Research On Some Mechanical Problems In The 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. Many complex situations and accidents, such as borehole wall caving, shrinkage, leakage, stickiness and reservoir pollution, are caused by the instability of wellbore in the process of drilling, seriously constraining the exploration and development of oil and gas field. Drilling costs are accounted for 50% to 80% of the total costs of exploration and development in the exploration and development of oil and gas. It is the key of reducing drilling costs to investigate and apply more advanced and applicable drilling technology.Since drilling process is an underground engineering and correlative with complicated geologic structures, it becomes worldwide and more sophisticated for the wellbore to lose stability, and receives the attention of petroleum drilling industry at home and abroad. In this thesis, the mechanism of wellbore stability is investigated from the perspective of mechanics, in geological mechanics, elastic-plastic mechanics, finite deformation elastic-plastic theory, fracture mechanics and damage mechanics.1. The strength (damage) criterion of rock material plays a fundamental role in drilling engineering. The strength criterion provides criteria for the damage of rock material in the case of hard and brittle rock, and is the important basis for further establishing the elastic-plastic constitutive relation of rock material in the case of plastic deformation rock. The strength criteria of rock material are reviewed in this thesis. When the material is homogeneous and isotropic, taking into account the continuity of the process for rock material to deform from elasticity to nonlinearity, it is believed that double parameter strength criterion used can bring the convenience of solving problems, and is reasonable in physical significance as well.2. When the wellbore is drilled, stresses around the wellbore are redistributed, stress concentration is formed around the wellbore. Under the application of external loads, wellbore internal region media enters the stage of plastic deformation because of the elasticity. Formation rock being thought as multi-aperture elastic-plastic medium, the theoretical analysis model of wellbore stability problem is established, considering the affect of pore pressure and seepage application, and the SD effect caused by different tension and compression yield strength of rocks. Self-similar assumption is used, elastic-plastic mechanics analysis is done for the rock around wellbores, and the distributions of stress and displacement are obtained.3. The investigation on the borehole shrinkage problem in rock salt and plastic mudstone formation is one of the problems widely concerned in drilling engineering. From the analysis of deformation order of magnitude and the results observed on site, it is found that the defect is obvious to analyze borehole shrinkage in small deformation theory. Finite deformation minor elastic-plastic theory being used, borehole shrinkage model is established, in order to provide theory reference for experimental determination of rock material, adjustment of drilling fluid density, prevention of wellbore instability.4. Aim at the borehole crush problem of the soft rock around borehole wall, which is produced in the process of finite deformation, the change process of borehole changing from circle shape to ellipse shape is investigated in finite deformation elastic theory, and the mechanics mechanism of borehole crush is discussed.5. There usually are a large number of random-distributed defects (cavities, micro-cracks, etc.) in rock formation. These micro-structures of rocks have fundamental influence on borehole stability. In drilling engineering, mudstone formations are investigated considering as quasi-brittle materials containing micro-cracks. The method of combining microscopic mechanics with fracture mechanics being used in the thesis, the borehole wall collapse and formation damage of borehole wall stability problems are analyzed quantitatively, the range of drilling fluid density is determined.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 thesis provides references to avoid wellbore instability.