Research On Risk Assessment Method For Deep Shaft Structure Design

Deep well drilling is facing with the complex geological conditions, the formation pressure is difficult to predict in the exploration phase, especially during in the early stage of exploration. On the consideration of costs and benefits, it’s impossible for well structure to pack each layer, Well bore structure design methods have limitations. These factors will cause drilling accidents, such as lost, overflow, collapse and sticking. The error of formation pressure is bigger; the likelihood of complex and accident appearance in drilling is greater. This will lead to increase the drilling cost and drilling cycle, reduce drilling efficiency. As a result, it will greatly restrict the drilling operation smoothly.To this end, this paper adopted a combination of theoretical analysis and numerical calculation, and established the deep well’s well structure design risk assessment model, by using deep well casing program design data, geological data, drilling data, and considering different levels of error of formation pressure. The risk assessment model can implement the quantitative evaluation of deep well’s well structure design, at the same time, it can provide the reference for optimizing the well structure when selecting the depth of casing and the density of drilling fluid.Based on the well bore structure design method, this paper first analyzed the uncertainty sources of pore pressure and fracture pressure and collapse pressure in deep well drilling, as well as the effects of angle, hole size, drilling depth for design parameters. The scope of the error can be calculated by using contrast method and molecular diffusion theory in the early stage of the exploration, using Monte Carlo simulation, formula of Taylor expansion in the later stage of the exploration.On the basis of uncertain source analysis and calculation of formation pressure and design parameters, how the pressure uncertainty influenced casing deep position and casing program design level limitations, formation lithology, safety, drilling fluid density window of drilling risk influenced the drilling risk have been analyzed, for the purpose of providing theoretical support for deep well casing design.The fuzzy membership function evaluation method has been used for classifying the possibility of accident and complex in drilling process as in [0,1]. Lost, overflow, collapse and sticking risk have been evaluated by the method. And the model of deep well casing program design risk evaluation has been established, which can obtain quantitative drilling risk under different degree of formation pressure error. According to the level of evaluation results, whether optimizing the well structure or not should decide. At the same time, it could provide the reference for choosing drilling fluid density when optimizing the well structure.On the basis of theoretical study, Visual Studio2010 and database were used to write the deep well structure design risk evaluation software. The software has been tested in two drilled wells in JLS structure——L16 well and L004-X1 well. It had also been tested in two drilled wells in NY structure——DS1 well and DF1 well. The results are consistent with the implementation, which verified the correctness of the model. According to the condition of higher risk for LI6 well, the suggestion of increasing a casing and adjust the density of drilling fluid has been throw out. The total risk value is 21.51% after optimizing, which down 32.12% comparing with the initial well structure.The present study has complemented the well structure design calculation method of risk assessment, realized well structure risk assessment under the condition of different degree of pressure error, provided the theoretical basis for the optimization of deep well casing program and site decision-making.