Micromechanics modelling for mineral volume fraction determination: application on a terrigenous formation

Rubén Nicolás-López,Enrique Coconi-Morales,Oscar C. Valdiviezo-Mijangos,José A. España-Pinto,Jaime Meléndez-Martínez,Carlos Couder-Castañeda,Alfredo López-Lena-Estrada

doi:10.1038/s41598-020-73775-w

Rubén Nicolás-López, Enrique Coconi-Morales + Show 5 more

Open Access

https://doi.org/10.1038/s41598-020-73775-w

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Abstract

This work presents a non-linear Self-Consistent (SC) micromechanics method to model the observed physical elastic properties of a terrigenous formation with the purpose to obtain its depth mineral volume fractions profile. In this approach, it is first assumed that the observed physical elastic properties obtained from well logs, such as the density rho_{o} and the elastic compressional Vp_{o} and shear Vs_{o} velocities, are a non-linear relationship of the unknown mineral volume fractions alpha. Then, a gradient descent algorithm is implemented to seek for those volume fractions alpha for which differences between modelled and observed physical elastic properties are minimum. It is assumed that quartz, calcite and clay are the main comprising minerals of the formation. Obtained volume fractions profile follow the same general trends to those estimated by implementing the Linear Least-Squares Inversion LLSI method which is widely used in petrophysical analysis to obtain mineral concentrations from density rho_{o}, photoelectric effect Pe_{o} and compressional slowness Delta tp_{o} well logs. Results also show that calcite and clay volume fractions from these two methods are highly correlated while quartz volume fractions show low correlation. Further comparison between clay concentrations from SC method with clay concentrations calculated from direct measurements of gamma ray GR well logs used as a guideline also exhibits high correlation. These results suggest that the SC method is better suited to obtain clay and calcite volume fractions rather than quartz volume fractions. However, SC method can provide with insights about the general distribution of quartz along the borehole.

Highlights

This work presents a non-linear Self-Consistent (SC) micromechanics method to model the observed physical elastic properties of a terrigenous formation with the purpose to obtain its depth mineral volume fractions profile
Since equivalent relationships to Eq (12) for quartz and calcite are not available in the literature, the total volume fractions of quartz and calcite from the GR well log can be estimated as follows: α(QTZ+CA) [GR] = 1 − αCL [GR]. The latter leads to values of P = 0.78 and P = 0.83 when correlating results from GR linear model with SC and LLSI methods respectively. These results suggest that SC method is best suited to estimate clay and calcite volume fractions as well as volume fractions of quartz + calcite mineral assemblages rather than estimating quartz volume fractions
A non-linear micromechanics SC method to calculate clay, calcite and quartz volume fractions from well logs obtained on a terrigenous formation have been implemented

Summary

Introduction

This work presents a non-linear Self-Consistent (SC) micromechanics method to model the observed physical elastic properties of a terrigenous formation with the purpose to obtain its depth mineral volume fractions profile. The system of non-linear SC micromechanics equations proposed in this contribution attempt to model the following rock elastic physical properties: the density ρo and both the compressional Vpo and the shear Vso velocities; where the velocities are calculated from compressional tpo and shear tso slowness well logs.

Results

Conclusion