Quantifying the impact of early calcite cementation on the reservoir quality of carbonate rocks: a comparison of 2D and 3D process-based models

Ola (Aleksandra) Hosa, Rachel Wood

Research output: Contribution to journalArticlepeer-review

Abstract

The reservoir properties of carbonates are controlled both by deposition and
diagenesis, and the latter includes early calcite cementation which can exert a
strong control on the evolution of subsequent diagenetic pathways. Here we investigate
early cement growth in grainstones to quantify the impact on evolving
pore space and partially to examine trends in the relationships between cementation
and permeability. We compare process-based models of early cementation
in 2D (Calcite2D) and 3D (Calcite3D). Both models assume polycrystalline and
monocrystalline grain types, upon which grow isopachous and syntaxial calcite
cement types, respectively. We also model two common rhombohedral calcite
forms: the blocky form 0112 and elongated form 4041.
Results demonstrate the effect of cement competition: an increasing proportion
of monocrystalline grains creates stronger competition and a reduction in
the impact of individual grains on the early calcite cement volume and porosity.
Isopachous cement is effective in closing pore throats and limiting permeability,
especially in the 2D model. We also show that the impact of syntaxial cement
on porosity occlusion and therefore flow is highly dependent on monocrystalline
grain location and direction of the grain crystal axis. This can lead to very
different permeabilities in samples of the same porosity in both the 2D and 3D results.
3D modelling shows that for samples with crystal form 0112 at constant
porosity, permeability becomes lower as the proportion of monocrystalline grains
increase, although this impact is relatively minor. Samples with crystal form
4041 produce inconclusive results.
Poroperm data generated by Calcite3D can be fitted with an exponential
curve with a high coefficient of determination, as observed in natural media.
This is in contrast to the 2D study, where the variability at any given porosity
spans up to two orders of magnitude. Moreover, the clustering of Calcite2D
model outputs with different widths of isopachous cement suggests that this
cement type is a strong control on the permeability. Results of the 2D modelling
(0.01 􀀀 8D) are in reasonable agreement with measured permeability reported
for grainstones (0.1mD 􀀀 5D) as well as for the plug data of the samples used
in modelling (porosity 22 􀀀 27%, permeability 0.2 􀀀 3D). Permeability results,
however, at any given porosity have a wide range due to the bias inherent to
the 2D flow modelling. Calcite3D is successful in modelling realistic changes in
cement volumes and in the pore space morphology, but permeabilities (0.01 􀀀
30D) are above the range reported due to very high permeability of the initial
synthetic sediment deposit (58.9D).
These results illustrate that subtle differences in early carbonate diagenesis,
such as the exact location and orientation of the crystal axes of the monocrystalline
grains, can have significant impact on the properties of cemented rocks.
We also highlight the importance of 3D, rather than 2D, digital rock modelling
and flow simulation to obtain reliable rock properties predictions.
Keywords: Digital rock, Synthetic rock, Carbonates, Porous Media, Porosity,
Permeability, Calcite cement, Isopachous cement, Syntaxial cement.
Original languageEnglish
Pages (from-to)89–104
JournalAdvances in Water Resources
Volume104
Early online date18 Mar 2017
DOIs
Publication statusPublished - Jun 2017

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