Rock physics inversion based on an optimized MCMC method.

Rock physics inversion is to use seismic elastic properties of underground strata for predicting reservoir petrophysical parameters. The Markov chain Monte Carlo (MCMC) algorithm is commonly used to solve rock physics inverse problems. However, all the parameters to be inverted are iterated simultaneously in the conventional MCMC algorithm. What is obtained is an optimal solution of combining the petrophysical parameters with being inverted. This study introduces the alternating direction (AD) method into the MCMC algorithm (i.e. the optimized MCMC algorithm) to ensure that each petrophysical parameter can get the optimal solution and improve the convergence of the inversion. Firstly, the Gassmann equations and Xu-White model are used to model shaly sandstone, and the theoretical relationship between seismic elastic properties and reservoir petrophysical parameters is established. Then, in the framework of Bayesian theory, the optimized MCMC algorithm is used to generate a Markov chain to obtain the optimal solution of each physical parameter to be inverted and obtain the maximum posterior density of the physical parameter. The proposed method is applied to actual logging and seismic data and the results show that the method can obtain more accurate porosity, saturation, and clay volume. [ABSTRACT FROM AUTHOR]