Study on mud weight window prediction model and safety additional value of horizontal well in shales considering effects of Well's extended-reach limit

Safety additional value is always added to the conventional mud weight window (MWW) to ensure the drilling safety in shales. However, the safety additional value is usually an empirical value. Since the horizontal well usually has a long horizontal section in shales, failing to accurately predict the wells MWW and corresponding safety additional value in shales may result in serious drilling hazards. In order to ensure that the horizontal well can reach the designed horizontal-section length in shales, the conventional MWW needs to be modified and the calculated value of safety additional value should also be given. In this paper, considering the effects of horizontal wells extended-reach limit and its horizontal-section limit (HSL) on the wells MWW in shales, the drilling process, tripping process and six constraint conditions are considered simultaneously. Results of the case study indicate that the corresponding safety additional value is calculated instead of using an empirical value. Moreover, the predicted MWW becomes narrower as the designed horizontal-section length is increased. In addition, in order to obtain a wider MWW, tripping velocity should be lower if the economic benefits can be satisfied. Additionally, the drilling pump with higher rated pressure is also necessary. Taking the maximum HSL as the objective function, there exists an optimal drilling fluid density, which is crucial for production in shales. The calculated value of safety additional value and the modified MWW can ensure the horizontal well reach the designed horizontal-section length, obtaining enough production and economic benefits as well as avoiding drilling hazards in shales.