--> Integrated Predrill And Real-Time Pore Pressure Prediction For Exploration Well Drilling In Xihu Depression, East China Sea

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Integrated Predrill And Real-Time Pore Pressure Prediction For Exploration Well Drilling In Xihu Depression, East China Sea

Abstract

The drilling of the exploration wells in Xihu depression in East China Sea becomes very challenging while the drilling is migrating to 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 abandment of some of the wells without completing the designed data collection task. Accurate pore pressure prediction becomes very crucial for the decision making and 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 mechanism 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 plan well, low, high and most possible pore pressure profiles cases were produced using all the available data including seismic velocity and depth-stretched logs from different offset wells to count for the geologicla and geophysicla uncertainties. During the drilling of the plan 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 adjusted in real time to assist the decision making in mud weight selection and casing depth changes.

The above approach was applied in the drilling of one of the exploration wells in Xihu depression. The well was drilled successfully by changing the casing design based on the real time pressure monitoring results. The data collection and pay zone appraisal was accomplished for the key zones after setting an extra 7” liner due to the prediction of high overpressure in the deeper formations. The overpressure was confirmed later by blind drilling a 6” hole for testing purpose.

After the completion of the plan well drilling, a post drill summary were conducted for the well to compare pressure models at different stages. In addition to the final detailed pressure model, an understanding in overpressure starting depth, overpressure mechanism and difference to the nearby area were also summarized which can be used and referenced in the drilling of future wells.