Velocity-Porosity-Mineralogy Model for Unconventional Shale and Its Applications to Digital Rock Physics

Jack Dvorkin,Joel Walls,Gabriela Davalos

doi:10.3389/feart.2020.613716

Jack Dvorkin, Joel Walls + Show 1 more

Open Access

https://doi.org/10.3389/feart.2020.613716

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Abstract

By examining wireline data from Woodford and Wolfcamp gas shale, we find that the primary controls on the elastic-wave velocity are the total porosity, kerogen content, and mineralogy. At a fixed porosity, bothVpandVsstrongly depend on the clay content, as well as on the kerogen content. Both velocities are also strong functions of the sum of the above two components. Even better discrimination of the elastic properties at a fixed porosity is attained if we use the elastic-wave velocity of the solid matrix (including kerogen) of rock as the third variable. This finding, fairly obvious in retrospect, helps combine all mineralogical factors into only two variables,VpandVsof the solid phase. The constant-cement rock physics model, whose mathematical form is the modified lower Hashin-Shtrikman elastic bound, accurately describes the data. The inputs to this model include the elastic moduli and density of the solid component (minerals plus kerogen), those of the formation fluid, the differential pressure, and the critical porosity and coordination number (the average number of grain-to-grain contacts at the critical porosity). We show how this rock physics model can be used to predict the elastic properties from digital images of core, as well as 2D scanning electron microscope images of very small rock fragments.

Highlights

Relations between the elastic properties of unconventional rocks and their volumetric properties, namely porosity, mineralogy, and kerogen content, are important in guiding reservoir development based on seismic data
This paper shows good agreement between model prediction and wireline log data in the Wolfcamp when reasonable assumptions are made about the elastic properties of kerogen and clay
We present applications of this RPM in the context of digital rock physics (DRP), where we compute the elastic properties as a function of porosity, mineralogy, and kerogen content obtained from digital images of core, as well as 2D SEM images of small rock fragments

Summary

INTRODUCTION

Relations between the elastic properties of unconventional rocks and their volumetric properties, namely porosity, mineralogy, and kerogen content, are important in guiding reservoir development based on seismic data. We compute velocity versus porosity curves according to this model for 100% water saturation case and a) using the minimum density and the bulk and shear moduli of the solid phase in the interval under examination and b) the maximum values of these inputs. By applying the RPM derived here from wireline data to petrophysical results from such core CT scanning, we can obtain high-resolution profiles of the dynamic elastic properties along the core For this purpose we assumed that the carbonate is pure dolomite, silica is quartz, and clay is illite. By applying a RPM to these outputs, we can estimate the elastic properties in deviated or horizontal wells where wireline measurements and core extraction are difficult and risky, if not impossible In the case under examination, these inputs match reasonably well providing a close match between the elastic properties measured in the well and computed using DRP

Findings

DISCUSSION

CONCLUSION