Effects of fluid seepage on wellbore stability of horizontal wells drilled underbalanced

The combination of underbalanced drilling (UBD) and horizontal wells produces high efficiency. However, UBD in horizontal wells is also risky – a lack of effective support from drilling fluid may lead to wellbore instability; formation fluid that flows from formation to the wellbore applies additional stresses to rocks around the wellbore; the existing stresses from fluid seepage which are usually ignored increase the chances of wellbore collapse. So the aim of this study is to analyze the effects of fluid seepage on wellbore rocks. Based on seepage mechanics and linear elastic theory, a new analytical model that takes fluid seepage into consideration is introduced to examine wellbore circumferential stresses, and then a new wellbore collapse pressure model is derived. When fluid seepage is taken into account, collapse pressure calculated by the new model is higher and more accurate than that calculated by the conventional model, which means that we will have a narrower mud-density window during UBD process. is of wellbore circumferential stresses shows that variation trends of radial stress and tangential stress are the same whether fluid seepage is considered or not, and that the minimum radial stress and the maximum tangential stress are reached at the wellbore wall. However, values of radial stress and tangential stress are greater when fluid seepage is considered. The effects from fluid seepage intensify with the decrease of effective fluid column pressure and diminish with the increase of radial distance. Meanwhile, sensitivity analysis has found that the borehole radius is an influencing factor and that pore pressure, cohesion strength and internal friction angle have certain impact on wellbore stability in both models.