碳酸盐岩超压岩石物理模拟实验及超压预测理论模型

The rock physics modeling experiment under overpressure and theoretical model for overpressure prediction in carbonate rocks

碳酸盐岩地层超压预测目前仍然是超压研究的难点问题,常用的碎屑岩地层超压预测方法是建立在Terzaghi有效应力理论基础上的、经验性的、且需要有明确响应超压的测井和地震参数(主要是纵波速度)。这些经验性的方法不适用于岩性致密且物性极不均一的碳酸盐岩地层的超压预测。通过碳酸盐岩样品超压岩石物理模拟实验剖析岩石弹性性质与孔隙流体压力和有效应力的关系,基于含流体岩石多孔介质弹性理论和广义胡克定律,从分析碳酸盐岩地层应力-应变-孔隙压力本构关系着手,建立表征孔隙压力与岩石弹性参数定量关系的超压预测理论模型(超压预测量化模型)。利用实测碳酸盐岩样品矿物组分含量并结合Voigt-Reuss-Hill模型计算岩石基质弹性模量,利用Wood模型和Patchy模型计算孔隙流体弹性模量,然后再利用碳酸盐岩样品岩石物理模拟实验得到的实际有效应力与岩石骨架弹性模量相关关系,根据Biot有效应力定律,计算得到岩石样品的等效骨架弹性模量。利用上述获得的碳酸盐岩样品各弹性参数,通过超压预测量化模型计算碳酸盐岩超压,并与碳酸盐岩样品岩石物理模拟实验加载的孔隙流体压力进行对比,验证了超压预测量化模型的合理性,提出了基于实测资料的模型校正方法。该超压预测理论模型所需的岩石弹性参数也可通过研究测井和地震资料计算获得,并可利用地震资料实现碳酸盐岩地层的超压钻前预测。

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.