The drilling of the exploration wells in Xihu depression in East China Sea becomes very challenging as the drilling campaign shifts towards the deeper part of the west slope. Due to the complex pressure compartments formed by highly faulted blocks, unexpected high pressure encountered in the targeted formations has caused the early abandonment of some of the wells without managing to properly evaluate the reservoir potential. Providing an accurate pore pressure prediction therefore becomes very crucial to enable the decision-making process required for drilling success.
An integrated approach was adopted to combine detailed predrill prediction with real-time pore pressure monitoring considering the uncertainties in the seismic velocities and varying geological conditions in different faulted blocks. During the predrill modelling, a few nearby offset wells were reviewed in detail to construct the pore pressure model and to understand the overpresure mechanisms and geological settings. Seismic velocities at offset well locations were also compared with the acoustic logs to understand the uncertainties in different datasets. For the planed well, a range of low, high and most possible pore pressure profiles were interpreted using all the available data including seismic velocity and depth-stretched logs from different offset wells to account for the geological and geophysical uncertainties. During the drilling of the exploration well, real-time pore pressure monitoring was conducted by integrating the Logging While Drilling (LWD) acoustic and resistivity logs, mud logging and drilling data. Pore pressure and fracture gradient profiles were updated in real-time to enable proactive decision making to optimize mud weights and casing points to reduce drilling risks and hazards.
This approach was applied in the drilling of one of the exploration wells in Xihu depression. The well was drilled successfully by optimizing the casing design based on the real-time pressure monitoring results. This enabled the operator to properly evaluate and appraise the reservoir potential owing to setting a liner just above the high overpressured section in the deeper formations that caused drilling issues in previous wells. The overpressure magnitude was confirmed later by blind drilling for testing purposes.
After the completion of the planed well drilling, a post drill summary was conducted to compare the pressure models at different stages. In addition to the final detailed pressure model, a clearer understanding and interpretation of the overpressure transition, overpressure mechanisms and pressure variations compared to the nearby area were summarized and correlated for reference, while planning for drilling of the future wells.