Theoretical Model And Application Of Overpressure Prediction In Carbonate Formation Based On The Poroelasticity Theory

Overpressure prediction in carbonate formation is still a difficult problem worldwide.Due to the high density and extremely heterogeneous physical properties,the empirical relationship between pore fluid overpressure and skeleton stress and the geophysical response to overpressure is not clear,which makes the prediction of overpressure very difficult.The research goal of this paper is to innovate research ideas and explore new theory and a technical method that different from the predict method of overpressure in clastic rock.Taking the Puguang-Maoba field located in the northeastern Sichuan Basin as the study area,the evolution mechanism of overpressure is analyzed in carbonate formation of the study area.The stress-strain relationship between rock and fluid in carbonate formation is analyzed by the rock physical simulation experiments such as overpressure elastic parameters and P-wave velocity experiments of carbonate rock,and the theoretical model for overpressure prediction in carbonate formation is established based on the poroelasticity theory.The rock elastic parameters required for overpressure prediction under formation conditions are obtained by using various rock physical models,P-wave velocity,and S-wave velocity.The overpressure prediction in carbonate formation is realized in the Puguang Maoba area of Northeast Sichuan by using drilling data,well-logging data,and seismic AVO data.The test and error analyses are carried out with the measured data.The main achievements are as follows:1.The carbonate rocks of Feixianguan-Changxing formation in Puguang and Maoba structures generally developed paleo-overpressure at 187 Ma?140 Ma,and the reservoir temperature is always above 150 ?.Oil cracking to gas is the main origin mechanism of the overpressure,and tectonic compression and uplift denudation may also contribute to the overpressure.At present,Maoba structure maintains the overpressure,while Puguang structure is in normal pressure state.The evolution characteristics of the pore pressure difference between Puguang and Maoba structures in the study area are summarized as "three differences and one closure",namely: ? Difference of sedimentary facies: In the Early Triassic,the Feixianguan and Changxing formation of Puguang structure has better initial porosity than Maoba structure.The better connectivity in the shallow burial stage is conducive to the infiltration and return dolomitization and mixed water dolomitization of the reservoir in Puguang structure.Thus,the differential evolution process of reservoir and overpressure of Puguang and Maoba structures is formed.? Difference of tectonic compression and uplift denudation: The deformation of Maoba structure is stronger than Puguang structure and the thickness of uplift and denudation is larger,which leads to the overpressure or maintain overpressure in Maoba structure under the sealing of gypsum caprock.? Difference of TSR effect: The decrease of fluid pressure can be attributed to the consumption of hydrocarbons by thermochemical sulfate reduction?TSR?and the dissolution and pore volume increasing caused by H₂S.The dolomitized hot brine in the Puguang structure provides SO₄^2-for TSR,so the Puguang structure is high in H₂S,dissolution and pore enlargement,and overpressure disappears.Except for Changxing formation of well MB-3,Maoba structure contains almost no H₂S,thus there is no such pressure relief mechanism.? Sealing of regional gypsum salt caprock: The regional high-quality gypsum salt caprock is a necessary condition for the overpressure preservation and gas reservoir in Maoba structure.2.The overpressure prediction in clastic rock is realized by establishing the empirical relationship between overpressure and different well-logging or seismic response parameters?mainly P-wave velocity?based on Terzaghi's effective stress theory.Due to the dense lithology and extremely heterogeneous physical properties,the unclear relationship between the overpressure of pore fluid and the stress of rock skeleton,and the unclear geophysical response of overpressure,it is very difficult to predict the overpressure in carbonate rocks.3.The physical simulation results show that the P-wave and S-wave velocities in the saturated carbonate rock are both have a response with the change of pore pressure.The decrease of the effective stress of the dried rock directly affects the elastic modulus of the rock skeleton,which indicates that the overpressure in the carbonate formation can still be predicted by the response of the P-wave and S-wave velocity and the elastic modulus.However,this response is not as significant as the overpressure response in clastic rocks.According to the poroelasticity theory and generalized Hooke's law,the stress-strain constitutive relation of rock under the action of pore pressure and confining pressure can be characterized by the change of the elastic modulus of saturated rock,rock matrix,skeleton,and pore fluid that forming carbonate rock unit,from which the quantitative model for