水合物储层水平井钻井井筒-储层耦合模型与井壁稳定性分析

Wellbore-reservoir coupling model and borehole stability analysis of horizontal well drilling in hydrate reservoirs

在深水浅层水合物沉积层中钻探水平井是一项具有挑战性的工作。由于天然气水合物的相平衡特性,钻井液侵入引起的温度和压力扰动容易造成井周水合物分解,增加井壁失稳风险。通过考虑泥饼孔渗的动态变化、水合物相变、储层力学性质的动态变化以及井筒多相管流与储层多相渗流的热质传递耦合,提出了水合物储层钻井井筒-储层耦合模型。通过与实验数据对比表明,该模型能够有效表征水合物储层钻井液侵入引起的井周温度和压力等参数的变化。基于中国南海神狐海域SH2井位钻探数据,分析了水合物储层长水平剖面的钻井液侵入特征及井壁失稳风险。研究结果表明,随水平段长度延长,井筒环空温度和压力增大,钻井液侵入深度和井壁失稳风险增加。水平井井位部署在储层水合物饱和度高的区域有利于缓解钻井液侵入程度,并降低井壁失稳风险。进一步模拟了有无泥饼、不同钻井液入口温度和盐度对水平井趾端井壁稳定性的影响。泥饼在减小水合物饱和度低的区域钻井液侵入深度和井周屈服半径方面效果更明显。钻井液盐度的变化主要影响井周屈服情况,水合物地层钻井过程中为减小井周屈服区域和屈服程度,应采用较低盐度钻井液。在计算条件下,钻井液盐度小于4.2%比较有利于井壁稳定。在相同计算条件下,井位①和井位②趾端发生屈服的临界钻井液入口温度分别为22.39℃和22.24℃,水合物地层钻井过程中为避免井周出现屈服,钻井液入口温度应小于临界屈服温度。

It is challenging to perform horizontal well drilling in deepwater shallow water hydrate sediment.Due to the phase equilibrium characteristics of natural gas hydrates,the temperature and pressure disturbance resulted from drilling fluid invasion can easily cause the decomposition of hydrates around the well,thus increasing the risk of borehole instability.To study this problem,this paper proposes a wellbore-reservoir coupling model.The model considers the dynamic changes in both mud cake porosity and permeability and reservoir mechanical properties,hydrate phase changes,and the heat-mass transfer coupling of wellbore multiphase pipe flow and reservoir multiphase seepage.The comparison with experimental data indicates that the model can effectively characterize the changes in parameters such as temperature and pressure around the well caused by the invasion of drilling fluid in hydrate reservoirs.Based on the drilling data of Well Site SH2 in Shenhu area of South China Sea,the paper analyzes the characteristics of drilling fluid invasion and the risk of borehole instability in the long horizontal section of hydrate reservoirs.The results show that with the extension of the horizontal section,the temperature and pressure in the wellbore annulus gradually go up,and the invasion depth of drilling fluid and the risk of borehole instability increase.The horizontal well location is deployed in the reservoir with high hydrate saturation,which helps alleviate the invasion degree of drilling fluid and reduce the risk of borehole instability.Further,the paper simulates the effect of mud cake,different drilling fluid inlet temperatures and salinities on the borehole stability at the toe of horizontal well.The results demonstrate that mud cake has a more obvious effect on reducing the drilling fluid invasion depth and the yield radius around the well in the low hydrate saturation area.The changes in drilling fluid salinity mainly affect the yield conditions around the well.It is suggested to use the drilling fluid with lower salinity,so as to reduce the yield area and yield degree around the well in the drilling process of hydrate formations.Under the calculation conditions in this study,the salinity of the drilling fluid lower than 4.2%is more conducive to borehole stability.Under the same calculation conditions,the critical drilling fluid inlet temperatures at the toe of well locations①and②are 22.39℃and 22.24℃,respectively.To avoid yield around the well in the drilling process of hydrate formation,the drilling fluid inlet temperature should be lower than the critical yield temperature.