多弱面地层水平井裸眼井壁垮塌量计算模型

Calculation model for bore-hole collapse volume of a horizontal openhole in multiple-weak-plane formation

针对裂缝性地层水平井试油过程中井壁垮塌的问题,建立了多弱面地层水平井裸眼井壁垮塌量计算模型,采用该模型分析了水平井裸眼井壁垮塌量的影响因素,并通过现场应用将其与单弱面模型进行对比分析。分析裂缝产状、井眼方位、地层应力各向异性、弱面数量及井内液体密度对井壁垮塌量的影响后发现:裂缝倾角较小时,井壁垮塌量较大,井壁稳定性差,裂缝倾角较大时,井壁垮塌量波动较大,井壁稳定性随井眼方位及裂缝倾向的变化而变化;裂缝倾向不同垮塌量极大值出现时井眼方位不同;地应力差越大井壁垮塌量越大;井壁垮塌量随井内液体密度的增加而减小,当井内液体密度在孔隙压力当量密度附近时垮塌量对井内液体密度的变化较敏感。现场应用结果表明:与单弱面模型相比,大多数情况下采用多弱面模型计算的井壁垮塌量更大,能更可靠地评估井壁垮塌风险。

In view of borehole collapse in testing of horizontal wells, a calculation model for borehole collapse volume of horizontal open-hole in multiple-weak-plane formation was set up to analyze the factors affecting collapse volume. It was compared with the single-weak-plane model by field application. After analyzing the influences of fracture occurrence, well-bore azimuth, in-situ stress anisotropy, number of weak planes and fluid density on well-bore collapse volume, this study reveals： When the fracture dip is small, the volume of well-bore collapse is big and the well-bore stability is poor; When the fracture dip is big, the collapse volume varies greatly with the change of well-bore azimuth and fracture trend; The maximum collapse volume appears with different well-bore azimuth if fracture trend is different; The greater the in-situ stress difference is, the greater the well-bore collapse volume will be; The collapse volume decreases with the increase of borehole fluid density, and is more sensitive to the change of fluid density when fluid density is near the equivalent density of pore pressure. Field application results show that, compared with the single-weak-plane model, the multiple-weak- plane model gets big collapse volume in most cases so it can assess the collapse risk more reliably.