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Carbonate Petrophysical Parameters Derived from 3d Images *
Mark Knackstedt1, Mahyar Madadi1, Christoph Arns1, Gregor Baechle2, Gregor Eberli2, and Ralf Weger2
Search and Discovery Article (2009)
Posted March 20, 2009
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008
1Australian National University, Canberra, Australia ([email protected] )
2Comparative Sedimentology Lab, University of Miami, Miami, FL. ([email protected] )
Carbonate
rocks are extremely diverse and their pore spaces complex and heterogeneous.
Large uncertainties in the petrophysical properties
of carbonates are due
to wide variations in pore type, pore shape and interconnectivity. Petrophysical
properties
such as acoustic velocity, permeability, and resistivity are
directly correlated to the amount and type of porosity, the dominant feature
size, and the interconnectivity of different porosity types. Accurately
measuring these attributes requires the quantitative 3D analysis of the
pore structure of carbonates. In this article we describe the imaging and
analysis of two types of carbonate core: a set of vuggy, recrystallized
dolostones and a set of oomoldic limestones. The structure and topology
of the pore space is accurately determined via micro-CT analysis and the
porosity consistent with experimental data. Acoustic velocity-porosity,
pore connectivity, and porosity permeability relationships are derived
directly on the image data via numerical simulation and compared with measured
data on the same rock. Acoustic velocity:porosity trends are good. Pore
structural
properties
(pore size, aspect ratios, pore and throat shape
and connectivity) are determined. The correlations between pore geometry
and topology and elastic and flow
properties
can be directly probed in
a systematic manner. Three dimensional imaging and analysis of carbonate
core
material
can provide a basis for more accurate petrophysical modeling
and improve carbonate reservoir characterization.
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Many
studies have demonstrated the importance of the pore structure
in carbonates on petrophysical
There
is now an opportunity to image and characterise the pore structure
of cores in 3D. This is based on coupling high resolution x-ray
micro-tomography and high end computational software methods, including
visualizing core
The
investigated samples are three recrystallized dolostones and three
oolitic grainstones with moldic pores and variable amounts of microporosity.
These samples were considered as they are end members in regards
to pore structure. In all samples the 2-D pore structure was analyzed
using the DIA methodology. In addition, porosity, permeability
and the elastic
In the recrystallized dolostone samples, the separation between pores and grains (crystals) is easily achieved. In the moldic limestones two pore size populations are detected; large, partly leached, round pores and small pores (1-2 micron). In these samples mercury injection indicates presence of micro- or disconnected porosity (Figure 1 ). The volume of the microporosity in these three samples is 0.3% of 14.1%, 8.3% of 24.8%, and 10.4% of 19.5% total porosity. The porosity is calculated directly from the image data. In all samples the image porosity is comparable to the measured porosity.
From the 3D images the
elastic
Macro/Microporosity and Macropore Phase Connectivity
The three dolostone samples
exhibit 100% connected porosity; in Figure
3
we show an example of the pore partitioning
within a slice of one dolostone sample and a 3D network structure
for the same sample. The 3D volume allows displaying the pore connectivity.
In the sample a number of pore structural parameters can be measured;
for example, the average pore connectivity is measured as 15.7, meaning
that a pore is connected to 15 other pores. The maximum pore connectivity
is 83. Connectivity, pore size information and pore shape can be
directly used to calculate permeability and multiphase flow
The oolitic limestone samples exhibit both resolvable macroscopic porosity and significant microporosity. Tests for connectivity of the resolved macroporous phase in the oolitic systems show that only one system is macroscopically connected at image resolution (see Figure 4 ). The other two samples exhibit disconnected macropores at the resolution of the image. Conclusion
The results of the 3D
imaging and analysis study show that the structure and petrophysical
1. High-resolution CT scans with a resolution of 2.5 microns provide a 3D quantitative structure of carbonate samples. Image porosity correlates well with measured plug porosity. 2. Calculated sonic velocities from FEM simulation based on information from high-resolution CT scans compare well to measured sonic velocities. 3. Recrystallized dolostones exhibit well connected porosity which can be directly quantified in 3D. The pore space topology and pore size information is directly measurable. Moldic rocks exhibit significant proportions of microporosity (porosity below image resolution of ~5 microns). Only one of three moldic samples exhibits connected porosity in 3D. 4. Permeability:porosity crossplots for the same rock type will be undertaken. Correlation of permeability to key geometrical attributes (e.g., pore connectivity and pore to throat aspect ratios) and to acoustic data will be undertaken. References
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