A Pore Pressure Calculating Method for Carbonate Formations Based on Fluid Velocity

Compared with clastic sandstone formations,carbonate formations exhibit more pronounced heterogeneity,with more developed pores and fractures and higher matrix rigidity.Therefore,the formation pore pressure calculation method based on the normal compaction theory is unsuitable for carbonate formations.In this paper,the method for calculating the pore pressure of carbonate formation was studied.It was indicated from acoustic velocity experiments and a theoretical analysis on carbonate rock samples that the change of P-wave velocity in carbonate rocks under different pore pressure was mainly caused by the change of P-wave velocity in pore fluids.The effect of small fluctuation of P-wave velocity in pore fluids on P-wave velocity in rocks was extracted and amplified by means of wavelet analysis method to identify the abnormal pressure layers in carbonate formations.Then,combined with the measured formation pore pressure,the characteristic parameters reflecting pressure were searched for and a model for predicting the pore pressure of carbonate formation was established.And thus a pore pressure prediction method of carbonate formation based on fluid velocity was developed.It was indicated from application cases that this method could predict the pore pressure of carbonate formations with a margin of error lower than 15%,which satisfies engineering requirements.It also provides a new method for predicting the pore pressure of carbonate formation.