Study On Creep Characteristics And Wellbore Stability Of Directional Well In Yangtak Salt Gypsum Formation

The geological conditions of the salt gypsum layer in the Yangtak area of Tarim Oilfield are complex.Due to structural compression,many sets of salt gypsum layers are developed vertically.The mineral composition of the salt gypsum layer is complex,and the mechanical behavior of rock varies greatly.In the drilling process,accidents such as shrinkage,leakage,and stuck occur frequently.It isn’t easy to run the cementing casing,resulting in a long drilling cycle.In some regions,the vertical distance between the salt gypsum layers and the target layer is short.When the directional well is used to drill through salt gypsum layers,it isn’t easy to control directional trajectory,expand the directional well section,and maintain drilling fluid density.This is mainly due to the unclear understanding of the creep mechanism of the salt gypsum layer in the Yangtake area,and it is difficult to establish a suitable wellbore stability model.Therefore,it is urgent to study the creep mechanism of salt gypsum layers under vertical and curved borehole conditions in the Yangtake area.The creep experiments and numerical simulations of salt-gypsum rock under different temperature and pressure coupling conditions are carried out to solve the above engineering problems.The main research work of this paper is as follows:(1)Based on XRD mineral composition analysis and physical and chemical properties experiment,the mineral composition distribution law of salt gypsum rock in the Yangtake area are obtained.The uniaxial and triaxial compression tests were carried out combined with gypsum rock to obtain the mechanical parameters such as elastic modulus and Poisson’s ratio of Yangtake gypsum rock,which provide basic parameters for establishing the geological mechanics model.(2)Aiming at the creep shrinkage of the gypsum layer in the Yangtake area,laboratory quasi-triaxial short-term creep test experiments were carried out on gypsum rock.By studying the creep mechanism of gypsum rock under low temperature and low confining pressure,it is revealed that the creep rate of gypsum rock is mainly affected by deviatoric stress and confining pressure under low temperature.By analyzing the creep characteristics of salt gypsum rock under high temperature and high confining pressure,it is clear that temperature and partial stress play a dominant role in the creep of salt gypsum rock.The influence of temperature on the creep of salt gypsum rock is much more significant than that of confining pressure.According to the creep experimental results,the creep constitutive model of salt gypsum rock in the Yangtak area is established.The creep parameters in the Yangtak area are obtained by nonlinear regression fitting the experimental results with the least square method,which provided the basis for the establishment of the geomechanical model.(3)A three-dimensional geomechanical model of composite salt gypsum rock creep shrinkage in the Yangtak area is established.The finite element method analyzes the creep mechanism of borehole wall rock under different well types and well inclination angles in the composite salt gypsum layer,and the borehole trajectory is optimized.The drilling fluid density and shrinkage rate charts are plotted according to the wellbore shrinkage law under different drilling fluid densities.The safe drilling fluid density window and drilling parameters were optimized.The problems of difficult control of directional well trajectory and difficult control of drilling fluid density in the Yangtak area are solved,which provides a theoretical basis for the optimization design of well wall stability in the salt gypsum layer in the Yangtak area.