WELLBORE STABILITY RESEARCH TO THE LEVEL SECTION OF HORIZONTAL DIRECTIONAL DRILLING

摘要

The problems such as sticking, tight pull when running casing occurred frequently in the horizontal hole during horizontal directional drilling in sandstone reservoir of a certain Bozhong block. Tests and theoretical analysis reveal that due to the characteristics of high porosity, high permeability, weak cementation and low strength, the sandstone reservoirs are subject to deformation under the effects of earth stress. It is the dominant of the wellbore instability which can be prevented by using proper drilling fluid density. This paper is based on a linear elastic and isotropic mechanics for stress distribution around direction wells and wellbore stability model is developed by comparing the caving pressure and fracture pressure which varied with deviation angle and azimuth. Considered the effect of sandstone viscosity-creep to the wellbore stability simultaneously, based on the experimental results of the creep for sandstone in site, the creep constitutive model and creep parameters is given. A numerical calculation of wellbore shrinkage rate is done by using FLAC3D code. From which the wellbore deformation with time of horizontal well be concluded. The conclusion is that the major cause of wellbore instability in horizontal section is the shear failure of sand stone, the effects of sandstone creep isn’t needed to be considered. Synthesized the two factors and fixed the proper drilling fluid density in the horizontal hole. And also make the conclusion that the optimum drilling angle is the direction of the minimal horizontal major stress.