| The exploration and development of shale gas reservoir have proven that formation pressure is not only the
sign of gas accumulation, but also the main driving force of gas migration. It is significant for drilling design, shale dessert
prediction, and horizontal well deployment with accurate prediction of formation pressure. The seismic formation pressure
prediction method can effectively obtain the vertical and horizontal pressure distribution characteristics within the research
area, by which it has become one of the main method for pre-drilling formation pressure prediction. The common seismic
methods for predicting formation pressure can be divided into two kinds: methods based on normal compaction trend lines
and methods based on the principle of effective stress. The first type requires the establishment of accurate normal
compaction trend lines which depend on the experience of different researcher. The second type only considers the
longitudinal wave velocity and ignores the influence of pore fluid on velocity, by which the prediction accuracy is difficult
to meet the needs of shale gas reservoir exploration and development. Therefore, on the basis of absorbing the advantages
of existing methods, this study constructs a new formation pressure prediction model based on the principle of effective
stress. By introducing shear wave velocity to reduce the influence of pore fluid, a new formula for calculating effective
stress in rocks has been derived, and a new formation pressure prediction model composed of longitudinal wave velocity,
shear wave velocity, and density has been established. This method effectively improves the accuracy of formation
pressure prediction and achieves quantitative prediction of shale formation pressure. The new method has been effectively
applied in the LU208 well block in the southern part of Sichuan Basin. After practical drilling verification, the predicted
results are highly consistent with the actual drilling, with a prediction error of less than 5%. This method is applicable to
overpressure formations caused by compaction. |
