Numerical Modeling And Optimum Engineering Design For Hole Shrinkage In Borehole Drilling At Salt And Gypsum Layers

Salt and gypsum layers are mainly consist of salt and gypsum. In salt basins, salt and gypsum layers are frequently confronted in drilling for oil and natural gas. Drilling into salt and gypsum layers(especially deep salt and gypsum layers) is a worldwide difficult problem. Serious hole shrinkage often occurs during drilling, which leads to the instability of borehole, the drill gets stuck and the annular tubes be pressed to break, which brings huge economic loss to the drilling. To explore oil and natural gas under salt and gypsum layers, and to drill through the thick salt and gypsum layers and make the borehole be stable for a relatively long period, we need to urgently solve this significant problem for drilling engineering.Salt and gypsum layers are rheological. The creep characteristics is especially obvious under high temperature and high pressure in deep for salt and gypsum layers. Being different from sandstone, shale and mudstone, the elasticity modulus of rheological layers is very small. They can not resist shear forces, large and continuous creep deformation may occur even under low horizontal stresses. When the salt and gypsum layer is drilled through in the drilling process, the relative balance of stresses in the primary layers, comparatively strong stress concentration is produced in the wall rock of the hole. Under the horizontal press, the salt and gypsum layers creep and deformed, and then, they drift and flow into the hole and finally leads to hole shrinkage and hole instability.Numerical modeling by computer is a important method to study hole shrinkage and hole instability in drilling through salt and gypsum layers, from which we can reproduce the creep and deformation process of salt and gypsum layers with time during drilling. For drilling process, time is important. By numerical modeling, we can unfold the temporal and spatial deformation process of hole shrinkage of salt and gypsum layers during drilling. So, foreigners have paid much attention to numerical modeling. Especially in recent ten years, based on study for the laws of rheology of salt and gypsum layers, large progresses have been achieved in the study of the deformation of salt and gypsum layers.Making quantitative study and numerical modeling for the entire process of creep velocity, spatial displacement and hole shrinkage of salt and gypsum layers is the key to successfully making design of drilling at salt and gypsum layers and to prevent and manage accidents in deep hole. Based on tri-axial creep experiments of core specimen, the constitutive law for salt and gypsum layers is established in this paper. Based on layers, structures and spatial distribution and attitude of salt and gypsum layers, the engineering model, geological model and numerical model of drilling hole are established. Numerical modeling for the entire process of creep velocity, displacement and hole shrinkage of salt and gypsum layers in drilling is carried out. The relationship between the intensity of hole liquid and the hole shrinkage of salt and gypsum layers is studied, and an optimum basis is provided for the design of drilling engineering at salt and gypsum layers.