overpressure prediction based on the poroelasticity theory is established.The elastic modulus can be obtained from the calculation of the P-wave and S-wave velocities.The prediction of the overpressure in carbonate rock can be realized by calculating the rock skeleton and the fluid elastic modulus respectively using the rock physical model combined with the physical parameters and other basic parameters.4.In the process of predicting overpressure by using well-logging data,due to the difference of frequency dispersion and attenuation in rocks caused by elastic waves with different frequencies,the calculation of elastic modulus of rock skeleton should be based on the frequency of acoustic data,and select the fluid-solid biphasic medium model can effectively reflect the dispersion and attenuation.The skeleton of carbonate rock is dense,and the lithology and physical properties change greatly because of the stong heterogeneity.The experimental data show that the dispersion and attenuation predicted by the BISQ model have a wide frequency distribution range?10³-10⁷Hz?.The BISQ model is more suitable for calculating the skeleton modulus with well-logging data.In addition,the calculation of key parameters in the theoretical model of overpressure prediction depends on the interpretation accuracy of physical parameters such as lithology,mineral composition and porosity.Therefore,the key that makes overpressure prediction more accurate is to carry out a comprehensive well-logging interpretation to get carbonate lithology and physical properties by analyzing the well-logging interpretation model applicable to carbonate formation.The overpressure prediction results of carbonate formation in typical overpressure drilling in Northeast Sichuan Basin show that the predicted pore pressure value is close to the DST value,the relative error range is 0.60 %?10.22 %,and the average error is 5.87 %.The predicted pore pressure changes obviously with depth,which better reflects the characteristics of frequent changes of overpressure consistent with the multiple heterogeneity of carbonate formation in Shuangmiao 1 well.The prediction results are close to the actual situation,which shows that the quantitative model of overpressure prediction based on the poroelasticity theory can be applied to the overpressure prediction in the actual carbonate formation.This model is not limited by the origin of overpressure.It is theoretically suitable for the overpressure prediction in the most sedimentary reservoirs.By studying the method of calculating rock elastic parameters with seismic data,and then using the quantitative model of overpressure prediction can realize predicting overpressure in carbonate formation before drilling.5.The velocity of P-wave and S-wave can be obtained by using the seismic AVO inversion technique,and then the rock elastic parameter for predicting pressure by the theoretical model can be calculated from P-wave and S-wave velocities.The pore pressure prediction results show that overpressure is developed in carbonate formation from the upper Permian to the Middle Triassic in Maoba area,while the carbonate formation in Puguang area is mainly normal pressure system.The case study shows that the theoretical model of overpressure prediction based on the poroelasticity theory,the models for calculating various elastic parameters,and related parameters can be used to predict the overpressure of carbonate formation.The accuracy of the prediction depends on the objectivity of various related parameters and the degree of conformity with geological condition.The error of overpressure prediction using well-logging data is relatively small,while that of seismic data is relatively large.The geological information contained in seismic data is multi-resolution and more complex.In the process of calculating pore pressure,many models must be used to obtain parameters with well-logging and testing data.The objectivity of these models and results has an important impact on the accuracy of pressure prediction,which needs further study.Based on the linear the poroelasticity theory,the overpressure prediction in carbonate formation belongs to the category of rock physics,and models for calculating parameter is complex,so it is difficult to be applied in practice.In the process of calculating overpressure with well-logging data,the interpretation of the basic physical property data?porosity,lithology composition,water saturation,etc.?is very important.It is necessary to select a comprehensive model of well-logging interpretation that suitable for carbonate rocks.we need to refine the interpretation steps and use the physical model of key elastic parameters to calculate the bulk modulus and then predict overpressure.The processing of seismic AVO data and the calculation of rock elastic parameters need to be more professional.The prediction of overpressure in carbonate formation by using the theoretical model based on the poroelasticity theory and seismic AVO technology also needs some statistical relations,so as to further improve the parameter acquisition method and the accuracy of pore pressure prediction.