Study On The Mechanism For Cooperatively Maintaining Shale Borehole Stability

Shale borehole instability problem is a worldwide problem in drilling engineering.Shale is a multi-field coupling thermodynamic open system.Complicated physical and chemical actions take place between the shale-soil mass and drilling fluid to change the shale mechanical properties and shale borehole stress distribution.Many models were developed to study the influence of drilling fluid on shale borehole stability due to hydration swelling of shale-soil mass.Among the models,many made few attempts to develop a comprehensive constitutive description of swelling at the macroscopic scale.Although the multi-field coupling shale hydration model incorporated constitutive description of swelling,parameters of the model was difficult to measure and cann`t be apllied in drilling engineering.To explain the influence of shale-soil mass hydration on shale borehole instability,research on shale hydration mechanism at the microscale was nessesary.The multi-field coupling model can be used to analyze shale borehole instability problem and the method to measure model parameter should be developed.The main conclusions drawn in this work included,The clay hydration inhibition mechanism for polyamine?PA?and K⁺,NH₄⁺ was studied.Montmorillonite absorbed less water after PA adsorption than that adsorbed K⁺,NH₄⁺.In addition,after PA adsorption,illite absorbed less water while kaolinite absorbed more waterBased on the model developed according to the balance of Helmholtz free energy combined with the first and the second law of thermodynamics for shale,multi-field coupling model was developed.As can be concluded that to reduce shale borehole normal stress decrease was another effective method to cooperatively maintain shale borehole stability for drilling fluid in addition to reducing shale strength decrease.Based on the model above,shale multi-field coupling device was developed to evaluate pore fluid flowing and solute diffusion in shale and sealing barrier of drlling fluid.Based on least square criterion,Monte Carlo random search method was adopted to estimate parameters of fluid flowing and solute diffusion model by Comsol Multiphysics finite element software.Shale swelling displacement was also measured to estimate parameters of mechanical equilibrium equation.The numerical results fitted well with the experimental data and as can be concluded the model and method to estimate model parameters was authentic.Based on the model and method to estimate parameters,shale borehole normal stress decrease was analyzed and as can be concluded the normal stress decrease due to shale-soli hydration and the reduction of normal stress decrease due to existence of sealing barrier should be considered when shale borehole stability was evaluated by analyzing wellbore stress distribution.