Research On The Influences Of Anisotropy On Borehole Stability In Bedding Formations

Nowadays,with the increasing depth of drilling engineering,the geological conditions become more complex,especially when the bedding rocks widely exist.Besides that,the high temperature,high in-situ stress and high pore pressure in deep formations make the borehole instability issue very prominent.On the other hand,higher requirements of borehole stability were put forward because of the popularization and application of highly deviated wells,extended reach wells and horizontal wells.Therefore,to deeply explore the key factors affecting the wellbore stability of anisotropic formation and make scientific explanations will provide more accurate theoretical guidance for engineering practice.First,the strength anisotropic wellbore stability model considering the influence of intermediate principal stress is established by combining the rock mass MogiCoulomb failure criterion with the weak plane Mohr-Coulomb failure criterion.Based on the field data,our model is compared with other common models,and it is found that this model we built is more suitable for evaluating the wellbore stability for bedding formation.It is further found that when the density of drilling fluid is high,the radial stress near the wellbore may become the maximum principal stress(the conversion of principal stresses),which results in failure around borehole.At this time,the rock on the wellbore wall is still in the state of compression instead of tension.In order to distinguish the collapse pressure commonly used in engineering,the "upper limit of collapse pressure" is proposed.It is pointed out that the upper limit of the safe mud pressure window in engineering should be the smaller one of the "upper limit of collapse pressure" and the "fracture pressure".Considering the elastic anisotropic characteristics of layered rocks,and using the coordinate transformation relations of material property tensor,stress tensor and direction vector,the inclined wellbore stability model is established.The necessity of considering the elastic anisotropy is clarified by analyzing the stress field and the failure situation near the wellbore.The influence of in-situ stress mechanism is investigated and it is more necessary to consider the elastic anisotropy when the reverse fault mechanism prevails.The reason why the safe mud pressure window can’t be obtained under certain circumstances is explained by analyzing stress changes.Numerical simulation is carried out and compared with the results of the established elastic anisotropic theoretical model.It is found that the numerical solution and analytical solution tend to be consistent when the grid is densified.Considering the coupling characteristics of high pore pressure,fluid flow and rock deformation in deep formation,based on the characteristics of layered rock and the introduction of equivalent permeability coefficient,an anisotropic seepage coupling borehole stability model under the theory of poro-elasticity is established.The stress analysis model of inclined wellbore is established by using specific analysis methods such as load decomposition and superposition.The results show that the permeability around the well is very different in the bedding formation.The anisotropic permeability further changes the collapse area by changing the maximum collapse depth around wellbore.The anisotropic permeability has influences both on the radial and circumferential stresses,and theses influences change with time.For some time and space domain,the collapse pressure calculated by considering anisotropic permeability is obviously larger than that of isotropic condition.Finally,considering the effect of drilling fluid circulation on the temperature distribution around wellbore,a thermal-fluid-solid coupling wellbore stability model with anisotropic permeability and heat transfer is established.Based on the Stehfest method and progressive method,this paper analyzes the influence of anisotropic heat transfer and anisotropic permeability on the wellbore stability in bedding formation.The results show that in a short time scale,ignoring the anisotropy of heat transfer and permeability to choose a lower mud pressure will lead to the collapse of the wellbore wall.In the formation with high temperature,the effect of anisotropic heat transfer on the stress distribution around the well is greater than that of anisotropic permeability.The effect of anisotropic heat transfer on collapse pressure varies with temperature gradient,time and location.