Pore pressure prediction in the Dongfang 1-1 gas field, Yinggehai Basin

Dongfang 1-1 gas field is a high temperature and high pressure area. An accurate pore pressure prediction is very important for safe drilling in this area. The authors have analyzed and classified the overpressure mechanisms on the base of loading and unloading, which are the relationships of vertical effective stress and acoustic velocity. Then, the logging responses for different overpressure mechanisms have been discussed. Based on the above, the new method of pore pressure prediction by means of the DT-density crossplot and DT-densitydepth crossplot has been proposed. The method has been applied to analyze the overpressure causes and predict a high temperature and high pressure well in DF1-1 gas filed, Yinggehai Basin. The results show that the overpressure in DF1-1 gas filed is caused by three factors: disequilibrium compaction, aquathermal expansion and hydrocarbon generation. Their contribution to the overpressure in different formations is different. By using the DT-density crossplot and the DT-density-depth crossplot the contribution can be distinguished and feasible prediction model can be chosen. The authors have proved that the Eaton method is only suitable for disequilibrium compaction; but the Bowers method is effective for the overpressure caused by disequilibrium compaction or fluid volume expansion. This method improves the pore pressure prediction accuracy in DF 1-1 gas filed effectively. It is of importance to the drilling engineering.