长水平段钻井泥岩井壁坍塌周期分析

Analysis on Time-dependent Wellbore Collapse for Long Horizontal Well in Shale Formation

泥页岩地层井壁失稳是国内外各油气田钻井过程中普遍存在且至今未能完全解决的世界性难题;但在致密泥岩长水平段钻进时,井壁失稳问题更加突出。考虑化学势差、水力压差和岩石骨架变形的耦合作用,同时考虑泥岩吸水引起的强度弱化,提出了泥岩流-固-化多场耦合坍塌周期分析模型。利用该模型,定量分析了泥岩渗透率、吸水扩散吸附系数、钻井液活度和膜效率对地层孔隙压力、地层强度以及坍塌周期的影响规律。结果表明:1泥岩渗透率越小,阻碍水力压力传递的能力越强,地层孔隙压力增长越慢,井眼坍塌周期越长;2钻井液化学作用对井周压力传递和坍塌周期影响显著,当钻井液活度高于地层活度时,井眼附近孔隙压力显著增大,井眼坍塌周期缩短;膜效率越大,则地层孔隙压力和井眼坍塌周期变化幅度越大;3泥岩强度弱化是影响坍塌周期变化的重要因素。随着吸水扩散系数增大,近井地层泥岩强度迅速降低,井眼坍塌周期显著缩短;4研究区域沿最小水平地应力方向水平钻进时,井眼坍塌周期最长,随着与水平最小地应力夹角的不断增大,井眼坍塌周期不断缩短,沿最大水平地应力方向时,井眼坍塌周期最短。

The tight oil and gas are abundant in China and the development potential is great. New plays have been discovered in the Ordos Basin,Bohai Bay Basin,Sichuan Basin and so on. However,the wellbore instability issue is severe during long horizontal drilling in tight shale. Considering the coupled effect of the chemical potential difference,hydraulic pressure difference,rock framework deformation and the strength weakening due to water absorption of shale,a fluid-solid-chemical-coupled multi-filed time-dependent wellbore stability model of shale has been proposed. With this model,the effects of shale permeability,water diffusion and adsorption coefficient,water activity of drilling fluids and membrane efficiency on the pore pressure,rock strength and time-lapsed wellbore stability are quantitatively analyzed. It has been shown that： 1The low-permeability shale has a strong resistance to transmission of hydraulic pressure,which slows down the pore pressure increase and extends the delay time of wellbore collapse. 2 The chemical effect of drilling fluids on pressure transmission around the wellbore and the time-dependent collapse is significant. When the water activity of mud is beyond that of formation water,the nearwellbore pore pressure increases sharply and the wellbore collapse cycle shortens; the change of formation pressure and delay time of collapse grows with increase of membrane efficiency. 3 The strength weakening of shale is an important factor influencing the delayed wellbore collapse. The strength of near-wellbore formation decreases quickly with high water diffusion and adsorption coefficient,which shortens the wellbore stable cycle. 4When the horizontal wellbore is drilled along the direction of the minimum horizontal stress in study area,the time-dependent wellbore collapse is most delayed. As the angle of the minimum horizontal stress increasing,the delay time for the time-dependent wellbore collapse is shortened.