Abstract
Drilling operations is risky due to narrow mud weight windows in deep wells. Different type of drilling events and wellbore instability have encountered frequently including inflow, drilling induced tensile fractures (DITF), losses and connection gas etc. As such to mitigate the problems, a robust pore pressure prediction is necessary with requires an understanding of the origins and distribution of overpressures in the area. The technical research process is divided into three steps: pre-drill pore pressure predication (PPP) modelling, real-time monitoring and post-drill validation. Efforts were made to understand the geological settings and temperature model. A pore pressure predication (PPP) model was built by integrating fully coupled geomechanical with thermodynamics modeling. Real-time monitoring information provides references and guidelines for PPP model optimization. During the post-drill stage, the updated PPP model was used to design a mud weight and casing program for the upcoming wells. The study area is located northwestern China, the deep formations that more than 7000 meters are ultra-high temperature (200-220 deg C). Thermal-related secondary pore pressure generating mechanism may become active leading to higher overpressure and difficulties in prediction. For the case study, an empirical relationship of overpressure impact factors versus temperature of sandstone and mudstone was proposed. An accurate PPP model is generated using available well-scale geomechanical model and overpressure impact factors. With an integrating fully coupled PPP model as foundation, the integrated approach helps to reduce serious wellbore instability caused by abnormal formation pressure, wellbore collapse and other complex drilling problems deep wells. A1 well was safely drilled guided by the study result and has no significant wellbore instability issues and has minimum reservoir damage due to optimal mud weight program. These findings will provide reference for overpressure mechanics study of deep wells. The multidisciplinary study results have created value by improving drilling performance and well delivery efficiency. It can also help operator reduce drilling costs and make development plan decisions.
