Abstract: Prediction of overpressure in carbonate formations is still a difficult problem in overpressure researches.The most common methods on overpressure prediction in clastic formations are empirical based on Terzaghi effective stress theory as well as logging and seismic parameters(mostly P-wave velocity) which have a clear response to overpressure.These empirical methods are not applicable to predict overpressure in carbonate formations with dense matrix and extremely heterogeneous physical properties.We selected carbonate rock samples to carry out the rock physics modeling experiment with overpressure, and analyzed the relationship of rock elastic property with pore fluid pressure and effective stress.Based on the poroelasticity theory and generalized Hooke's law in fluid-bearing rocks, a theoretical model for overpressure prediction(a quantitative model of overpressure prediction) was established to reflect the quantitative relationship between pore pressure and rock elastic parameters by analyzing the constitutive relation between stress, strain and pore pressure in carbonate formations.The elastic modulus of rock matrix was calculated by Voigt-Reuss-Hill model and mineral composition content in carbonate rocks is obtained through lab measurements.The Wood and Patchy models were used to calculate the elastic modulus of pore fluids, and then the equivalent elastic modulus of rock framework is calcula-ted indirectly by the Biot effective stress theory and the correlation between effective stress and elastic modulus of rock framework was acquired through the rock physics modeling experiment.The above obtained elastic parameters were in turn used to calculate overpressure in carbonate rocks by the quantitative overpressure prediction model.In comparison with the pore fluid pressure artificially loaded in the carbonate rock physics modeling experiment, we verified the applicability of the quantitative model of overpressure prediction in carbonate rocks, and proposed the idea for correction of the theoretical model of overpressure prediction.The rock elastic parameters required for the theoretical model of overpressure prediction can also be calculated by studying logging and seismic data, and the pre-drilling overpressure prediction with seismic data could also be achieved in carbonate formations.

Key words: poroelasticity theory, effective stress, elastic wave velocity, rock physics modeling, overpressure prediction, carbonate formation