Research On Borehole Collapse Pressure In Elliptical Wellbore

The wellbore instability is always a main problem for drilling engineering,a large number experience and theory research had been carried out by researcher at home and abroad for this problem.However,the most analysis treat wellbore shape as circle.As we all known,the wellbore shape is not a standard circle when the stress is inhomogeneous and layer is anisotropic,on the contrary,the wellbore shape is more likely an elliptical wellbore rather than a circle wellbore.As a result,elliptical wellbore stability is different from circle wellbore stability.This paper is supported by National Science Foundation(NO:51574202 and 51474186).And the actual work as follow:first,analyzing the reason of elliptical wellbore produced base on logging date and finite element;second,establishing vertical and direct elliptical wellbore collapse pressure model,and systematically analyzing the effect law of wellbore shape,rock mechanic parameter,well track and weak plane attitude;Thirdly,the collapse pressure model of elliptical wellbore under the joint influence of weak plane and anisotropic seepage is established to find out the influence factors of weak surface effect and seepage to collapse pressures.Finally,field data is used to verify the validity of the model.The result are as follows:(1)The finite element software analysis results show that an elliptic wellbore is formed under the effect of non-uniform stress,and the ratio of the ratio and the short axis of elliptic wellbore increases with the increase of the inhomogeneity of ground stress;The diameter measurement results of horizontal wells of the Da Niu Di gas field show a clear irregular wellbore characteristics.(2)Analysis of collapsed pressure model for elliptic wellbore considering the effect of weak plane in vertical wells/directional wells shows that the collapse pressure will increase with the increase of the ratio of major and minor axis of the ellipse;the increase of internal friction angle and internal friction at the weak plane will make the wellbore wall more stable,and the collapse pressure increases with the increase of the dip angle of the weak plane,increases first and then decreases with the increase of the weak plane inclination;the larger the well angle(?b),the more unstable the borehole wall is.For well azimuth angle(?b),the collapse pressure first decreases and then increases when 0°<?b<180°.At the same time,when 180°<?b<360°,the change law is consistent with 0° ?b<180°.(3)The results show that under the conditions of normal fault ground stress,when ?b =135°and 315°,the collapse pressure reaches the maximum value,at the same time,the collapse pressure stays increasing;Under normal fault-sliding ground stress conditions,when ?b =135°and 315°,the collapse pressure reaches the maximum value;Under the conditions of strike-slip stress,when ?b = 105°,285°,the collapse pressure is minimal value,when ?b =45°,225°,the collapse pressure is maximal value.When ?b =135° 315°,and ?b =90°,the collapse pressure reaches the minimal value,and when ?b =45°,225° and ?b =90°,the collapse pressure reaches the maximal value under the strike-slip-retro fault in-situ stress conditions.In the case of reverse faults,the borehole wall is most likely to stabilize when ?b = 45° and ?b = 135°,315°.When?b = 45°,225°,the collapse pressure reaches the maximum.(4)Analytical results of elliptic wellbore collapse pressure model considering horizontal isotropic and anisotropic seepage flow show that both isotropic seepage and anisotropic seepage will aggravate borehole instability,The collapse pressure obtained by anisotropy seepage is greater than that of isotropic seepage;the collapse pressure considering the influence of both weak surface and seepage is greater than that of just considering unilateral factors.(5)The prediction results of elliptical wellbore collapse pressure model established in this paper are in good agreement with the actual drilling conditions of the Anji Haihe Formation in the Hu 2 Well in Xinjiang,which proves the accuracy of this model,and also shows that the elliptical well collapse pressure model is more accurate than that of the circular one.The research results of this paper not only enrich the basic theory of wellbore stability research,but also accord well with the actual conditions of drilling,which has important theoretical significance and application value.