to a conservative estimate, the losses to the petroleum industry caused by the wellbore instability are computed at $500-600 million. Since drilling process is an underground engineering and correlative with complicated geologic structures, and down-hole accidents caused by wellbore instability are harmful to the drilling engineering, it becomes worldwide and more sophisticated. It receives the attention of petroleum drilling industry at home and abroad. Many engineers in petroleum drilling circles devote themselves to solving the problem persistently.It indicates in the studies of the mechanism of wellbore stability that coupled influence of mechanical and chemistry factors results in the instability of the wellbore, also chemistry aspect may be ascribed to mechanical problem ultimately. Based on the investigation on the collapse stress and fracture stress of the stratum, from the point of view of mechanics the mechanism of wellbore stability is analyzed in present paper according to the theory of geomechanics, elasto-plastics, fracture mechanics and damage mechanics.Firstly, according to the fracture mechanics and microscopic damage mechanics, from the point of view of microscopic damage mechanics, the condition under which micro crack expands is examined, then the expanding direction and the deformation mechanism of brittle rocks are studied. The stress condition around the wellbore is analyzed. Based on the failure criterion of rocks, the computational mechanical model for the collapse stress and fracture stress ofthe stratum is established, where the influence of mechanical features, fracturetoughness ( ^c) , microscopic dimension of the crack on the stresses is given. Another method is used to investigate the instability of the wellbore during drilling.Secondly, after establishing the nonlinear constitutive damage equations, the damage fields around the wellbore are given with the initial damage pressure condition setup by microscopic mechanics. It is verified that the dimension and the stress distribution of damage region are related with the material mechanics feature, micro structure parameter and geometrical size, which gives a new method for the investigation of damage and collapse mechanism and law of the wellbore instability.Thirdly, drilling is a process during which rocks are crushed and wellbore forms. Forming the wellbore disturbs the originally relatively stable stress fields. At the same time, strong stress concentration occurs on the rocks around the wellbore caused by the far-field stress. Reaming and shrinking of the wellbore usually occur on the haliteand plastic mud stratums. According to the deformation magnitude, shrinking of the wellbore belongs to the category of finite deformation. Elasto-plastic finite deformation theory is introduced to analyze the reaming and shrinking of the wellbore, which provides accurate reference for drilling engineering.Fourthly, the mechanics, physics-chemistry, construction techniques mechanism for the wellbore instability is discussed; the relationship between the maximum, the minimal drilling fluid densities and the stress distribution around the wellbore is given before the rocks are crushed, proper actions to avoid the instability are provided on the drilling site. In practical engineering, instabilitycould be prevented and treated duly if theory and practice are combined together according to physical situation on the oil field.The instability of wellbore is a complex problem which needs further investigation. With the more need of energy and the development of petroleum industry, it is urgent to solve the problem. The theory in present paper provides references to avoid wellbore instability.