Study on open-hole extended-reach limit model analysis for horizontal drilling in shales

The published extended-reach well (ERW) limit theory can be used to predict how far the horizontal ERW can extend. However, the use of this theory in shale formation remains poorly understood. Therefore, an open-hole extended-reach limit model of horizontal drilling in shales must be established based on the published ERW limit theory and models. First, the published models are modified for convenient practical calculation, and the core of this modified model is to predict the horizontal-section limit of horizontal ERW. The horizontal-section limit and the final open-hole extended-reach limit primarily depend on the annular pressure drop and fracture pressure in shales. Secondly, the characteristic of shale hydration swelling is considered. The stress state of the wellbore wall changes when the hydration stress is treated as a part of the effective stress. Moreover, the hydration stress can also significantly affect the horizontal-section limit and final open-hole extended-reach limit. In addition, a case study is performed, and the results indicate that the horizontal-section limit is 7292 m when there is no hydration stress, whereas it decreases by several thousand meters if the hydration swelling effect is considered. Finally, through the sensitivity analysis of the main factors in the modified model, the following results are obtained: the longer horizontal-section limit can be achieved in conditions with smaller flow behavior index, lower mud density, smaller mud flow rate, and lower temperature; meanwhile, a reasonable eccentricity range is also necessary, which is approximately 30-35 mm in this paper. The model in this paper provides a theoretical guidance for the optimization design method and operating parameters of horizontal ERW in shales to achieve a longer horizontal-section limit, which is significant for the use of shale gas.