基于D-P准则的天然气水合物井壁稳定性研究

Research on Gas Hydrate Wellbore Stability Based on D-P Criterion

钻井过程中,钻井液对水合物地层产生热交换作用,同时钻井液侵入会影响水合物稳定,两者均引起井周应力场变化,导致井壁坍塌等复杂情况,严重制约着天然气水合物资源高效开发。基于线性热弹性多孔介质理论建立了热流固耦合的水合物斜井井周应力模型,引入欧拉变换考虑三维地应力大小和方向的随机性,采用D-P准则强化中间主应力对水合物井壁稳定的影响,并开展了井斜角/方位角、水合物饱和度、井壁渗透性以及钻井液温度等因素对水合物井壁稳定的研究。研究结果表明:井斜角对井壁的稳定性具有更大的影响,井斜角每增加30°坍塌压力变化0.155 MPa,方位角每增加30°坍塌压力变化0.112 MPa。水合物从高到低饱和度的分解过程,前期地层强度缓慢降低,后期快速降低。水合物地层的有效孔隙度增加使得钻井液侵入量增加,致使地层有效应力降低,导致地层的变形和破裂,增加井壁失稳风险。水合物地层对高于相平衡温度(285 K)的钻井液较为敏感,每升高1 K坍塌压力当量密度约增加0.0069 g/cm^(3),相反则表现较为迟钝。在进行钻井作业时,选用合适的高抑制性、低温钻井液有助于控制水合物的分解以及减少钻井液侵入,进而降低水合物井壁失稳风险。研究结果可为天然气水合物井壁稳定性研究提供参考。

During drilling operations,drilling fluid generates heat exchange with hydrate formation,and the invasion of drilling fluid affects the stability of the hydrate.Both situations cause changes in the stress field around the well,leading to complexities such as wellbore collapse,which restrict the efficient development of gas hydrate resources.In the paper,a thermo-fluid-solid coupling circumferential stress model of deviated hydrate well was built according to the theory of linear thermoelastic porous media.Then,by introducing the Euler's transformation and considering the random magnitudes and directions of 3D in-situ stress,the D-P criterion was used to strengthen the influence of intermediate principal stress on hydrate wellbore stability,and understand the influences of the factors such as borehole inclination/azimuth,hydrate saturation,sidewall permeability and drilling fluid temperature on hydrate wellbore stability.The results show that the inclination has a greater influence on the wellbore stability.For every 30°increase in inclination,the collapse pressure changes by 0.155 MPa;for every 30°increase in azimuth,the collapse pressure changes by 0.112 MPa.In the decomposition process of hydrate from high to low saturation,the formation strength slowly decreases in the early stage and rapidly decreases in the later stage.The increase in effective porosity of hydrate formation leads to an increase in drilling fluid invasion,resulting in a decrease in effective stress of the formation,which further leas to the deformation and fracturing of the formation and an increase in the risk of wellbore instability.Hydrate formation is more sensitive to drilling fluids above the phase equilibrium temperature(285 K).For every 1 K increase,the equivalent density of collapse pressure is increased by about 0.0069 g/cm^(3);conversely,hydrate formation is rather dull to drilling fluids.At the time of drilling,the selection of appropriate high inhibitory and low-temperature drilling fluids can help control the decomposition of hydrates and reduce drilling fluid invasion,thereby reducing the risk of hydrate wellbore instability.The research results provide reference for investigating gas hydrate wellbore stability.