Study on the model for predicting maximum allowable measured depth of a horizontal well drilled with underbalanced operation

In order to overcome the disadvantage that the previous model for predicting maximum allowable measured depth (MAMD) of a horizontal well only applies to overbalanced operation, a model to predict the horizontal well's MAMD by underbalanced operation is presented in this study. It is of great significance for obtaining higher horizontal section length as well as the oil and gas production. The prediction model is established based on the dynamic pressure balance of bottom hole. The pore pressure, instead of the leak off pressure or fracture pressure in overbalanced drilling, is taken as the critical point at which horizontal wells must stop extending to maintain the underbalanced state of bottom hole. Moreover, the water-based liquid coupled with the nitrogen is used in the simulation process. Furthermore, a horizontal well of C-A14H in China, drilled by underbalanced operation with aerated mud is analyzed to predict its MAMD, and some optimization control measures are also put forward to obtain higher well's MAMD. Results show that the MAMD can be achieved when the bottom hole pressure reaches the pore pressure of drilled formation. The predicted MAMDs are totally different with different water flow rates and gas flow rates. The well's MAMD first increases and then decreases with the increase in gas injection rate, and there exists the optimal gas flow rate. Its MAMD keep decreases with the increase in water flow rate. However, the underbalanced state will never be satisfied within the allowable range of gas flow rate if the water flow rate is too large. Moreover, there is a minimum gas flow rate under a certain water flow rate, and the underbalanced state cannot be maintained if the gas flow rate is too small. The study also shows that the underbalanced operation does not guarantee the improvement of horizontal well's extension ability, which mainly depends on the relationship between the bottom hole pressure and its corresponding critical point. Therefore, in order to obtain greater MAMD, optimum gas flow rate and lower water flow rate are all necessary. This work provides a practical tool for predicting the horizontal well's MAMD by underbalanced operation with aerated mud and achieving greater MAMD. The study is also significant to obtain good economic benefits of oil and gas development by underbalanced operation. The model established in this paper allows us to understand the well's extension capability before drilling, which is also crucial for avoiding drilling risk in underbalanced conditions.